Long-Term Performance of Natural Stone Cobbles for Paving Raised Junctions: Findings from over a Decade of Use

Abstract

Raised junctions (RJs) are chosen more and more frequently by town planners and road designers in traffic calming (TC) projects. This choice is supported by analyses of the existing transport systems in question. Where a few raised junctions have been designed for the project, use of different paving options may be worth consideration. This article describes a situation where a few RJs were placed on a short street section, all of which were provided with a cobblestone or cobblestone pavement (SBP). In order to verify the commercial viability of this option, we examined the traffic conditions, assessed the severity of condition of the pavements under analysis, and estimated the vehicle accelerations (ACs) and decelerations (DEs) in two chosen study areas, further referred to as the first and second study area. Two noise surveys were also carried out for the purposes of this research: the first one after a few years of operation and the second one after another ten years. Considering the problems of sustainable road construction, many environmental factors were taken into account in the studies. Based on the experimental results, we could assess the effectiveness of using RJs as the only TC measure in 30 km/h zones and check whether the severity of pavement condition depended on the AC and DE values and if SBP can be recommended to mitigate environmental impacts of street traffic. Finally, two pavement structures are proposed to choose from, depending on the local transport system conditions and streetscape characteristics. In addition, the authors recommend the use of solar-power elements at RJs to enhance their visibility and increase the traffic slowing effect.

1. Introduction

Traffic calming schemes are implemented to protect residential areas, especially in suburban areas, from traffic impacts—including traffic-generated noise—which have become an issue due to the rapid pace of economic growth and the associated ever-growing number of motor vehicles on the roads. In the so-called 30 km/h or 40 km/h zones or home zones, we can find horizontal or vertical traffic calming measures. The zone 30 speed limit tends to be exceeded, as has been shown in a report from speed surveys carried out in the Netherlands [1]. Vertical speed control elements (VSCEs) are preferred over horizontal speed control elements (HSCEs) in areas constrained by limited availability of public space around existing streets. The former include various vertical deflections (VDs), including speed bumps, speed humps, speed tables, raised crossings and, last but not least, raised junctions. On the other hand, horizontal deflections, such as chicanes and road chokers, which do not impose such rapid braking as is the case before speed humps, are much preferred by drivers. These, however, take more land. Taking the above facts into consideration, raised junctions, which care for the safety of pedestrians and drivers and provide a better quality of public space design, are becoming a more and more popular option among designers [2,3,4,5,6,7,8,9,10]. They are particularly recommended and chosen in suburban areas on minor or less important junctions, where through traffic streets intersect minor, local streets and where high pedestrian traffic occurs, typically near public buildings and facilities [8,11,12,13].

Raised junctions improve traffic safety and encourage drivers to yield to pedestrians intending to legally cross the street [9,10,13,14,15]. Merging of the carriageway and footway into one level surface is often designed to encourage slower driving through the junction. It is not required to mark such pedestrian crossings, unless they are not at the same level as the adjacent footway [8,9,10,11,12,13,14,16]. In order to obtain one, a level surface approach (AR) and departure ramps (DRs) must be designed with appropriate inclinations [8,9,12,13,14,17]. These inclinations and the end height may vary strongly depending on the country and the local conditions [15,18,19,20,21,22,23]. Where dealing with high traffic volumes, additional solar LED road studs or different pavement colours or even surface types may be used, typically on the main junction legs.

Examples of suburban road systems where raised junctions are typically used are shown in Figure 1. Suburban town squares of square, rectangular, or round shape with radially arranged local streets leading to them can serve as the first example (Figure 1a). A town square of this kind may host community events or accommodate recreational facilities or playgrounds. The area should be occupied primarily by single-family buildings or other residential developments. The second example is a through street intersecting a few minor streets of local importance or linking smaller residential areas (Figure 1b). Residential buildings may be accompanied by associated retail outlets. In this case, the key design parameter is the streetscape character and traffic composition. In the case shown in Figure 1a, only passenger cars and municipal vehicles are allowed on the street. In the case shown in Figure 1b, the traffic also includes heavy vehicles, mass transit vehicles, and a high volume of cycle traffic [24,25]. This traffic composition must be reflected in the raised junction design, which should take into account the different characteristics of the road users in question.

Another issue pertaining to the use of raised junctions in 30 km/h zones is surfacing design and construction. To take into account the problems of sustainable road construction, the surroundings and streetscape characteristics may impose the usage of certain materials, conventional or recycled, as noted in [26,27,28,29]. The guidance provided by the available engineering and scientific literature on the subject is limited to providing general information on the selection and application of various natural stone cobbles or specification of mechanical and physiochemical performance requirements for this material [30,31]. However, the conclusions of [30] and recommendations given in [32,33,34] indicate that one of the main problems intrinsic to cobblestone pavements is stabilisation by the setting action of the joint filling material, which, in addition, provides the primary resistance to traffic loading of these pavements. Analysis of studies and recommendations contained in [30,31,32,33,34], taking into account the problems of sustainable road construction, indicate the importance of this filling in cobblestone pavements in stabilising the units, transmitting and dissipating loads, and controlled draining of surface water. Other problems intrinsic to cobblestone pavements include distress caused by mechanical and physical–chemical factors, design flaws due to incorrect assumptions, or construction defects [30].

When considering the implementation of raised junctions in 30 km/h zones, it is also important to investigate the impacts of such vertical deflections on driving comfort. These investigations should consider the driving style and behaviour of drivers [35] and engineering aspects by measuring acceleration and deceleration values before and past the vertical deflection in question. These issues were investigated in a number of research projects carried out with the use of special equipment for determining the driving parameters across speed humps [36,37,38].

Another important issue in the analyses of sustainable road construction carried out in the 30 km/h zone regarding the use of cobblestone pavements is road noise [39,40,41,42,43,44], be-cause, as shown in [40,42], it may considerably affect the comfort of living or even health of the residents [40,42]. The noise impacts on the well-being of the affected people, including irritability and sleep disorders, were thoroughly studied in [41]. In this connection, the street surroundings are rated in different sound absorption classes [39,45] and sound absorption coefficients are determined for the surrounding vegetation, taking into account its arrangement in the adjacent area [46,47]. According to Wikosz-Mamcarczyk [48] and Dmitrowa et al. [49] and Gonzalo et al. [50] and [51], the sound distribution through space is influenced by the local topography, layout of buildings, adjacent development, and the effect of vegetation (including type, density and height). Noise analyses related to implementation or impact assessment of traffic calming schemes should be conducted for the traffic calming measures in place [52,53,54] and also speed limit signs used alone [55]. The pavement type and construction method are also highly relevant in this respect [44]. Considering the findings reported in [56,57], it should also be checked if the imposed speed limit would influence, and if yes to what extent, the sound environment in the vicinity. Based on the data given in [56,57], a reduction in driving speed from 50 km/h to 40 km/h decreased the traffic noise level by 1.4 dB(A) on straight sections of the analysed street with lower density buildings, while no further noise reduction was observed after further reduction from 40 km/h to 30 km/h. Thus, when choosing traffic calming measure, one should take into account, on an equal basis, the desired slowing effect and streetscape, road system features, and residential development characteristics.

