1.2.5. Vehicle Speed, Braking Characteristics, and the Dilemma Zone

The issue of traffic safety from the point of view of the mechanics of vehicle movement when an SCT is turned off is described in [27]. Analyzing mathematical relations, it was found that the GSCT did not increase the vehicle speed above the minimum. A psychological factor was not included in the article. Research in [45] showed a reduction in the speed of vehicles reaching a crossing during the yellow signal. A negative impact on the speed of vehicles at the approach was found in Kayseri (Turkey) [12]. Vehicle speed at intersections with a GSCT was also analyzed in New Delhi [46]. The proportion of vehicles exceeding the speed limit at the intersection entrance was higher (approx. 4% compared to approx. 13%). On the other hand, the speed of vehicles crossing the intersection with a GSCT during the amber signal was almost twice as high.

An analysis of an SCT functioning in the Philippines is presented in [47]. As part of the study, drivers were surveyed in Manila and at intersections in Quezon and Pasig. It was observed that the use of countdown timers caused no low-speed vehicle passages in the final period of the amber and green signals, while there were soft brakings and stops of cars on the amber signal and starts from the stopping line before the green signal. Research also shows that drivers' declared and real behavior during the amber and green signals differed significantly. For example, drivers declared that they decelerated during an amber signal when they were accelerating or moving at a constant speed.

An analysis covering decisions made by drivers while driving through an intersection with and without an SCT was conducted in China [48]. A logistic model was used for the research, taking into account the probabilities of such behaviors as acceleration, braking, and speed maintenance. Determined from the time displayed on the GSCT, the distance from the stop line and the vehicle's speed was assessed. The study showed that at intersections with an SCT, the proportion of stopping or slowing vehicles was greater than at intersections without an SCT, while the percentage of accelerating vehicles decreased.

Differences in drivers' behavior at traffic lights with an SCT and a CCTV system in summer and winter were examined in [49]. The research was conducted in Changchun, China, at eight intersections where traffic violation cameras were installed. The tests covered 90 m long sections of intersection approaches. Based on video observations, braking speeds and decelerations were determined. It was observed that in winter, drivers braked at a greater distance from an intersection equipped with a GSCT, while at intersections not equipped with a GSCT, drivers braked when an amber signal was displayed.

GSCT tests were also carried out with the use of a vehicle simulator [50]. The study included an analysis of drivers' behavior in the dilemma zone and braking deceleration. With a GSCT in place, drivers who were in the dilemma zone decided to stop more often, and at the same time, braking was performed with less deceleration. Research in [25] also showed a shortening of the dilemma zone. The influence of GSCT on vehicle traffic parameters used in car-following models (the method used to determine how vehicles follow one another on a roadway) was also investigated [51]. Numerical simulation showed that GSCT significantly affected drivers' behavior before traffic lights and should be considered during simulations.

The technique used for measurements has also developed over the years. Article [36] describes SCT tests performed with the use of UAVs. Video recording from the air has proven successful and, as a result of the study, a solution was obtained to reduce the start-up lost time (by 16–39%) and increase the speed of crossing the intersection (by 10–28%).

### 1.2.6. Reaction Time

Another influence of the SCT is a shortening of the brake perception-reaction time. This is indicated by research in [52] carried out in Harbin, China, at six intersections with an amber signal duration of 3 or 4 s and a cycle length of 107–170 s. The presence of a GSCT significantly reduced the driver's reaction time to changing signals. The article analyzed reaction time distribution, and the median reaction time was reduced by 30%.

A broader issue was described in the article [53] concerning various solutions about the end of the green signal such as a green blinking signal and an RSCT. The article presents five models concerning the driver's decision to drive or stop. The analysis concluded that the driver's reaction with the application of GSCT was faster than that resulting from the use of a flashing green signal, and the slowest response occurred in the case of the standard sequence of signals (green/amber/red).

