Washington, September 12 (ANI): A group of researchers studying drivers' behaviours as they approach yellow lights tried to determine signal times for intersections that are safer and still efficient.
If a driver decides to stop when instead of proceeding, rear-end crashes could occur. If a driver proceeds instead of stopping, collisions with side street traffic could occur.
Although observation-based research shows that only 1.4 percent of drivers cross the stop line after the light turns red, more than 20 percent of traffic fatalities in the United States occur at intersections.
If the yellow time is not set correctly, a dilemma zone is imminent, Hesham Rakha from the Center for Sustainable Mobility at the Virginia Tech Transportation Institute said.
The dilemma zone occurs when the driver has no feasible choice.
In other words the driver can neither stop nor proceed through the intersection before the light turns red. This can also occur if the approaching vehicle is travelling faster than the posted speed limit and/or if the driver's perception and reaction time is longer than the design one-second value, Rakha said.
In most cases, the yellow time is set for 4.2 seconds on a 45 mph road. The time is longer for higher-speed roads.
These timings are based on two assumptions, Rakha explains.
Namely, the driver requires one second to perceive and react to the change in signal indication and that the driver requires 3.2 seconds to stop from 45 mph at a comfortable deceleration level, assumed to be 3 meters per second squared (3 m/s2 ) or 10 feet per second squared, he said.
For his studies, Rakha used Virginia's Smart Road, located at the Virginia Tech Transportation Institute. The Smart Road intersection has a signal that can be controlled for length of the red, yellow and green lights.
We can study driver behaviour by changing the signal when the driver is a certain distance from the intersection, he said.
Center for Sustainable Mobility researchers have determined that half of drivers make the stop-go decision three seconds before the stop line.
Of those that go, few clear the intersection before the light changes to red. In Virginia, if you are in the intersection when the light turns red, you are not running a red light. However, there is still risk.
The specific findings from the Smart Road study are that 43 percent of drivers who crossed the stop line during the yellow time were not able to clear the intersection before the light turned red.
At 45 mph, it takes 1.5 seconds to clear a 30-meter (98.4 feet) intersection.
If the all-red interval is the minimum conventional one second, then there is a potential risk that the legal yellow-light runners would not be able to completely clear the intersection at the instant the side-street traffic gains the right-of-way, the researchers said.
People over 60 years of age have a longer perception-reaction time, so they have to brake harder to stop. But they are more likely to try to stop, compared to younger drivers. However, if they keep going, they are unlikely to clear the intersection, the researchers report.
The research determined that the perception-reaction time is slightly longer than one second, but that driver deceleration levels are significantly higher than the deceleration level assumed for traffic signal design.
The findings of the study were presented at the Transportation Research Board Annual Meeting in 2010. (ANI)