Therefore, as we can see, not all the issues pertaining to the design of raised junctions in 30 km/h zones have been resolved as yet. For example, the effectiveness of using exclusively cobblestone paved raised junctions in the 30 km/h zones has not been covered by the literature on the subject, not to mention the issue of their deterioration under traffic after years in service. In order to fill this gap, we have formulated the following research questions, which we intend to answer in this research:

• Research question ①—“Will raised junctions be effective when used as the only traffic calming measure in 30 km/h zones?”
• Research question ②—“Will the use of raised junctions as the only traffic calming measure in 30 km/h zones have any bearing on the condition of the cobblestone pavement after over a dozen years of use?”
• Research question ③—“Can cobblestone paved raised junctions be recommended for use in 30 km/h zones to mitigate environmental impacts?”

Several analyses were conducted to answer these research questions, which are schematically represented in Figure 2 below. In the first step, the most appropriate 30 km/h zones were chosen from among a few dozen ones located in the town of Szczecin, including its suburbia. The primary consideration was transparency and representativeness of the study area, bearing in mind applicability of the results of this research for future planning of 30 km/h zones in other towns. Two sites were identified as the most representative for the existing traffic systems and thus chosen for further analysis. Next, traffic surveys were carried out at these two sites to answer research question ①. These traffic surveys enabled the determination of horizontal forces acting on the pavements, whose condition was assessed by visual examination. The analysis of horizontal forces and condition evaluation of the analysed pavements allowed us to answer research question ②. A statistical analysis of the obtained horizontal forces and pavement condition parameters showed that in the studied areas, particular attention should be paid to the streetscapes around the raised junctions as a factor highly relevant to the pavement design. Another important issue related to the use of cobblestone pavements is their environmental impacts, especially in traffic calming projects. In this connection, noise surveys were carried out at different pavement condition degrees to answer research question ③.

2. Materials and Methods

2.1. Study Area

As mentioned, two study areas were chosen in this research project. The first of them was a town square located in the suburbs of Szczecin, in a historical district with architecture echoing the architecture of Berlin Charlottenburg. There are two-way streets of local importance meeting at the square. The properties around the square are architecturally diverse multi-family buildings surrounded by landscaped gardens. In 2010, cobblestone paved strips were installed on both sides of the 6.3 m wide road running around the square. The inner strip was 0.8 m wide, and the outer strip was 1 m wide. Five raised junctions were installed in the square area in the same year. The square has an outer diameter measuring 116 metres. The peak hour traffic does not exceed 100 veh./h there (only passenger cars and occasional passages of garbage truck). The narrowing strips and raised junctions were paved with 15 × 15 cm split face cobblestones, laid on a 5 cm thick cement/sand bed.

The second study area was a through street running through the suburban part of Szczecin between residential and commercial ones. The two-way street is ca. 870 m long and has 3 m wide travel lanes. On the way, the street has a few junctions with side streets leading to small gated communities or home zones. Four raised junctions were installed on this street in 2010. There is one signalised junction at one end, where it meets a dual carriageway of high importance in the road system. On the opposite end, there is a small roundabout whose exits are all part of 30 km/h zone. On one side of the street, there are many public buildings, hypermarkets, and industrial premises with large underground car parks. On the other side stand residential buildings separated from the street by fences and vegetated strips The traffic in this street includes heavy vehicles and city buses. At one raised junction, there is a bus terminus where all five bus services have a stop. The peak hour volume ranges 600–700 veh./h (the share of heavy goods vehicles does not exceed 10%, including up to 24 buses per hour). The narrowing strips and raised junctions have been paved with 18 × 25 cm reclaimed cobblestones, laid on a 5 cm thick cement/sand bed.

In both study areas, the lower parts of the pavements are composed of 20 cm thick lean concrete and 15 cm thick unbound crushed aggregate layers. Kerbs were not installed at approach ramps and departure ramps. The sections between the raised junctions have asphalt pavement.

The design parameters of the analysed raised junctions are given in Appendix A and Appendix B to this report. Tachometric measurements in x, y, z coordinate system on the respective raised junctions were carried out using pole-mounted GNSS receivers. This equipment allowed us to determine the survey point positions to 0.001 m accuracy and GPS enabled spatial positioning of these points and for the relevant conventional survey maps to be updated.

2.2. Traffic and Noise Surveys

The slowing effect of raised junctions was determined based on the speed surveys carried out as part of this research. For this purpose, we used SR4 traffic detection devices that can be mounted on the existing signposts or on temporary posts that are removed after surveys. In this project, the latter mounting option was chosen (Figure 3). SR4 devices were placed sufficiently ahead of the approach ramp lower edge, in order to measure the driving speeds before the start of the ramp. The second measurement point was located in the initial part of the raised junction. At the opposite end, i.e., the departure ramp, the detectors were positioned in the same way as they were ahead of the approach ramp. The placement of detectors is shown in Figure 3. The detectors were time-synchronised, to measure speeds travelled by one vehicle at a time. The information gathered by SR4 detectors included date, time, speed, vehicle type, headway in seconds, and headway in metres. These data enable accurate determination of four driving speeds for each vehicle at the approach ramp and four at the departure ramp, respectively. In addition, the time logging function of SR4 devices (to 0.1 s accuracy) and recorded time headways enabled us to filter out vehicles in free traffic flow conditions, i.e., with the road ahead free of obstacles for a distance covered in 7 s, as per [58].

This method of speed measurement enabled the determination of horizontal forces and for us to analyse their influence on the pavement condition. This assumption adopted for the planned analyses entailed a comparison of the calculated horizontal forces ahead of and past raised junctions with the same forces calculated on a street section without any vertical deflection. Increased horizontal forces imposed on the pavement would then be represented by the D ratio value (Equations (1)–(3)).