#### 1.2.7. Safety (Number of Crashes, Vehicle-Vehicle Conflicts)

Safety characteristics of SCTs were investigated in [54] for not only a GSCT and an RSCT but also CCT a (continuous countdown timer—displaying all signals) The literature was researched using the PRISMA method and 79 references were analyzed, of which 14 were thoroughly investigated. It was found that the effect of countdown timers is dependent on the type of device, and different parameters in different studies indicated improvement or lack thereof. In particular, the impact of SCTs on road safety was not unequivocal.

Article [55] describes the advantages and disadvantages of SCTs and presents survey results. The opinions of the Ministry of Infrastructure are also presented, and the cities using these devices are listed (albeit, in a manner inconsistent with applicable law). Article [56] covers an analysis of SCTs installed in Toru ´n (Poland) at four intersections. The number of crashes on intersections was used as a measure of safety. The test results did not show any improvement or deterioration in traffic safety.

In other research, standard traffic lights, signals with different types of light rules, and numerical SCTs were compared. Traffic safety studies were carried out using the pre and post analysis method. The results show a reduction in the number of crashes by approximately 35% [24].

Research in Słupsk (Poland) [57] included only questionnaire studies on the opinions of road users in terms of safety, driving comfort, and driving behavior. However, as noted in [47], there were significant discrepancies between the survey results, and the article did not contain a verification.

The values displayed on the SCT impact the driver and can be analyzed in conjunction with road crashes. This issue is described in article [58], which includes a case study for a selected intersection in Krakow. However, the article's conclusions indicate that further research is needed related to traffic safety with the use of an SCT. Study [59] considers the time display when modeling traffic with a cellular automaton. The model was implemented for a two-lane road section.

A detailed analysis of the causes of GSCT-related road incidents is presented in [60]. The research was carried out for five intersections in Delhi (India). The intersections were characterized by a cycle length of 220–240 s, and characteristics related to red light violations and conflicts between road users were determined as part of the research. The use of a GSCT reduced the number of red-light violations and the number of conflicts by almost half.

The influence of RCST and GCST on road safety in China was also analyzed [61]. In 2000–2007 such devices were installed at 1036 intersections in China. The tests were

conducted in Taipei in four periods: preinstallation, 1.5 months after installation, and 3 and 4.5 months after installation. The research showed a negative impact of the GCST on the dilemma zone and traffic safety.

The psychological aspect of using SCT is critical, and can be studied based on statistics gathered by surveys. One such survey took into account the gender, age, and driving experience of drivers. It included 32 questions regarding opinions on various SCT solutions, driver's behavior, such as engine shutdowns when stopped at traffic lights, and the scope of the displayed information [62].

Innovative technical solutions in the field of RSCT are presented in [63]. Various graphic SCT solutions were presented in which the passage of time was displayed as an LED ruler, a circle segment, and a change in the diameter of the signal. The proposed solutions were tested on a group of 12 road users and assessed in terms of the perception of the information provided and assessment of the impression of the device. Doubts as to the practical application of the solutions were raised, because of difficulties regarding meeting the requirements of the norm [64].

#### 1.2.8. GHG Emissions

The impact of SCT on greenhouse gas (GHG) emissions was investigated in [65]. The study was conducted in Haeundae-gu, Korea. As part of the study, a driver behavior model was developed and 340 trips were made to test the effect of SCT on such behavior. Then, a model of the Haeundae-gu road network was made in the Vissim program, and based on the simulation results, the GHG emission from idling vehicles was determined. It was shown that with the use of an SCT it is possible to significantly reduce greenhouse gas emissions resulting from a reduction in the number of stops and the possibility of turning off the engine.

#### 1.2.9. Cycling

A study of the impact of an RSCT on cycle traffic was analyzed in [66]. The study was conducted in Groningen, the Netherlands. Research showed that cyclists observe the SCT and adjust their riding style to the displayed value. After using an SCT, a small number of cyclists approached the intersection during the amber signal, and eye-tracking studies indicate that they observed the SCT from a distance of about 60–70 m. However, when standing in front of a signal head, eyesight is more focused on the SCT than on the signal head.

