*4.2. Red Light Violation*

The first issue examined was the proportion of vehicles crossing during a signal other than green (Figure 6). After switching on the SCT at intersections 1 and 2, the

number of vehicles crossing on other signals than green increased. On the other hand, at junction 3, the opposite situation was observed. When analyzing this situation in more detail, it was noticed that in all cases, after switching on the GSCT, the number of vehicles entering an intersection on the amber signal decreased. This is consistent with other research [12,38,44,47]. The results of the research on the red and amber signal are similarly confirmed by previous studies [26,30,31,37–40,47]. An increase in the number of entries during the red and amber signals was also observed in Płock. No such cases were observed with the RSCT turned off, but they occurred at each intersection with the RSCT turned on. The increased proportion of crossing on the green signal at intersection No. 3 can be explained by the large proportion of entries on the amber signal with the GSCT turned off. The improvement of the results at intersection No. 3 is mainly due to the reduction of a large number of entries during the amber signal.

#### *4.3. Headways*

In the case of headways measurements (Figure 8), the following phenomena were observed:


Exceptionally high headway values for junction No. 2 were most likely because there were no marked lanes at the 5.8 m wide approach, which increases the headway and significantly impacts capacity; this was taken into account in the capacity calculation methods [95]. Headways for subsequent vehicles decreased according to a similar pattern at other intersections, but these were much higher values than those recorded at different intersections. Intersection No. 2 is also located outside of the network of national and provincial roads.

The headway values at intersections No. 1 and No. 3 were also high compared to foreign surveys [12,14,68], and higher than the values recorded during other surveys carried out in Warsaw (the capital of Poland). They are above the median for studies conducted in Polish cities, similar to the results obtained at rural intersections [99]. A possible reason is the size of the city of Płock and the length of the trip related to it. Studies conducted outside Poland were often conducted in metropolises with a population of millions, such as Bangkok [30], Shanghai [21], Kuala Lumpur [19,20], Kayseri [12], Sydney [69], and New Dehli [46], but Płock has only 120,000 inhabitants and is a county city.

Improvement of traffic conditions after the use of RSCT was observed only at intersection No. 3. This is consistent with the results of studies presented in the literature (11 cases of no effect [19,21,24–26,30–32,34,38,61] and four cases of improvement [20,22,23,28]). Improvement was observed only at the largest of the examined intersections.

#### *4.4. Entering after the Green Signal*

The boxplot analysis (Figure 9) with the green signal entry times indicates that the median time decreased after the application of a GSCT (intersection No. 1 and No. 2) or remained the same (intersection No. 3). For intersections No. 1 and No. 3, the maximum values of these times increased, which is an unfavorable situation. The speed of the vehicles was not recorded in our study, but this situation may be related to an increase in speed at the inlets equipped with GSCTs, which was observed in previous research: [12,37,44,46,48]. For intersection No. 2, the number of registered entries after the green signal was only two, which does not allow statistical analysis. At that intersection, there was the lowest traffic volume among the examined intersections.

The chi-square (Pearson) test for compliance with a normal distribution was performed for the data. The test results for individual populations are presented in Table 3.


**Table 3.** Pearson test of compliance of travel times after the green signal.

*p*-Value < 0.05 indicates that the population is likely not normally distributed. For this reason, the comparison of the distribution of entry times was carried out using the nonparametric Wilcoxon-Mann-Whitney test. The results are presented in Table 4.

**Table 4.** Pearson test of compliance of the travel times after the green signal with the GSCT on (ton) and off (toff).


The test results show no indication of a shift between the distributions of entry with GSCT on and GSCT off. However, there is a noticeable shift in the modal value of the distribution to the right for the situation with the SCT off (Figure 10).

#### *4.5. Change in Fuel Consumption during Use of SCT*

The effect of saturation flow on the road capacity, and consequent fuel consumption, was direct. Studies in 11 cases indicated that the use of SCT does not affect saturation flow. Research made by the Faculty of Transport at Warsaw University of Technology indicated that such an impact only occurs in Płock at one intersection out of three. Only four studies [20,22,23,28] showed an improvement in capacity. Typical initial saturation flow values, depending on the capacity calculation method, were 1700–1900 veh./h. In the research, the speed of vehicles was measured at intersections or directly before them. However, the average speed over a longer distance and maintaining a constant speed significantly impacts fuel consumption. Therefore, it is not possible to conclude the effect of the SCT on fuel consumption based on these measurements.