Horizontal force acting on pavement during acceleration: F = m a and F₀ = m a₀

(1)

Horizontal force acting on pavement during deceleration: F = m b and F₀ = m b₀

(2)

$$D=\frac{F_{\left(a,\hspace{0.33em}b\right) }}{F_0}=\frac{\frac{G}{g}\left|\hspace{0.33em}a \mathrm{or}\hspace{0.33em}b\right|}{\frac{G}{g}\left|a_0\hspace{0.33em}\mathrm{or}\hspace{0.33em}{b}_0\right|}=\frac{\left|a\hspace{0.33em}\mathrm{or}\hspace{0.33em}b\right|}{\left|a_0\hspace{0.33em}\mathrm{or}\hspace{0.33em}{b}_0\right|}$$

(3)

where F—horizontal force acting on the pavement of street including raised junctions during acceleration (F_a) and braking (F_b), kN, F₀—horizontal force on street section without any vertical deflection s, kN, m—vehicle weight G/g, N, a—acceleration calculated from speed data, m/s², a₀—acceleration in free-flowing traffic on a street section without any vertical deflection, m/s², b—deceleration calculated from speed data, m/s², and b₀—deceleration in free-flowing traffic on a street section without any vertical deflection, m/s².

The next of the planned research tasks was to compare the horizontal forces calculated with Equations (1)–(3) with the raised junction pavement condition parameters.

In the application of sustainable design principles in traffic calming projects, the focus is put on improvements in traffic safety and slowing to the desired speed of 30 km/h or 40 km/h, as appropriate. However, mitigation of environmental impacts by reducing noise, exhaust gas, and other emissions [9,10,14,15,59,60] must not be forgotten in this context of sustainable design. Thus, noise measurements were scheduled to be carried out as part of this research at both study areas. Noise measurements were carried out using an SVAN 971 class 1 A-weighted sound level metre with FAST time weighting, in compliance with IEC 61672-1 [61] and ISO 1996-2 [62]. The noise survey time-span was 15 min. The sound level metres were placed on the pathways at approach ramps and departure ramps before and after each ramp (Figure 3). In addition, considering the progressing deterioration of the analysed pavement, we also decided to measure the sound level on raised junctions.

The maximum daytime noise level given in [63,64] for multi-family buildings surrounded by landscaped gardens is 65 dB(A).

In this case, the pavement condition becomes less important in favour of adjacent development and carriageway width of the main and side legs [44,48,49,50,52]. The following streetscape characteristics are deemed to have a considerable bearing on the impact of noise [46,47,48,49,50,51]:

• Differing levels of urban intensity,
• Distance to carriageway edge and spacing of buildings on both sides of the street,
• Height and front wall style (flat with windows or architecturally and spatially diverse, featuring balconies, recesses, etc.), and
• Land use and soft landscaping features composed of different plants of varying height on both sides of the street.

2.3. Assumptions Relating to Assessment of Cobblestone Pavement Condition on Raised Junctions

In view of the lack of cobblestone pavement assessment guidelines, the methods used in [30,65,66,67,68] were used to assess the condition of cobblestone pavements on the raised junctions under analysis. The analysed pavement deformations included: subsidence and surface waves and mechanical damage types included: joint gaps, units affected by cracking and edge chipping and units with spalled edges, and cracked or chipped corners.

In this study, the deformation areas are delineated along the outer edges of the cobblestones concerned (Figure 4a—subsidence up to 2 cm is marked by a white line and above 2 cm by a red line). Joint gaps and subsidence depth were measured, taking a non-deformed surface as a reference plane, by measuring a gap under a 2 m long straightedge applied on the pavement (Figure 4b). In the case under analysis, we applied the findings of Autelitano et al. [30] that the texture of cobblestone sides and joint width play a major role in stabilising the pavement and transferring the loads imposed thereon. In the case of wide joints, gaps in the joint filling considerably reduce the transfer of traffic-induced stress onto adjacent units [30], hence the joint width limit of 1.2 cm recommended by old guidelines [30,69,70], which, in some cases, may remain in force to this day [71,72]. However, the joint width range recommended by Autelitano et al. in their most recent research on contemporary joint filling materials [30] is 2.5–3.0 cm, and this update is reflected in Figure 4. All the above-mentioned deformations and mechanical defects have been rated as less severe (LS), moderately severe (MS), and very severe (VS).

3. Results and Discussion

3.1. Traffic Survey Result on the Control Section

The analysis of vertical and horizontal forces acting on the pavement presented in [73,74,75,76] was used to represent horizontal forces exerted on the analysed pavements by vehicles moving in free-flowing traffic (Figure 5a,b). Following the assumptions mentioned in Section 2 above, regarding comparison of horizontal forces recorded on the analysed raised junctions, free-flow speeds were measured on the control section, i.e., a cobblestone paved street without any traffic calming measures. Acceleration and deceleration values were calculated for the speeds measured on the control section using Equations (1) and (2), as represented in Figure 5c. These data are shown in the bar graph in Figure 5c in the order corresponding the sequence of measurements. The analysis of the result reveals that free-flow speeds do not exceed the limits of a₀ = 0.6 m/s² and b₀ = –0.6 m/s². In line with the conclusions given in [66,68] and the assumptions of this research described in Section 2, the loading of pavement was considered “increased” where acceleration and deceleration exceeded, by more than two times, the above values of a₀ and b₀. We believe that this loading could lead to early deterioration of the pavement in question.

3.2. Strength Parameters of Reclaimed Cobblestones

For cobblestone pavements, it is allowed to apply a limited scope of testing [77,78,79]. Thus, the scope of testing in this research was limited to visual assessment of the analysed pavements and the units were subjected only to in situ structural tests. Considering the identified mechanical defects and the compressive strength recommendations given in [77,80,81,82,83], appropriate strength tests were carried out in the laboratory on the units sampled at four different locations. The units sampled from the first study area were designated No. 1 and No. 2 and units sampled from the second study area were designated No. 3 and No. 4. As it transpires from the graph representing the compressive strength results (Figure 6), all the units sampled from the analysed pavements satisfied the grade II compressive strength requirement of 120 MPa [80,81,82]. These results confirmed their fitness for use in both study areas under analysis.

3.3. Test Results for Raised Junctions on the Analysed Square with Circular Traffic System

The geometric parameters of the raised junctions located in the first study area are given in

Table A1. Parameters of raised junctions in the first study area. Source: own work.

