*2.4. Other Methods Used for SCT Research*

The following simplified formula is used in traffic modeling computer programs such as Synchro [94] and Transyt to analyze fuel consumption:

$$F = TotalTravel \cdot k1 + TotalDelay \cdot k2 + Stops \cdot k3 \tag{1}$$

where: *<sup>k</sup>*<sup>1</sup> = 0.075283 − 0.0015892·*Speed* + 0.000015066·*Speed*2, *<sup>k</sup>*2 = 0.7329, *<sup>k</sup>*<sup>3</sup> = 0.0000061411·*Speed*2, *<sup>F</sup>* = fuel consumed in gallons, *Speed* = cruise speed in mph, *TotalTravel* = vehicle miles traveled, *TotalDelay* = total signal delay in hours, *Stops* = total stops in vehicles per hour. Measurements in metric units were converted to imperial units. Based on fuel consumption, it was also possible to estimate the emission of pollutants CO, NOx, and VOC.

Parameters such as Delay and Stops are strictly related to the way traffic lights function [95]. An analysis of the SCT literature showed that they may have a potential impact on factors influencing fuel consumption (Table 1):


An analysis of the research subject matter showed that many of the problems related to SCTs have not yet been investigated. Many road authorities are not aware of the risks associated with the use of SCTs.

The use of SCT in traffic-actuated control reduces the effectiveness of traffic control. The use of long countdown times makes it impossible to shorten this time at any time and speed up the passage a road user. However, it could be possible for traffic reasons, for example, when the vehicle stream has ended at one of the approaches. A display of the phase duration is only possible with fixed-time control. This solution was adopted in [9]. However, there are known cases of disabling traffic-actuated control and working in the fixed-time control mode because of an SCT installation. Such a situation should be assessed unfavorably due to control efficiency in off-peak hours. Although some studies indicate a slight improvement in control efficiency locally by reduction of headway [20,22,23,28], and globally [65], there are no comparative studies between fixed-time control with SCT vs. actuated control. It is particularly interesting how the change of control would affect efficiency in a period of lower traffic and using algorithms with priority for public transport. The use of such algorithms allows shorter travel times, but also benefits in terms of energy consumption [96]. Problems with displayed time values may also occur with fixed-time signaling due to offsets changing during the exchange of signaling plans on coordinated arterial roads. The use of SCT requires developing special control programs and extension of the time of offsets. It is also impossible to display the values on the SCT in the case of control algorithms allowing for the extension of a certain phase without time limits (until a notification occurs). An example of such signaling is the all-red algorithm and the "return of green signal to the main road".

Efficiency of control, depending on the SCT solution used, can be tested using simulation methods. It is possible to analyze the countdown of all signals, only selected ones, or to limit the countdown period. Such methods make it possible to determine the number of implementations of a given phase and its duration depending on road conditions. This allows assessing the impact of additional control limits on measures of effectiveness.

Additionally, ref. [3] specified in Art. 23, that "Subject to the provisions of paragraph 12 of this Article, the only lights which may be used as light signals for regulating vehicle traffic, other than those intended solely for public transport vehicles, are the following ( . . . )". Use of an SCT is, therefore, contrary to the provisions laid down by the signatories of this Convention.

The behavior of road users in an SCT failure mode situation is completely unexplored. Such situations include the shortening of the signal (red or green) in relation to the displayed time, as well as lengthening of these signals with respect to the time displayed on the SCT. A failure mode may also occur in the event of bulb burnout or other failures of the traffic control system. In this case, the signaling is switched to a flashing amber signal. This seems to be the most dangerous situation when using a GSCT. The driver assumes they will pass the intersection on a green signal, while due to a signal failure, they must give way to pedestrians or vehicles. The behavior of other road users must also be considered, such as braking, or an attempt to drive through the intersection during a signal failure. Traffic lights are derived from the signals used on railways, where the priority is traffic safety in the event of a failure [97]. Similarly, in the case of road traffic, the introduction of a new solution in traffic control should be preceded by detailed tests of traffic lights in failure mode situations.

The behavior of drivers causing crashes, or forcing other road users to avoid collisions, is very rare, since this is possible only through monitoring systems. It is necessary to record SCT indications, signals of traffic lights, and vehicle traffic. Such recordings can be analyzed as a case study. The number of crashes on a single intersection is usually low, which does not allow for statistical analysis. A case study of a road incident is presented in this article.

Scientists must consider that the benefits of an SCT can be assessed in different ways, e.g., by extending the red and amber signal to 2 s. Such a signal was used in Poland in the years 1990–1994 [98]. Similarly, the amber signal could also be extended. In the years 1990–2003, the amber signal in Poland had a duration depending on road speed. In the case of the green signal for trams, this can be extended if there is a tram in the dilemma zone. Such a solution is used in Warsaw, e.g., at the intersection of Marymoncka and Zabłoci ´nska streets.

#### **3. Results**

#### *3.1. Respect for the Red Signal by Drivers*

During the measurements, the number of vehicles crossing the intersection was determined. Then, the number of vehicles passing during individual signals was monitored and broken down into:


• red signal with amber (entries before the green signal)—wrong behavior.

The proportion of particular behaviors were determined in relation to the number of all crossings through the intersection. The results are presented in Figure 7. The results are divided into individual intersections and the SCT status (On/Off). It should be noted that for the turned-off SCTs, not a single red-amber signal entry was recorded. For intersections No. 1 and No. 2, the proportion of entries on the green signal decreased after switching on the SCT, while for intersection No. 3, it increased. However, the proportion of entries both on red and red with amber signals increased for this intersection. At all intersections, the percentage of entries during the amber signal decreased.

**Figure 7.** Percentage of vehicle entries on each signal.
