Traffic Safety & Signal Control
New Data-Driven Dilemma Zone and Intersection Safety Strategies Needed for a Vision Zero Future
Despite advances in traffic control technologies and systems, safety at the “dilemma zone” remains operationally problematic. According to Gazis, Herman, and Maradudin’s original 1960s dilemma zone research, a dilemma zone is defined as the area in which a driver is unable to stop safely before the stop bar, or proceed through the intersection before the signal turns red. 1 This is known as the “Type I” dilemma zone.
In the 1970s, further research by Zegeer and Deen considered distances before an intersection relative to vehicle velocity and driver behavior. This is known as the “Type II” dilemma zone and is often referred to as the ‘decision zone.’ 2 The Type II dilemma zone consists of the area in which a driver decides to either stop or proceed through the intersection before the signal turns red. This zone is the most critical point in ensuring safe passage at the intersection.
Intersection Safety & the Dilemma Zone
The enormous impact of road traffic crashes, particularly at intersections, is a major global challenge to Vision Zero. It is well-documented that road crashes claim around 1.35 million lives each year with approximately 50 million people sustaining serious injuries and living with long-term adverse health consequences. 3 Worldwide, road traffic crashes are the leading cause of death for children and young people aged 5–29 years, and rank as the eighth leading cause of death worldwide, resulting in significant burdens for economies, health systems, and transportation networks, in addition to the immeasurable and unimaginable generational emotional and trauma consequences it can have on families. An urgent resolution is required.
Intersection collisions and conflicts are for the most part due to driver misjudgment of the traffic signal transition from green-to-yellow-to-red while approaching the intersection at speed. This misjudgment most often occurs when the driver perceives the traffic signal change ahead of an intersection and must decide to stop before the intersection or proceed through it, possibly violating a red signal. It is often referred to as the dilemma zone. Key statistics, such as the latest report from the U.S. Insurance Institute for Highway Safety (IIHS), underscore the importance of dilemma zone protection. According to the IIHS, red-light running collisions caused 1,109 deaths in the U.S. in 2021, along with approximately 127,000 injuries. 4 Half of the fatalities were other vulnerable roadway users, including pedestrians, bicyclists, and other motor vehicle occupants hit by the drivers who violated the red signal. These findings emphasize the global challenge of dilemma zone and intersection safety.
Under the principles of Vision Zero, fatalities and serious injuries due to roadway traffic collisions, including intersection conflicts, can be prevented. In the past, designing safer infrastructure and incorporating road safety features into land-use and transport planning, as well as improving the safety features of vehicles; improving post-crash care for victims of road crashes; setting and enforcing laws relating to key risks; and raising public awareness have collectively proven to enhance intersection safety incrementally. However, dynamic data-driven strategies are needed to make more substantial and sustainable gains for dilemma zone protection and safety at intersections.
The Dilemma Zone
From a transportation management and traffic control operations perspective, one critical area that continues to be a persistent safety concern and substantial challenge to realizing a Vision Zero future is the ‘dilemma zone.’ For the purposes of this paper, the Type II dilemma zone, the distance beyond which 90 percent of all drivers would stop if presented with a yellow signal, or about five seconds of travel time, will be the reference. The length of the zone itself is determined at the furthest point of the area between a ‘likely stop’ decision and the nearest point where a ‘likely go’ decision is made by each driver faced with a green or yellow signal indication at the upcoming intersection.
The Type II dilemma zone, sometimes referred to as the ‘decision zone’, is the most critical point in ensuring the safe passage of vehicles approaching an intersection. Zegeer, et al., were the first to attempt to quantify this area in 1978 when they identified the start of the dilemma zone as the point at which 90 percent of drivers began the vehicle stopping process, and the end of the zone as the point at which only 10 percent of drivers began the stopping process. Later, Chang, Messer, and Santiago defined the boundaries in terms of travel time to the stop bar. They observed that 85 percent of drivers stopped if their vehicles were three or more seconds from the stop bar, but almost all drivers continued through the intersection if they were two or fewer seconds from the stop bar. They also observed that drivers more often stopped at wide intersections and proceeded through narrow intersections. 5
Quantifying the Decision Zone
The Signal Timing Manual Second Edition, which provides trend-based guidance from previous decision zone studies, indicates the decision zone usually exists between 5.5 and 2.5 seconds of travel time from the stop bar. 6 The Signal Timing Manual quantifies these values in stop bar distance, dictated by various common approach speeds. This travel time-based definition is the simplest quantification of the decision zone but should be used with caution as most of the factors involved in the decision zone encompass individual human variables, including perception and reaction time (Figure 1):
Figure 1: The Signal Timing Manual 2nd Edition Decision Zone Based on Vehicle Speed
| Approach / Vehicular Speed (MPH) |
Beginning of Decision Zone (5.5s from Stop Bar) |
End of Decision Zone (2.5s from Stop Bar) |
|---|---|---|
| 35 | 285 feet | 125 feet |
| 40 | 325 feet | 145 feet |
| 45 | 365 feet | 165 feet |
| 50 | 405 feet | 180 feet |
| 55 | 445 feet | 200 feet |
The Role of Signal Timing
It’s obvious that signal timing is arguably the most important component to intersection safety operations. The appropriate length of yellow signal change and red signal clearance timing, paired with accurate detection technology for green or red signal extension activation, mitigates that a driver will encounter a conflict at an intersection with other vehicles, as well as vulnerable roadway users such as pedestrians and bicyclists.