Parameter	Symbol	Raised Junction
		A	B	C	D	E
Length between ramps, m	l	21	20	12	25	23
Length of approach, m	d	30.9	97.3	17.2	65.0	32.9
Length of approach ramp, m	lb	1.14	1.03	1.05	1.02	1.20
Height of approach ramp, cm	hb	12	13	6	10	11
Slope, %	ib	10.5 1 in 9.5	12.6 1 in 7.9	6.0 1 in 17.5	10.0 1 in 10.2	9.2 1 in 10.9
Length departure ramp, m	le	0.92	1.01	1.14	1.10	1.62
Height of departure ramp, cm	he	10	3	7	7	8
Slope, %	ih	10.7 1 in 9.2	3.0 1 in 33.7	6.1 1 in 16.3	6.4 1 in 15.7	4.9 1 in 20.3

in Appendix A. The approach ramp and departure ramp inclinations varied among the analysed raised junctions and depended on the presence of private entryways. Nevertheless, on all the analysed raised junctions the pavement was merged into one level surface with the adjacent footway.

The speed surveys were carried out on business days between 10:00 and 14:00 h. This was a one-way section and the traffic detectors were placed along the outer part of it. Since further on in this study the acceleration and deceleration values were determined for each ramp, in all the cases a few traffic detectors were placed ahead of and past each approach ramp and departure ramp. All these detectors were time-synchronised to obtain four speeds for each passing vehicle, which were required to calculate acceleration and deceleration, using Equations (4) and (5).

Acceleration AC: a = (v_e − v₀)/t

(4)

Deceleration DE: b = (v_e − v₀)/t

(5)

where a—acceleration, m/s², b—deceleration, m/s²; v_e—end speed, m/s, v₀—start speed, m/s, and t—passage time calculated from SR4 data, s.

Considering the small traffic volumes in this section, the surveys were discontinued upon gathering the speeds for 100 vehicles riding in free-flow traffic conditions. The speed data were then processed using conventional statistical tests, as typically applied in road traffic studies. All the speed populations were found to have normal distribution. The speed data are represented in Figure 7. Not exceeding 30 km/h on any raised junction under analysis, the measured speeds are considered low. The lowest speeds were noted on raised junctions A and B, most probably due to the presence of parked vehicles (Figure 8a). The highest speeds were noted ahead of raised junction C, most probably due to no parked vehicles on the carriageway running around the square and the short length of the section between raised junctions B and C (17 m). The properties located by this section have capacious parking spaces where up to a few cars can park (Figure 8b). This allows us to conclude that in suburban town squares, raised junctions may be considered an effective traffic calming measure. Therefore, it is justified to answer research question ① that in the analysed case raised junctions accompanied by road chokers effectively calmed the traffic and thus may be recommended for use in similar streetscapes. In Figure 7, the graphs are accompanied with photographs of the respective raised junctions, included there for illustration.

Next, acceleration and deceleration values were calculated before and after each ramp of each raised junction. The acceleration and deceleration values in the order of recording of speeds by SR4 detectors are represented in Figure 9. Small acceleration and deceleration values were obtained only at departure ramps of raised junctions A and B, probably due to a large distance between them (Table A1 in Appendix A—ca. 97 m) or lack of vehicles parked past raised junction B that would reduce the travel width (Figure 8b). On the other ramps, values greater than a = 0.6 m/s² or b = −0.6 m/s² were recorded only occasionally. This shows that almost all the obtained acceleration and deceleration values did not exceed acceleration a₀ and deceleration b₀ on the control section. Higher deceleration values were obtained only at approach ramps of raised junctions A and B. Still, they did not exceed the deceleration value recorded on the control section by more than two times. This allows us to conclude that horizontal forces at approach ramps and departure ramps of the analysed raised junctions are comparable to the horizontal forces acting on the control section pavement. This means that increased loading of the pavement in comparison to loading imposed by free-flowing traffic did not occur in the first study area.

In line with the research assumptions described in Section 2 above, the pavement condition was assessed through visual examination only. Neither subsidence nor joint gaps were found in the analysed raised junctions. Thus, the pavement condition after fourteen years of exploitation was rated as very good. Small surface waves were noted only on the exit legs from raised junctions A and C to side roads. However, they were limited to 3 m² in area and outer wheelpaths in space. This defect could be due to some construction faults as no ponding of stormwater was observed and the pavement surface had appropriate longitudinal and transverse slopes. Thus, based on the horizontal forces analysis and the visual assessment of pavement condition, we can answer research question ② that in 30 km/h zones in the first study area, the raised junctions, when used as the only traffic calming measure, do not affect pavement condition after over a dozen years of exploitation.

The last analysis concerns environmental impacts, noise in particular. The noise survey results are represented in Figure 10. The analysis of the results revealed that the measured values are generally below the allowable limit of 65 dB(A). Streetscape characteristics are the most likely primary cause of that, and a very good condition of the pavement on the analysed raised junctions and other parts of the carriageway may also have some bearing. Taking this into account, Figure 10 also includes satellite imagery in the background, which clearly shows streetscape variability around the analysed raised junctions. The central island and the landscaped gardens in the adjacent properties on the outer side of the street include very diverse vegetation of different heights (Figure 11). Residential buildings standing around the square are spaced away from the carriageway and the footway (Figure 11b), which limits reverberation [42,43,49,50,52,84,85,86] and thus the overall noise level. In addition, the central island has its surface lowered by 1 to 4 m below the carriageway (Figure 11a), which also contributes to noise mitigation [48,49,50,54].

Now, the noise results represented in Figure 11 enable us to answer research question ③ by stating that split face cobblestone paving may be used on other planned raised junctions with no adverse environmental impact in the immediate vicinity, as long as diverse vegetation of different heights is used on both sides of the carriageway running around the square. It is worth noting that an uneven top surface of split face cobblestone pavement discourages driving faster than 30 km/h (Figure 7). Although a split face surface increases the noise generated by the passing vehicles, its propagation may be effectively mitigated by the presence of diverse vegetation of different heights (Figure 10). This reduces the impacts of traffic calming schemes experienced by the local residents and pedestrians where raised junctions are used as the only traffic calming measure.

However, the scheme used in the first study area has some drawbacks from a sustainable construction point of view. The access way to the central island recreational area includes ramps, provided there to facilitate access for cyclists, residents with strollers heading to the playground located in the central island area, and for people on wheelchairs (Figure 11a). Note that pedestrian crossings on the way leading to wheelchair access ramps should be paved with stones featuring a flamed, i.e., more even, top surface. The existing, uneven split stone surface causes some discomfort to wheelchair users riding on it. Asphalt pavement would be an option in this place, yet it would increase the construction costs considerably, possibly above the project budget.

The second drawback of the analysed traffic calming design is a reduced lighting of the approach ramps and departure ramps in dusk hours and at night due to the different height vegetation. This can lead to drivers experiencing undesired shock vibrations when passing through hardly visible ramps. Therefore, bearing in mind the principles of sustainable design, we recommend installation of solar road LED studs before raised island approach ramps to alert the drivers of the oncoming obstacle.