### 1.2.10. Pedestrian Passing on Flashing Green

Research [67] conducted in China was also related to pedestrian decision-making before crossing an intersection. The study was carried out at crossings equipped with PSCTs. Pedestrians were divided into groups according to their behavior. While the study did not directly address the impact of PSCTs on road traffic, the methodology could be used for this purpose.

A study done in Korea concerning numerical and graphical SCTs monitored the number of pedestrians entering the road during a flashing green signal. A reduction in the number of pedestrians passing during a flashing green signal was indicated, which is not allowed in Korea [68].

Extensive PSCT studies have been conducted in Sydney, which concluded that the use of PSCT did not reduce the number of entries during the "Flashing Don't Walk" (blinking) signal and reduced the proportion of late finishers (pedestrians entering the crossing in the last seconds of the green signal) [69]. Other research based in Sydney [70], indicated an increase in the proportion of pedestrians entering the crossing during the "Flashing Don't Walk" signal after the launch of a PSCT. Similar results were obtained in a study [71], also carried out in Australia. An increase in the number of entries to the pedestrian crossing was observed during the "Flashing Don't Walk" signal.

A study conducted in the USA [72], indicated that PSCTs increased the number of pedestrians entering during a "Flashing Don't Walk" signal. Similar results were described in research carried out in India [73], indicating an increase in the number of pedestrians entering the crossing in the final period of the green signal and during the yellow signal. Research [45] showed a reduction in the proportion of pedestrians reaching a pedestrian crossing during the "Flashing Don't Walk" signal and shortening of the dilemma zone.

Research conducted in the United Arab Emirates [74] showed that after using a PSCT, the number of people crossing according to regulations increased, i.e., crossing at the end of the "Flashing Don't Walk" signal. The research also included a survey in which the majority of respondents supported the use of the PSCT.

In other research, PSCTs were analyzed concerning the time counted down during green and red signals [75]. The study was conducted at five intersections in Shanghai. Signal cycle lengths ranged from 128 to 200 s. The use of a PSCT reduced the number of pedestrians over 50 years old crossing during a flashing green signal. In all age groups, the number of pedestrians finishing the crossing during the flashing green signal increased. The reaction time of pedestrians at the beginning of the green signal also decreased, especially for people aged 50+.

A study in China researching children's' behavior showed no influence on the ratio of entries during the green blinking signal. Still, slightly more children finished crossing during the blinking green signal. The use of a PSCT did not affect crossings started during the blinking green signal [76].

A team from the Silesian University of Technology in Poland researched PSCTs [40]. The research was carried out in Zabrze. With the PSCT turned off, the proportion of pedestrians entering the crossing was greater during the flashing green signal (allowed in Poland) and at the beginning of the red signal.

#### 1.2.11. Pedestrian Red-Light Violations

Research held in China [77], investigated the behavior of pedestrians, dividing them into four groups. The results showed a statistically significant increase in the proportion of pedestrians obeying signaling when a PSCT was used. Children's behavior related to PSCT is described in article [76]. The PSCT counts down the duration of the green flashing signal and the duration of the red signal. The research was conducted in Jinan (China) at two intersections. During the study, it was found that at an intersection equipped with a PSCT, the proportion of children entering during the red signal was greater.

Other studies conducted in Europe (Greece, Thessaloniki) [78] indicate numerous incorrect crossings of pedestrians regardless of various factors. The proportion of pedestrians crossing during the red signal in this study are greater than those presented in other articles.

PSCT research was also conducted on two intersections in New Delhi [79]. The use of a PSCT significantly increased the number of pedestrians crossing during the red signal. At crossings without a PSCT, the proportion of people crossing during the green signal was higher, while after installing a PSCT, the percentage of people crossing during the amber signal also increased. The use of a PSCT did not affect the waiting time of pedestrians before the crossing or times of arrival at the pedestrian crossing.

In Poland, the inclusion of a PSCT reduced the number of entrances at the beginning of the red signal but resulted in pedestrians to intrude on the crossing at the end of the red signal [40].