The first objective when identifying a solution for a dilemma zone is to determine the green signal extension indication. Traditionally, the green signal timing interval is extended by drivers passing through advanced detection zone(s) positioned at various key locations along the roadway ahead of the intersection stop bar based on the posted speed limit for the roadway. Each detector has its own green extension timer with preset timing pertinent to the travel time from that dilemma zone location in the designed or appropriate approach speed to the stop bar. This means the timing is unique to each detector location with a simple multiplication of speed and distance to yield the travel time in seconds. Green signal extension timing should provide enough additional green time for a driver to reach the end of the decision zone (closest to the stop bar) to safely pass through the intersection, or alternatively provide enough time for the driver to safely stop at the stop bar of intersection.
However, at a certain point it becomes unsustainable to continue extending the green signal interval as a result of continuous approaching traffic. Zegeer, et al., suggested terminating a traffic movement’s green signal extension when the total extension length reaches 99 seconds to prevent excessive minor movement delay. Zegeer, et al., also advised against using green signal extension during peak congestion. Under these conditions, the intersection relies on an appropriately timed yellow signal change and red signal clearance interval and extending the red signal interval until all conflicting and potentially conflicting drivers have safely passed through the intersection.
Traditional signal timing for dilemma zone protection rarely takes into account drivers exceeding the posted speed limit. In order to address the dynamic nature of the roadway and implement traffic signal timing to significantly elevate dilemma zone safety, higher resolution, real-time data-driven strategies are needed.
The Importance of Detecting Velocity and Tracking
Green signal time, yellow signal change, and red signal clearance intervals work together in addressing the dilemma zone protection. Red signal clearance helps ensure safe clearance through the intersection. Chang, et al., (1985) presented an equation in terms of the driver’s position approaching the intersection in relation to the distance that the driver can travel at a constant approach speed given the yellow signal change and red signal clearance time:
x + w + l ≤ v(Y + AR)
Where x is the distance from the stop bar, w is intersection width, l is vehicle length, v is approach velocity, Y is yellow signal duration, and AR is red signal clearance duration. This determines that the distance a driver must travel to reach the far side of the intersection must be less than or equal to the distance traveled at a constant speed during yellow change and red clearance.
In addition to the distance traveled, Chang, et al., (1985) also noted that stopping distance is just as important to determining appropriate signal timing to enhance dilemma zone safety:
xs = v·t + v22d
This stopping distance formula includes t as the driver’s perception-reaction time and d as the deceleration rate. If the stopping distance (xs) is greater than x (the distance traveled during yellow + red clearance), the driver cannot stop safely before the intersection, rendering the clearance interval insufficient.
Factoring in perception-reaction time and deceleration rate is critical. The Signal Timing Manual 2nd Edition provides a formula to calculate the yellow signal change interval, and the Institute of Transportation Engineers has provided an updated formula that includes factors for turning movements:
Y = t + 1.47v2(a + 32.2g)
Variable Definitions:
- Y = Yellow change interval (seconds)
- t = Perception-reaction time to yellow indication (seconds)
- v = Approach speed / velocity (MPH)
- a = Deceleration rate in response to yellow indication (ft/s²)
- g = Grade percentage divided by 100 (uphill positive, downhill negative)
The National Cooperative Highway Research Program (NCHRP) 731 report specifies a deceleration rate of 10 feet per second per second and a perception-reaction time of 1.0 second. 7 However, Chang, et al., (1985) recommended longer perception-reaction times than the NCHRP 731 report, suggesting 1.2 seconds, with some agencies applying a longer reaction time factor of 1.5 seconds for perception-reaction time.