3.4. Results for Raised Junctions Located on Collector or Link Streets

Similar speed analysis was carried out for the raised junctions located in the second study area (Figure 12). As can be seen in the graph in Figure 12, the 85 percentile speeds exceeded the 30 km/h speed limit in both traffic directions on all the analysed raised junctions. This means that in the second study area, raised junctions provided as the only traffic calming measure on the through street failed to slow the traffic down to the desired speed. This results in a negative answer to research question ①. Therefore, in our opinion, when contemplating the use of raised junctions as the only traffic calming measure in traffic calming projects involving through streets, a 40 km/h zone should be implemented rather than a 30 km/h zone. Then, the v₈₅ values would not exceed the posted speed limit. In the analysed case, it was not possible to use other traffic calming measures, such as horizontal deflections between the successive raised junctions due to the narrow right-of-way of this street bounded by plots occupied by residential and public buildings. The narrow right-of-way of through streets that are bounded by built-up plots is a commonly encountered problem in many towns and cities. Thus, we recommend implementing a 40 km/h rather than 30 km/h zone where raised junctions are used as the only traffic calming measure in through street traffic calming projects. The streetscape of this street is presented for illustration in Figure 12 below.

The same as for the first study area, accelerations and decelerations were calculated for all the ramps, based on the measured speeds. Figure 13 represents the calculated acceleration and deceleration values in the order of measured speeds in free-flowing traffic (bar graphs closer to the carriageway). The obtained acceleration and deceleration values are much higher than obtained in the first study area on the one hand, and comparable to the results obtained by D’Apuzzo et al. [36] and Cantisani et al. 2010 [37], who carried out their surveys using special vehicles. The acceleration and deceleration values obtained in the second study area were much higher than in the control section (Figure 5). In both cases, the driving style in free-flow conditions (acceleration or braking on the approach to or departure from raised junction) was found to depend highly on the drivers’ personality and driving habits. That said, the acceleration and deceleration values obtained in this case are similar to the results reported in [36,37]. However, in line with this study and the assumptions given in Section 2 above, the analysed acceleration and deceleration values should be representative of free-flow traffic conditions, and thus Figure 12 represents, for comparison, the acceleration and deceleration values calculated for continuous traffic flow on two raised junctions (A and B), where numerous subsidence spots and joint gaps were found. The purpose of this comparison was to determine how the pavement actually behaves when subjected to traffic loading in free-flow and continuous traffic conditions and thus answer research question ②. The bar graphs relating to free-flow traffic conditions show the results of ca. 100 measurements and continuous traffic results have been, for the sake of clarity, limited to half-hourly measurements, giving 150 speed values in total. Considering the conclusions of [30], the different AC and DE variation ranges may be deemed to represent the actual effect of traffic loading on the pavement and thus may be used to explain its premature deterioration (Figure 13). This approach to interpretation of traffic survey results is a novelty in the literature on the subject. Thus far, the acceleration and deceleration values were analysed only in relation to free-flow conditions or to the data obtained from over a dozen passes of a special test vehicle. Commenting further on Figure 13, we should also note that the bar graphs represent the actual pavement behaviour with varying stress levels and rapid changes from compressive to tensile forces in response to braking and acceleration of the subsequent vehicles. Most characteristic variations in horizontal forces are shown, for example, in the acceleration/deceleration chart for continuous traffic conditions at the departure ramp of raised junction B (eastbound direction), where we can see decelerations of over a dozen vehicles, most probably due to the presence of a bus driving out of the terminus. A similar set of over a dozen deceleration values can also be seen at the approach ramp of raised junction B, most probably due to a bus driving into the terminus or other vehicles driving into any of a series of capacious parking lots before the hypermarket, warehouses, and public buildings. On the other hand, in the westbound direction, a strong influence of the surrounding streetscape, where drivers are encouraged to accelerate by a lack of exits after raised junction A, can be seen on the bar graphs for both free-flow and continuous traffic conditions.

All the calculated accelerations and decelerations were subjected to standard statistical tests. The acceleration and deceleration distribution parameters are compiled in

Table 1. Acceleration and deceleration values in second study area. Source: own work.

	Ramp	Acceleration and Deceleration, m/s2
		amax	a85/D	aav	bmax	b85/D	bav
Eastbound traffic
Raised junction A	Approach ramp	7.1	3.3/2.8	1.7	−7.3	−4.1/3.4	−2.0
	Departure ramp	6.9	3.8/3.2	2.2	−6.8	−4.8/4.0	−2.4
Raised junction B	Approach ramp	2.6	1.4/1.2	0.8	−2.2	−1.5/1.3	−0.9
	Departure ramp	3.9	2.5/2.1	1.5	−6.0	−3.1/2.6	−1.9
Raised junction C	Approach ramp	2.2	1.2/1.0	0.6	−4.9	−1.7/1.4	−1.1
	Departure ramp	2.3	1.9/1.6	0.9	−7.2	−4.3/3.6	−2.5
Raised junction D	Approach ramp	1.4	1.1/0.9	0.6	−3.5	−2.0/1.7	−1.1
	Departure ramp	2.8	1.4/1.2	0.7	−7.3	−3.7/3.1	−2.2
Westbound traffic
Raised junction D	Approach ramp	2.8	1.4/1.2	0.9	−1.6	−1.2/1.0	−0.6
	Departure ramp	3.9	2.6/2.2	1.5	−7.5	−4.9/4.1	−2.6
Raised junction C	Approach ramp	2.5	1.8/1.5	0.9	−2.8	−1.5/1.3	−0.8
	Departure ramp	6.1	4.3/3.6	2.5	−3.6	−2.1/1.8	−1.0
Raised junction B	Approach ramp	2.3	1.4/1.2	0.8	−2.2	−1.3/1.1	−0.8
	Departure ramp	6.4	3.3/2.8	2.1	−5.4	−2.5/2.1	−1.5
Raised junction A	Approach ramp	4.9	2.6/2.2	1.7	−7.2	−4.5/3.8	−2.4
	Departure ramp	6.8	3.6/3.0	2.2	−4.8	−3.0/2.5	−1.6

. In addition, Table 1 gives the values of D calculated with Equation (3).

For illustration, acceleration and deceleration values for all the analysed raised junctions, separately for the two directions of traffic, are represented in a box plot in Figure 14.

Next, the pavement condition after over a dozen years of exploitation was assessed through visual examination, separately for the two traffic directions. The results of this examination are compiled in

Table 2. Results of visual assessment of the pavement condition in the second study area. Source: own work.

