Rakha and his team at Virginia Tech have been studying drivers' behavior and reactions as they approach yellow lights since 2005. Their goal is to determine signal times for intersections that are safer than those currently in use, while still keeping those signals efficient and practical to use.
According to Rakha, if yellow light timing is not set correctly set at an intersection, a "dilemma zone" is created. Rakha's so-called dilemma zone occurs when the driver approaching an intersection can neither stop in time before entering an intersection, nor can they safely make it through before the light turns red. In this situation, drivers who try to implement an emergency stop increase their risk of a rear- end collision, but if they decide to go through the intersection, a collision with side street traffic could occur. The choice can be a risky gamble. The research Rakha conducted found that while only 1.4% of drivers actually cross the stop line after the light turns red, more than 20% of traffic fatalities in the U.S. occur at intersections.
According to the article, current yellow light times are set for 4.2 seconds on a 45 mph road. Rakha explains that timings are based on the assumptions that drivers require one second to perceive and react to the change in signal, and that drivers requires 3.2 seconds to stop from 45 mph at a comfortable deceleration level of 10 feet per second squared.
To test this, Rakha used Virginia's Smart Road, located at the Virginia Tech Transportation Institute. The road has signals that allow the researchers to change the length of the red, yellow, and green lights. This let Rakha and his team study driver behavior by changing the signal when a test driver is a certain distance from the intersection. The research revealed that 43% of drivers who crossed the stop line during a yellow light duration were not able to clear the intersection before the light turned red.
Rakha's research also determined that the perception-reaction times were slightly longer than one second, but that driver deceleration levels are significantly higher than the deceleration level assumed for traffic signal design. However, if road conditions are poor, drivers react 15% slower because they are processing more information, which reduces deceleration by 8%. To remedy this, the intersection would require a longer yellow light interval.
Rakha and his team developed a procedure to compute and change yellow light interval duration that can account for drives being caught in the dilemma zone. The team created tables for light vehicles at various speeds for dry and wet roads to enable traffic planners to set traffic signals that can be adjusted to road surface and weather conditions. They also created tables for driving characteristics for different age groups, since studies showed that elderly drivers were not able to react to changing signals as quickly as younger drivers were.
Beyond their tables, Rakha and his team are moving on to development of caution lights that can hopefully be used on existing traffic signal structures. These warning lights will alert drivers when a green light is about to change to yellow. This would provide drivers with longer times to react. The team also hopes to take this a step further by implementing vehicle-to-vehicle and infrastructure-to-vehicle communication that can create a warning visible on a vehicle's in-car display, which can be customized to each driver's reaction time. The goal is to provide a four- or five-second warning when approaching an intersection.
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