Right-turn conflicts at signalized intersections
by Thomas Skallebæk Buch
Traffic & Roads
The interaction between right-turning motor vehicles and pedestrians walking straight ahead does not always proceed smoothly. This article presents a study of right-turn conflicts in signalized intersections, where the process leading up to the conflicts is investigated. The aim has been to investigate the characteristics of the conflicts in relation to time in the signal phase and road user behavior. The study shows several interesting results.
Right-turn accidents between right-turning motor vehicles and straight-ahead bicycles or small mopeds are one of the most frequent injury accidents at signalized intersections. Most often the right-turning vehicle is a car or van, but especially in the case of a right-turning truck, the consequences can be fatal for the cyclist and the moped rider.
Accident data rarely contain information about the events leading up to the accidents, as such information is not systematically recorded. This means that it is difficult to investigate when in the signal phase the accidents occur and what behavior the road users have had prior to the occurrence of the accidents.
With support from the Bicycle Pool, Trafitec has therefore investigated situations with conflicting behavior. The purpose has been to obtain new knowledge that hopefully can be used in connection with the development and implementation of safety-enhancing measures and in connection with information to road users. Based on the study, a detailed report, "Right-turn conflicts in signalized intersections", has been prepared and is available on Trafitec's website (www.trafitec.dk). The following article presents the main results of the study.
The following four questions have been addressed:
- When in the signalling phase do the situations occur?
- What is the speed of footpath users entering the intersection?
- How do path users appear?
- What is the driving behavior of motor vehicles?
The questions are answered by examining situations of conflicting behavior (hereafter only referred to as "conflict") between right-turning motor vehicles and pedestrians driving straight ahead.
Use of conflicts
In this study, conflicts are selected by reviewing the collected video material. Thus, situations are included where the video material shows a sudden reaction, which must be assumed to be an evasive maneuver to prevent an accident. The method is based on the Swedish conflict technique, but no time distance to the accident is calculated and no distinction is made between the severity of the situations.
There may be a bias in the data between the registered conflicts and accidents, especially in relation to the characteristics of path users. This is because conflicts are averted accidents, and some users may be better at avoiding accidents than others. Conversely, accident data may also be skewed in relation to the actual number of accidents, as the police's registration rate of accidents involving cyclists and moped riders is low.
Video observations were made at 10 intersections with a bicycle lane. For each intersection, video recordings were made on a weekday between 7:00 and 17:30. The intersection legs were selected so that the study includes intersections with different design solutions and variations in terms of road user composition.
On the video material, the behavior of road users up to and through the conflict area is observed, as well as the signal phase during the approach. A number of parameters are recorded for each conflict in order to answer the study questions. The parameters are also filled in for a control group consisting of simultaneous arrivals between right-turning motor vehicles and pedestrians driving straight ahead.
The conflicts are compared with the control group and, where there appear to be interesting differences, the relative risk is calculated using the odds ratio (OR). It is also calculated whether the exposed group has a significantly higher (or lower) relative risk than the non-exposed group.
The study includes 45 conflicts involving 44 different right-turning motor vehicles and 46 different straight road users. It is not possible on the basis of this study to look at conflicts involving right-turning buses and trucks, as they only occur in 2 conflicts. The control group consists of 1,352 simultaneous arrivals involving 203 right-turning motor vehicles and 1,322 straight path users.
Results Question 1
Only 2 out of 45 conflicts were preceded by both parties stopping at a red light before proceeding through the intersection. In the control group, this applies to 544 out of 1,352 simultaneous arrivals, and there is thus a significantly reduced relative risk of such a simultaneous arrival developing into a conflict (OR=0.07; significant). This is particularly interesting as safety enhancement measures in intersection design primarily focus on the start of the signal phase, where both the right-turning motor vehicle and the straight-on pedestrian stop at the red light before start-up. In 8 out of the 10 intersection legs studied, there are such measures: either retracted stop lines, pre-green for cyclists or a combination of these two. However, the study cannot determine whether the design solutions work as intended or whether right-turn conflicts simply rarely occur in this part of the signal phase.