	Ramp	p1, m2	p2, m2	p3, m2	g1, m2	g2, m2	k, szt.
Streetscape in the eastbound direction (car park access points, warehouse driveways, bus terminus entry and exit)
Raised junction A	Approach ramp	35.1	10.1		76.3	18.5	1
	Departure ramp	39.0	8.0		68.7	15.7
Raised junction B	Approach ramp	25.4	6.2	12.0	56.8	12.1
	Departure ramp	22.9	6.0		51.2	13.3	1
Raised junction C	Approach ramp	15.2			44.6	2.7	1
	Departure ramp	16.9			40.1	2.0
Raised junction D	Approach ramp	14.3			42.0	3.2
	Departure ramp	15.9			37.8	2.5
Streetscape in the westbound direction (driveways to gated residential communities, to a home zone, and to a local street leading to a small housing estate)
Raised junction D	Approach ramp	12.7			28.8	1.5
	Departure ramp	10.2			26.7	1.3
Raised junction C	Approach ramp	8.1			30.6	1.4
	Departure ramp	10.8			28.3	2.0
Raised junction B	Approach ramp	12.2			39.8	4.8
	Departure ramp	16.3			36.8	6.0
Raised junction A	Approach ramp	31.2			52.3	13.0
	Departure ramp	29.4			48.4	12.0

Designations: p1—at a depth of 1 to 2 cm subsidence; p2—at a depth of 2 to 5 cm subsidence; p3—subsidence at the terminus entry point; g1—up to 2 cm joint gap, g2—over 2 cm joint gap, k—number of broken stone cubes.

below.

A detailed analysis of the visual examination results showed strong variation in the severity of the following:

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Deformations, most probably related to the traffic conditions and poor construction over a dozen years ago.

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Damaged or joint gaps, aggravated by wide joints and lack of stagger between rows of cobblestone units.

The cobblestone pavement construction guidelines, covering the joint width, depth of cement/sand joint filling, and laying pattern, were described in the literature years ago [30,31,32,33,34,71,72]. More recently, research projects on cobblestone joint filling and laying patterns and on traffic-induced horizontal forces were carried out by Autelitano et al. [30], D’Apuzzo et al. [36], Cantisani et al. [37], Garilli et al. [79], and Mampearachchi [87]. Particularly relevant, and useful for our research, are the findings of Autelitano et al. [30]. This article elaborates on the issues covered there by relating them to joint gaps and different joint widths, as shown in Figure 15. Taking into account the severity of pavement condition—in particular at deeper subsidence spots—and linking this with the presence of joint gaps, we have come to the conclusion that 4–5 cm wide joints affect the resistance of the individual cobblestones to displacement under traffic loading, with this including sinking, shifting, and tipping. These observations are consistent with and validate the findings of Autelitano et al. [30]. In joints as wide as that, a lack of joint filling mortar and ingress of moisture into the joint space considerably affect the vertical interlock responsible transferring shear loads to adjacent cobblestones through the joint filling material. Also, the horizontal interlock responsible for dispersing forces induced by braking, turning, and acceleration becomes less effective. These processes, leading to deformation of cobblestone pavements after over a dozen years of exploitation, are illustrated in Figure 15b,c. In addition, the cobblestones should be staggered between rows by at least one-quarter of the unit width [30,31,32,71,72]. This staggered pattern is obtained by using edge units at the kerb line [30,31,32,71,72,79]. Apparently, the shifting and tipping of units observed in the pavement under analysis was due to incorrect laying (Figure 15d–f).

Considering the fact that the analysed pavement was constructed of reclaimed cobblestones, noise surveys were carried out twice in the second study area (Figure 16). The first noise survey was carried out in 2016, i.e., after 6 years of use of the pavement when the condition was limited to very slight subsidence and up to 1 cm deep joint damage. The second noise survey was carried out in 2024, i.e., after the street had been trafficked for fourteen years, which led to many subsidence spots and damaged joints throughout the pavement. The red dashed line in Figure 16 represents the allowable noise level as per [63,64]. As it can be seen, this limit was exceeded in both surveys [63,64]. As it transpires from the graphs in Figure 16, the 55–100% noise percentile levels exceeded the allowable noise level already in 2016. Since 2024, this limit has been exceeded starting from the 30% percentile. After over a dozen years of exploitation, the highest noise levels were noted in raised junction A, i.e., the place showing most severe deterioration of pavement. Now, based on these findings, we can answer research question ③ by stating that the level of traffic-generated noise on cobblestone paved carriageways depends on the severity of pavement deterioration, aggravated by the use of reclaimed cobblestones during construction, incorrect joint widths, and the laying pattern. To showcase the bearing of the surrounding streetscape on the noise levels, the noise graphs in Figure 16 are presented on a satellite imagery background.

Figure 17 represents control equivalent noise levels Leq measured in the 2016 and 2024 surveys. These data show an average increase in Leq by about 2 dB(A) in places of the smallest subsidence areas and no joint gaps and by up to ca. 4 dB(A) in places where the largest subsidence areas and most severe joint damage were found. However, there are also other factors influencing the noise level and Leq variations, such as two big buildings built near the carriageway at the departure ramp of raised junction A, reflecting the noise propagation wave and thus giving the highest noise increase levels. The influence of streetscape on noise levels can be seen, for example, at raised junctions A and B, where one side of the street is lined with parking spaces for passenger cars over almost the entire raised junction length. The cars parked there are an obstacle to noise propagation. In addition, the street is lined on both sides with trees and hedgerows in front gardens, which also disrupt the noise wave propagation, giving different Leq on the two sides of the street. Since speed reduction to 30 km/h was not achieved in this street and v₈₅ values oscillate above 40 km/h, it can be assumed that raised junctions used as the only traffic calming measure, in line with the data given in [56,57], decreased the noise level in the initial period after construction by ca. 2 dB(A), in comparison to the noise level before the traffic calming project. However, pavement deterioration increased the noise level by ca. 2–4 dB(A). Thus, we can say that the traffic calming project had an adverse effect on the environmental conditions after over a dozen years of exploitation.

4. Statistical Inference

In summary of the data on the issue raised in research question ① on the basis of the data presented in Figure 6 and Figure 11, it can be concluded that the use of raised junctions alone in 30 km/h zones is effective for squares located in single-family and multi-family residential areas with only passenger car traffic not exceeding 100 veh./h. However, for through streets with a traffic volume of approximately 600–700 veh./h and heavy vehicle traffic and bus services, the use of raised junctions as the only traffic calming measure is hardly effective. The authors are of the opinion that when raised junctions alone are used on a straight section of an arterial street, a 40 km/h zone should be used, or other traffic calming measures should be provided in addition, in order to achieve slowing of traffic to 30 km/h.