On the other hand, start-up lost time applies only to the first vehicle in the queue. The effect on increasing the capacity and, consequently, on fuel consumption is smaller the longer the time of the green signal. For intersection No. 3, an analysis of the impact of the headway change on indicators related to fuel consumption was carried out. The analysis included vehicles in the 4th–9th position in the queue due to the large variation in the headway in the initial period of the green signal. Stabilization of saturation flow during start-up of the vehicle column is a typical phenomenon. For the situation with SCT turned off, the mean headway was 2.24 s, and with the SCT turned on was 2.04 s. This corresponds to a saturation flow 1607 veh./h and 1764 veh./h (for one lane), respectively. However, these are the actual values of saturation flow, considering inter alia, lane width, heavy vehicle occurrence, approach inclination and lane position. For analytical needs, the change of the starting saturation flow in the range of 1600–2000 veh./h was modeled. For

such data, selected measures of effectiveness were calculated using the Synchro program. The results are presented in Table 5.


**Table 5.** Fuel consumption at the intersection depending on the output saturation flow.

The results presented in the table refer to the fuel consumption of all vehicles in the model within the intersection with inflow sections. The results are the sum of all four approaches, not only for the SCT test approach. They show that reducing the headway (and consequently increasing the saturation flow) has a beneficial effect on fuel consumption. However, this effect was observed only at one of the three examined intersections. Similarly, headway variability occurs between different surveys, as shown in Figure 8.

#### *4.6. Threats Resulting from Improper Operation of the SCT: A Case Study*

A published regulation [9] introduced safety requirements for SCT use including device operation supervision and switching off signaling in a failure mode. Switching off the SCT should take place within 0.3 s from the occurrence of a failure mode situation. However, the lack of a displayed value on the SCT, also in a failure mode, may contribute to the occurrence of a road accident. It should be considered whether it is not justified to switch the signaling to the warning operation mode (amber flashing signal) in the case of SCT failure and then start the signaling with the SCT turned off.

Since the introduction of the regulations stated in [9], the number of SCTs installed in Poland has decreased, as the offered products did not meet the new requirements. SCT supervision within standards [100,101] compliant with national requirements is quite complex. This requires designing the device and communication with the controller from scratch.

#### **5. Conclusions**

Based on the conducted research, it is impossible to state unequivocally that the use of SCTs is beneficial for road traffic safety and efficiency. It has been observed that SCTs increase the number of red-light violations (crossing after the end of the amber signal) and on the red-amber signal. This is a negative situation and is confirmed by other studies. On the other hand, the number of crossings on the amber signal with SCTs is lower, which is also consistent with the conclusions of other studies. The modal value of the distribution of vehicle entry times after the end of the green signal with the GSCT turned on is shifted to the left, but the tests performed do not indicate that the shift of the entire distribution is statistically significant. The study results show that the impact of SCTs on the effectiveness of traffic control depends on the geometry of the intersection. The literature review also indicates that the use of SCTs results in reduction of start-up lost time (positive impact) and increases vehicle speed (negative impact). Just as the effect of using an SCT on efficiency varies, the effect on fuel consumption can vary. If the MOEs improve in a given situation, it is also possible to observe a reduction in fuel consumption. It should be noted that assistive steering devices are positively assessed, but confidence in them varies from

country to country [102]. In the event of their use, particular attention should be paid to the intelligibility of the messages conveyed.

The limitations of the study are the short measurement period and the date of research (during holidays), due to the consent of the road administration to turn off the SCT. Other limitations include conducting the study in a medium-sized city on roads with an urban traffic and a small number of heavy goods vehicles. Further tests on approaches with long queues are advisable to ensure more significant measurements. Research should also be carried out in larger cities and on rural roads, especially national roads.

The conclusions of the article can be addressed to various stakeholders. It is important for drivers to be aware that they should always follow the indications of the signal heads, not the SCT. There are known situations where the indications of these two devices are divergent, which can cause road crashes. Road authorities and local authorities need to point out that the use of SCTs does not always bring the expected effect. For road traffic engineers, it is important to know the impact of SCT on traffic safety and control effectiveness. This allows assessment of the SCT impact on traffic and making a rational decision whether or not to use an SCT. Research shows that an SCT does not always fulfill its role in improving road safety and control efficiency.

Other unexplored problems are associated with SCT use related to drivers' behavior on intersections with traffic lights functioning in a failure mode. The case study shows that such a situation may be an indirect cause or a factor contributing to a road accident. For ethical reasons, these issues must be investigated safely. This is not possible in regular road traffic. Such situations can only be tested with a vehicle simulator.

**Author Contributions:** Conceptualization, T.K. and J.S.; data curation, T.K. and K.F.; methodology, T.K. and J.S.; visualization, T.K.; writing—original draft, T.K. and K.F.; writing—review & editing, T.K., K.F., M.C. and J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Warsaw University of Technology.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy issues.

**Acknowledgments:** The authors would like to thank Agata Krezyman and Kamil G ˛aska for making video recordings and for work put into their analysis. The authors would like to thank the initiator of this research Marek Buda, who passed away in 2017. Thanks also to the Office of the City of Plock for agreeing to conduct the research.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