In the middle and at the end of the path users' green phase, the majority of conflicts occur. In 9 conflicts, a path user crosses the yellow signal stop line. 6 of these 9 path users cross the stop line more than 2.5 seconds after the signal has changed to yellow, so they have had plenty of time to see the signal and stop (see example in Figure 1). Path users who run the yellow have an elevated relative risk (OR=6.06; significant).
The behavior of road users around the switching on and off of a green right turn arrow for motor vehicles seems to cause some conflicts. This is seen both when the green arrow is switched off before path users get the green, and when the bicycle signal changes to yellow and red while the main signal remains green.
Since the survey was conducted, bicycle pockets have been relaunched as bicycle boxes. The purpose of the bike box is to make pedestrians on a straight path more visible to drivers of right-turning motor vehicles when both parties are stopped at a red light. Based on this study, it is worth considering how many more conflicts (and accidents) a bike box can prevent if it is installed in a location where there are already retracted stop lines and a forward bicycle lane. Developing design solutions that focus on the middle and end of the signal phase is likely to have much greater potential.
Results question 2
Path users who stop before proceeding through the intersection have a lower relative risk of entering into conflicts than other path users (OR=0.12; significant). On the other hand, path users with a speed above 25 km/h when crossing the stop line have a significantly higher relative risk than other path users (OR=4.14; significant). This is also the case if path users who stop before moving forward are excluded (OR=2.48; significant).
Results Question 3
Several results suggest that the visibility of path users has an impact on the risk of entering into a conflict. Path users have an increased relative risk if they ride alone with a minimum distance of 2 seconds to the nearest path user (OR=9.82; significant). There is thus a clear sign of "Safety in numbers": for the individual path user, it is safer to pass the intersection at the same time as other path users. Footpath users wearing a black or dark jacket/shirt also appear to have an increased relative risk, excluding footpath users who stop at a red light before entering the intersection (OR=1.79; tends to be significant). One explanation could be that such a jacket, even in daylight, makes the footpath user blend in with the surroundings, including the dark gray asphalt.
The conflict area is the area where the right-turn conflicts at each intersection leg may occur. The point of conflict between two road users can therefore be anywhere within this conflict area. If the footpath user arrives in the conflict area at the same time as or a maximum of 2 seconds after a right-turning motor vehicle, there is an increased relative risk of conflict (OR=4.27; significant). If the footpath user arrives more than 10 seconds after the motor vehicle, there is a reduced relative risk (OR=0.42; significant). If the footpath user has a head start when entering the conflict area, no conflicts are observed. This suggests that footpath users are more likely to be involved in right-turn conflicts if they are driving next to or just behind the right-turning motor vehicle.
Results Question 4
The video recordings collected allow to study the driving behavior of motor vehicles. From this it is not possible to say much about why some end up in conflicts with path users while others do not. The reason is probably that the answer is to be found inside the motor vehicle itself, in terms of driver orientation, visibility from the vehicle and possible distraction.
However, it was found that a motor vehicle is more often involved in conflicts if it stops and blocks the bicycle lane while turning (OR=5.16; significant). The blockage typically occurs when the motor vehicle crosses into the bicycle lane and yields to pedestrians. An example illustrating such a conflict is shown in Figure 2. There is some evidence to suggest that this behavior may give rise to misunderstandings between the driver of the motor vehicle and pedestrians approaching from behind.
The study identifies some characteristics of right-turn conflicts. It is likely that these characteristics will also be found in right-turn accidents.
The conflicts occur mainly in the middle and end of the green time. This shows where the potential is to introduce new design solutions.
The visibility of path users to the driver of the motor vehicle is probably crucial for the safety of path users. Path users who ride alone on the path and/or just behind the right-turning motor vehicle appear to be at increased risk. However, path users themselves can reduce their risk by
- failing to yield to a yellow light
- refrain from speeding at intersections
- avoid driving with black/dark jackets
Conflicts are observed if motor vehicles stop and block the bicycle lane. Thus, the right turn should only be initiated when there is space in both the bicycle and pedestrian lane. If a motor vehicle still has to stop in the bicycle lane, the driver must be aware of cyclists approaching from behind before starting the turn.