#### 1.2.12. Pedestrian Speed

A study prepared in Sydney indicates that the speed of pedestrians passing through the crossing increases with a PSCT in place [69]. During the "Flashing Don't Walk" signal, pedestrians move at a higher speed than when the PSCT is off. Nevertheless, pedestrians positively assessed the use of PSCT in the survey [70]. Similar results were obtained in a study [71], also carried out in Australia. An increase in the speed of pedestrians at

the crossing in the final period of the green signal was observed [71]. In Shanghai, the speed of pedestrians who started crossing on the green signal after 5 s increased [75]. An increase was also observed in the research described in [45]. In research conducted in New Delhi [79], no influence of PSCT on the speed of pedestrians crossing the road was found.

### 1.2.13. Pedestrian Safety (Vehicle-Pedestrian Conflicts)

A study in Sydney indicated that it is not possible to unequivocally assess the impact of a PSCT on the number of pedestrian-vehicle conflicts [69]. Studies conducted in Canada do not show a statistically significant influence of PSCT on the number of pedestrianvehicle collisions [80].

The safety aspects of PSCTs studied at 106 intersections with traffic lights in North Carolina, USA [81]. There was a statistically insignificant reduction in the number of road incidents with pedestrians after installing PSCTs. In contrast, the total number of road incidents decreased in a statistically significant manner.

PSCTs were also researched in a study related to road traffic accidents of older people [82]. The research was conducted at 190 intersections, and included the analysis of road incidents before and after installing a PSCT. A radius of 250 ft from the intersection was assumed as the intersection area. The study showed a reduction in the total number of incidents injuries after installing the PSCT.

However, when PSCTs were used in India [73] and China [76], the number of pedestrianto-vehicle conflicts was more significant. Canadian studies [83] showed that PSCTs increased the number of all crashes with pedestrians by 7.5%. There was no significant influence on the number of crashes with injuries or fatalities.

Research was also conducted in the USA on the use of pedestrian buttons at intersections with a PSCT [84], which showed the use of pedestrian buttons reduced the number of traffic incidents with pedestrians. The use of a PSCT alone, without buttons for pedestrians, did not bring about any effect on improved safety.

The behavior of pedestrians using crossings with pedestrian islands in Belgrade (Serbia) has been investigated [85]. A PSCT had a positive effect on reducing the number of pedestrians entering the first pedestrian crossing, but did not affect pedestrians leaving the second crossing.

Study [86] shows that the use of a PSCT had a statistically significant effect on the number of rear collisions and the number of incidents involving pedestrians. The analysis of PSCT assembly costs and road incident costs shows that the use of PSCT is economically justified.

#### 1.2.14. Literature Review Summary

The above literature analysis shows that there are significantly different results concerning the effects of STCs. Table 1 presents the essential information about each of the studies.

A '+' sign indicates a positive impact on road traffic (understood as improvement of safety, improvement of indicators related to safety, or improvement of measures of effectiveness). For example, this symbol denotes a decrease in headway or an increase in saturation flow. The symbol '−' indicates a negative impact on road traffic, while symbol 0 shows no significant effect. In the "others" column, L stands for literature analysis and research methodology development, S survey research, T theoretical analysis, M modeling or data collection for modeling, C comparison SCT with similar devices.



**Table 1.** Summary of research results on SCT (set chronologically and divided into SCT for drivers and pedestrians).



Based on the number of articles, a rating was calculated considering publications with a positive impact of an SCT on a given factor with a weight of '+1' and a negative impact with a weight of '−1'. On this basis, it can be concluded that in terms of positive effects (rating 4–11) of the use of SCT, the most frequently mentioned are:


The factors assessed negatively (score from −3 to −9) in order of the most frequently mentioned:


The remaining factors were examined in individual studies. The results were inconsistent, or no significant influence of SCT was demonstrated.

Due to the often-divergent results, research gaps remain. Most of the research was conducted in large urban centers. It is therefore important to conduct research in smaller cities and outside the cities. There are visible differences in drivers' behavior between individual countries, related to, for example, cultural norms. Research carried out in Poland is sparse, so it is advisable to conduct research to a greater extent. It is interesting to consider whether the results obtained mainly in Asia, America, and Australia will be similar in Poland. There are no studies related to traffic safety during failures in the use of SCTs or the traffic light system.