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In order to answer research question ② as to whether the use of only raised junctions in the 30 km/h zone affects the condition of the pavement, statistical inference and multi variant regression analyses were performed. A detailed analysis of the test results showed that the pavement condition should be analysed separately at approach ramps and separately at departure ramps in a given case. At the approach ramps, drivers tended to brake in most cases, while at the departure ramps they accelerated. The magnitude of the recorded acceleration and deceleration values should also be analysed separately depending on the streetscape characteristics, i.e., Number of parking and side road access points,

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Straight and turn movements of heavy goods vehicles and buses,

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Bus terminus location,

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Individual access points to small enclosed housing estates or home zones.

Outcomes of preliminary analyses to support this division are shown in Figure 18. A detailed analysis of the data presented in Figure 18 shows that pavement subsidence areas do not depend only on accelerations and decelerations but also on other factors characterising traffic conditions and the streetscape characteristics on both sides of the raised junctions in question. Similar scatterings of the analysed data were also noted for the surface areas of joint filling damage up to 2 cm in depth.

Figure 19 shows another part of the analyses on the dependence of the subsidence area (up to 2 cm in depth) on accelerations or decelerations, carried out on the approach ramps. In the case in question, the data under analysis were also split between the individual ramps and by the traffic conditions and the character of the streetscape on both sides of the raised junctions. The analysis of the data obtained in Figure 19a showed that, on approach ramps, the subsidence area of up to 2 cm depended largely on deceleration and to a lesser degree on acceleration, but also on the other factors mentioned above, whereas for another streetscape, a lack of any dependence of the subsidence area (within a range of up to 2 cm in depth) on accelerations or decelerations was demonstrated (Figure 19b).

Figure 20 shows analogous results of the analyses carried out for departure ramps. In this case, the dependence of a subsidence area up to 2 cm in depth on the parameters of accelerations on the departure ramp from a raised junction with a varied number of driveways for parking, side roads, and truck and bus turn movements was also demonstrated. In turn, in the case of the streetscape with one access to a closed small housing estate or home zone, varied dependencies of the subsidence area up to 2 cm in depth on acceleration and deceleration parameters were shown (Figure 20b).

Similar analyses were performed for the dependence of joint filling areas with joint gaps up to 2 cm and above 2 cm deep on the magnitude of accelerations and decelerations, and similar results, i.e., the values of correlation and determination coefficients, were obtained. In addition, a possible impact on the subsidence areas and the cavities in joint fillings may have been due to the previously mentioned construction flaws. Since the observations and selective surveys in the second study area were carried out over several years of its operation, Figure 21 shows the condition of the pavement in the three selected periods of operation. In the initial period of operation, the condition of the pavement was rated very good (Figure 21a), with a noted tight filling of joints. In 2016, after 6 years in service, the first signs of joint filling damage were already visible, especially on the wheel paths (Figure 21b). And Figure 21c shows a clearly visible poor laying pattern, resulting in a lack of resistance of the pavement to units moving out of position, damage to joint filling, and pavement subsidence. Incorrect laying pattern results most of all from the following:

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Lack of stagger where in subsequent rows the joints should be staggered by at least one-quarter of the unit width and lack of end units at kerb line.

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Too wide joints.

5. Pavement Structure Design

An analysis of the condition of the pavement in the past years has shown that the design of the pavement structure was correct, as the same paving used in the first study area has not deteriorated over the same period of operation. However, the two analysed study areas differ significantly in terms of traffic volumes and, above all, in terms of turn movements, as well as in the construction quality. Taking the above results into account, in order to provide a constructive answer for research question ② at various locations, Figure 22 shows the differences in the impact of various factors on the selected three pavement condition parameters. The analysis considers the following separately:

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Approach ramps and departure ramps.

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Traffic volumes of turn movements in their vicinity, with particular attention paid to heavy and bus traffic (symbolically depicted with wheels).

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The streetscape in a given traffic direction, i.e., the number of exits, car parks, type of development, etc. (symbolically depicted with stylized vehicles).

The above-mentioned factors are symbolically shown in Figure 22, which also presents, on a chromatic scale, the strength of the correlation, taking into account the magnitude of the coefficient R. An analysis of the correlation coefficients obtained (Figure 22) shows that distress and deformation are highly dependent on horizontal forces, with heavy traffic in turn movements and diverse streetscapes featuring numerous elements such as driveways, parking lots, local markets, and public buildings. On the other hand, with little streetscape variations, distress and deformation were found not to depend on acceleration and deceleration. In other cases, the correlation strengths of the various factors influencing the condition of the pavement vary.

Given the above analyses and the condition of the pavement in the two study areas, the authors re-examined the pavement design and proposed two new pavement designs that are likely to ensure good pavement condition throughout its standard service life. For the first study area, i.e., the one with a one-way carriageway, low traffic volumes, and single- and multi-family housing, the authors proposed the pavement structure shown in Figure 23a. The proposed pavement, owing to base course reinforcement, should provide a better distribution of the stresses induced in the pavement from dynamic change and load impact. It is also important to use a kerb at the bottom edge of the ramp to prevent shifting of cobblestones under horizontal forces. Depending on country-specific guidelines, appropriate ramp heights and gradients should be specified in raised junction designs. In the first study area, steeper gradients should be used on the approach ramp in order to ensure speed reduction, while gentler slopes should be used on the departure ramp in order to mitigate shocks negatively perceived by drivers and passengers [11,13,59]. And for a through street with high levels of heavy vehicle and bus traffic, numerous car parks, etc. (second study area), the authors proposed the pavement structure shown in Figure 23b. In this case, it is also suggested that the heights and gradients on both ramps be specified according to the guidelines in force in the country concerned. However, where the street carries bus traffic, a gradient of 1 in 15 is recommended. And if a crossable median dividing both directions of traffic was used in the raised junction area (Figure 24), then different gradients can be used on both ramps [9,11,25,59]. As for the pavement structure, in addition to the reinforcement of the sub-base layers, the authors proposed to apply a 1 m long transition zone before the ramp (Figure 23b). The proposed transition zone is made of the same cobblestone paving as the raised junction pavement. This design will help the accumulated stresses to better distribute in the pavement due to different distribution of horizontal and vertical forces on the ramps that occur during braking and acceleration. Figure 23b also shows a non-recommended pavement structure, the condition of which is shown in Figure 21.