The Importance of Leading-Edge Sensors
Fortunately, new capabilities powered by leading-edge sensor technologies that instantaneously sense geopositional vehicle information, specify vehicle classification, and provide vehicle tracking data can enable new strategies for dynamic dilemma zone protection. High-resolution real-time sensor data combined with innovative traffic controller software that can dynamically apply instantaneous signal timing changes, like extending green-light and all-red-light timing, offers the promise to effectively address the dynamic conditions of the roadway to eliminate intersection collisions and conflicts.
New sensors and their accompanying software can continuously detect the velocity and position of all approaching vehicles along the roadway, including the dilemma zone. This can provide new dynamic signal timing strategies based on real-time conditions.
By leveraging real-time vehicle data from these advanced sensors, dynamic adjustments can be triggered to extend green signal intervals for all approaching vehicles in the dilemma zone, while protecting other roadway users from potential intersection conflicts. In addition, this geo-position and tracking capability can be used to overcome the variables of driver perception-reaction time.
New Sensor Technology Enabling New Dilemma Zone Safety Capabilities
Leading-edge radar and video sensors are now offering alternative solutions to fixed-point detection for dilemma zone configurations and protection. Existing detector layouts with additional software features enabled in these new sensors can provide more intelligent and dynamic traffic signal timing capabilities, which in turn can inspire new dilemma zone safety strategies.
In addition, these new sensor and controller software solutions can provide situational awareness, including vehicle Estimated Time of Arrival (ETA), trajectory, tracking, and classification. Advance detection zones can be created as ‘motion’ zones to accommodate highly variable traffic condition elements to control the zone’s output. Motion zones can then be leveraged to create ETA destination point detection for multiple vehicles.
Moreover, radar and video sensors are providing conditional outputs for the motion zone to identify vehicle approach speed, class, and direction/trajectory data. This effectively addresses the dynamic nature of vehicles traveling below or above the posted speed limit—which require significantly different decision zones and thus would have very different ETAs—as well as different vehicle classes (for example, providing more extension time for heavy trucks). The implications of these capabilities on future dilemma zone protection, traffic signal operations, and safer mobility strategies are profound.
Driving the Future of Dilemma Zone Safety & Vision Zero
With sensors providing situational awareness through high-resolution, real-time vehicle data—including speed, location, trajectory, classification, and ETA of all approaching vehicles—dynamic traffic signal change strategies can now be deployed. This enables real-time green signal extensions and red signal clearance intervals to be applied dynamically for vehicles with varying classifications and ETAs. Consequently, green signal extensions are supplied only to vehicles unable to safely stop before the stop bar.
These advanced capabilities can also dynamically trigger red signal extensions or external warning devices in real time if a vehicle is detected traveling at unsafe speeds through the dilemma zone. By employing speed-filtered advance detection or motion zones, timed red signal extensions help avoid severe intersection conflicts.
This dynamic response capability will prove essential in managing new and emerging modes of transportation, including Connected and Automated Vehicles (CAVs). Ultimately, the availability of real-time geo-positional vehicle data opens the door for comprehensive, far-reaching safety strategies that accelerate our progress toward a Vision Zero future.
References
- Denos Gazis, Robert Herman, Alexei Maradudin, (1960) The Problem of the Amber Signal Light in Traffic Flow. Operations Research 8(1):112-132
- Charles V. Zegeer, Robert C. Deen, (1978) Green-Extension Systems at High-Speed Intersections. Kentucky Transportation Center Research Report 835.
- World Health Organization (WHO), Safety and Mobility (SAM), Social Determinants of Health (SDH), (2023) Global Status Report on Road Safety 2023. ISBN: 978-92-4-008651-7
- Insurance Institute for Highway Safety (IIWS), (2023), Red Light Running, https://www.iihs.org/topics/red-light-running
- Myung-Soon Chang, Carroll J. Messer, Alberto J. Santiago, (1985) Timing Traffic Signal Change Intervals Based on Driver Behavior. Transportation Research Record: Journal of the Transportation Research Board (TRB), No. 1027, TRB of the National Academies, Washington D.C.
- Tom Urbanik, Alison M. Tanaka, Bailey Lozner, Eric Lindstrom, Kevin Lee, Shaun Quayle, Scott Beaird, Shing Tsoi, Paul Ryus, Doug Gettman, Srinivasa Sunkari, Kevin Balke, Darcy M. Bullock, (2015), Signal Timing Manual, 2nd Edition, Report: National Cooperative Highway Research Program (NCHRP) 812, Transportation Research Board (TRB) of the National Acadamies, Washington D.C.
- Transportation Research Board, National Cooperative Highway Research Program (NCHRP) Report 731: Guidelines for Timing Yellow and All-Red Intervals at Signalized Intersections offers guidance for yellow change and all-red clearance intervals at signalized intersections (2016). DOI: 10.17226/22700, Project Number: 03-95