#### *1.3. Article Content*

The article is structured as follows. Section 1 presents an in-depth literature review and analysis of the research carried out so far. Section 2 presents the research methodology of the conducted research. Section 3 presents the results of the field research. The latter part of the article (Section 4) discusses the results in relation to the literature analysis and previous research. Unexplored issues related to the functioning of SCTs are indicated. Selected case studies associated with SCTs are also presented. The aim of the paper is to conduct a comprehensive analysis of the literature, to conduct field research carried out covering many criteria, and to identify previously unexplored issues related to SCTs that have not been described in previous publications. The article is summarized in the conclusions presented in Section 5.

#### **2. Methods**

#### *2.1. Place and Method of Conducting Research*

Field measurements were carried out to experimentally evaluate the impact of countdown timers on the movement of vehicles. The research aimed to assess the validity of the use of countdown timers and their impact on the driver's decision to brake and start the vehicle. The research included making direct observations of traffic at the approaches of selected intersections. The study was conducted in Płock, Poland (Masovian Voivodeship). Płock is a county city with a population of approximately 120,000. Traffic lights are installed at 57 intersections; 32 are equipped with SCTs and 11 with PSCTs. An SCT for vehicles performs the functions of an RSCT and GSCT.

The following factors were taken into account when selecting the study site:


Because of the above requirements, three intersections were selected:


Later in the study, intersections were marked with the numbers mentioned above. The location of intersections on the Płock map is shown in Figure 2.

**Figure 2.** Location of measurement objects in the city of Płock. (Background map © OpenStreetMap contributors, [87]).

All intersections have four approaches, with traffic lights operating in a fixed-time mode. There are pedestrian crossings at each of the intersections at all legs. Moreover, during the morning and afternoon rush hours, queues of vehicles of an appropriate length form at intersections.

At each of the intersections equipped with an SCT, measurement points were selected to observe the approach. The measurements were made using an image recording method. Vehicles crossing the stop line at the selected approach were recorded [88,89]. A Sony camera was used, which was placed on a tripod at a convenient observation point. The camera was set in a place that did not attract the attention of drivers, and as little visible to the drivers as possible. The field of view for the camera used for the measurements was 155 degrees and included the stop line, a distance of about 10 m in front of the stop line, and signals. The focal length of the camera was f = 29.8–298.0 mm (16:9). Films were recorded in HD quality. Time was superimposed on the image with an accuracy of 0.1 s. Vehicles on all lanes were visible.

Based on previous measurements of vehicle traffic, the duration of the morning and afternoon peaks was determined: morning peak hours 6:45 a.m. to 8:45 a.m. and afternoon peak hours 2:45 p.m. to 4:45 p.m. The research was carried out in both rush-hour peaks and the off-peak period (11:30–13:30). Traffic at junction No. 1 was monitored in the morning rush hour, in the off-peak period at junction No. 2, and in the afternoon rush-hour at junction No. 3. A total of 12 h of recordings were made and were used in full to compile the results.

During the measurements, the weather conditions were very good (sunny, rainless). Due to administrative procedures related to obtaining appropriate approvals, measurements were taken on 19 July and 20 July 2017. This is the period of the summer holidays. The test results were analyzed manually using a method of observation and analysis of the obtained video material.

Unfortunately, the traffic volumes and lengths of queues during the holiday season (the Road Administration agreed to such date of the study) were not high. Therefore, only six vehicles from the queue were registered for intersection No. 2, data for eight vehicles were recorded for intersection No. 1, and for intersection No. 3 observations only nine vehicles were included.

Courtesy of the Płock City Hall, the research was conducted in the same conditions with the SCT turned on and off. This made it possible to compare the behavior of drivers in the following days. Both drivers' behavior at the green timer (GSCT) and the red timer (RSCT) were tested. The research focused on the following three issues:


Apart from speed, which could not be measured by the adopted method, these are the most frequently studied factors related to the functioning of a GSCT.