In addition, considering the turning traffic of heavy vehicles and buses at parking areas and side streets, in the opinion of the authors, the joints should be filled with resin grouts with a strength of more than 50 MPa to obtain a monolithic; resistant to abrasion, mechanical cleaning, frost, and salt; and weed-free cobblestone surface. Sand and cement joints are porous and thus undergo a natural process of shrinkage or crumbling of the cement paste, which can result in a faster occurrence of joint gaps, especially in the area of frequent turning manoeuvres of heavy vehicles and buses. The epoxy grouts recommended in this article are a non-porous, durable material. Figure 25 shows the condition of joints filled with resin grouts over several years of pavement operation, during which it carried similar straight and turning traffic volumes.

Another problem that should be addressed, apart from the pavement design recommended above, is the additional use of state-of-the-art technology and materials for signage, which would have a more restrictive effect on drivers’ behaviour in order to achieve the expected speed reduction. In similar traffic circulation systems with a through street which carries heavy vehicle and bus traffic, there have been experiments involving the use of various traffic calming measures and road markings to make drivers slow down to the posted speed limits. For example, in the studies described in [88,89,90,91], rumble strips made of thick-layered plastic compounds, high-density polyethylene materials, or of various coloured stone units were used in front of the raised junction. In line with the conclusions described in [90,91], the use of rumble strip alert systems of preformed plastic compounds can additionally contribute to speed reductions between ca. 2 km/h to 15 km/h, depending on traffic conditions and streetscape characteristics.

The use of thermoplastic coatings on old stone pavements for horizontal markings on ramps can be problematic due to the uneven surfaces and non-monolithic character of such surfaces. These conditions lead to faster disintegration of the thermoplastic material and its faster wear and tear under traffic loading.

Bearing this in mind, and taking into account the fact that there are generally major land constraints in the second study area, Figure 26 shows possible solutions involving the additional use of solar road LED studs [92], rumble strip alert systems, or chockers. The proposed additional solutions may contribute to a better and faster raised junction perception by drivers and to the desired speed reduction as a consequence. The solar LED road studs can be placed in the kerb faces to give an illusory impression of a narrowing of the carriageway, and, in addition, directly before the approach ramp (Figure 26a). Alert systems (Figure 26b) can be the cheapest option, while the proposed combinations of solar LED road studs (Figure 26a) are slightly more expensive. In contrast, the most expensive solution is the proposed termination of the crossable median before the raised junction and the use of a choker along the length of the raised junction, as shown in Figure 26c. This solution, however, requires the right geometrical conditions to achieve a narrowing on both sides, but it has the greatest effect on drivers’ perception by visual narrowing of their swept path on the raised junction. With this solution, Corben and Duarte [88] propose an abrupt termination of the median/central island and the use of coloured, yellow/black kerbs. The proposed additional road marking elements should be based on high-density polyethylene materials, where alert strips are used [88]. Alternatively, solar panels powering solar road elements or high-intensity LEDs should be used.

In the first study area, on the other hand, a different aspect has drawn the authors’ attention. Namely, despite good illumination of the carriageway by street lamps, the change in the level of the raised pavement is poorly visible to drivers at night or under the conditions of limited visibility, which may result in their lack of reaction and failure to slow down before entering the raised junction (Figure 27). Figure 27a shows the existing night lighting conditions and Figure 27b visualises the proposed use of LED lights in the kerb of the centre island, solar-powered LED road lights before the approach ramp, and Figure 27c further adds LED verge marking posts at the corners of the intersection, for the purpose of alerting drivers of the presence of the approach ramp on the raised junction. The proposal to use LED elements in the kerb face is an innovative design option [93,94].

6. Conclusions

The analyses presented in this article fill a gap identified in the sustainable design of traffic calming in 30 km/h zones and in the use of sole raised junctions in various traffic circulation system configurations. The analytical results presented here can help researchers and designers in the further sustainable planning of traffic calming schemes and in the implementation of correct pavement design principles. The article demonstrates the importance of selecting the right stone elements and the proper type of joint filling mortar for cobblestone pavements used on raised junctions in 30 km/h zones.

The findings of this research allowed us to answer the research questions posed earlier in this article:

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On the basis of the conducted surveys and analyses, the use of raised junctions as the only traffic calming measure in 30 km/h zones can be recommended in areas featuring low traffic volumes of up to 100 veh./h, located in suburban districts with single- and multi-family housing, distributing traffic to local side streets with low traffic volumes.

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Based on the research and analysis carried out, it can be concluded that the use of raised junctions as the only traffic calming measure in 30 km/h zones on through streets with heavy vehicle and bus traffic is not effective in slowing down the traffic as desired. Instead, this arrangement is likely to prove effective in 40 km/h zones. To slow down the traffic to 30 km/h, it would probably be necessary to use rumble strip alert systems before approach ramps or install solar road LEDs in the road pavement and in kerb faces, aided additionally by verge marker posts, to induce in drivers an illusion of the narrowing of the carriageway at the raised junction entry point.

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In a 30 km/h zone located in a suburban district with single- and multi-family housing, the use of only raised junctions in the squares, distributing traffic to local side streets with low traffic volumes, has little effect on the condition of the stone block pavement after over a dozen years of service.

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In a 30 km/h zone in a street with heavy vehicle and bus traffic, the use of only raised junctions has a significant impact on the condition of the stone pavement after over a dozen years of service; as an alternative, it may be possible to sort the reclaimed pavers to ensure correct installation.

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Following a thorough analysis, the authors do not recommend the use of reclaimed cobblestone for constructing pavements on the raised junctions installed as part of traffic calming schemes, due to their irregular and non-standard shape, which often makes it impossible to apply the recommended standard joint width and the recommended stagger between rows, leading to the persistence of moisture in the joints, subsidence, and premature pavement deterioration.

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The research and analyses performed have shown that when traffic calming is implemented on arterial streets with heavy and bus traffic with the sole use of cobblestone paved raised junctions, a reinforced pavement structure and epoxy grouts of the latest generation should be used and special transition zones of 1 m length should be designed outside the ramps for correct distribution the accumulated stresses from the increased horizontal forces in this area.

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In 30 km/h zones located in suburban districts with single- and multi-family housing, the use of cobblestone paved raised junctions as the only traffic calming measure in squares distributing traffic to low-traffic local side streets has little effect on noise if vegetation varied in terms of height and species composition is present on both sides of the carriageway.

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In 30 km/h zones on streets with a high volume of heavy vehicle and bus traffic, the use of reclaimed cobblestone paved raised junctions as the only traffic calming measure has a significant effect on the pavement condition and, consequently, on the traffic-generated noise level. Taking this into account, flamed stone block paving should be used in this case, and the joints should be filled with epoxy grout.

The above study has some limitations, namely it lacks an arterial street with wide belts of greenery along the way. The authors are currently awaiting the completion of such a case and plan to conduct analogous research on it in a few years’ time.