Programs & Labs
Winter 2002
Research investigates effects of advance warning flashers at signalized intersections
(top) The type of intersection under investigation; (bottom) an intersection in the virtual environment
Drivers approaching a signalized intersection when the signal light changes from green to yellow are presented with a choice: slow down and prepare to stop for the red light, or continue on—possibly speeding up—to pass through the intersection. Depending on vehicle speed, the "decision zone" where drivers decide whether to stop or continue extends some distance ahead of the intersection itself.
Advance warning flashers are generally positioned before the decision zone and activated before the signal light itself changes from green to yellow. Surveys have shown that advance warning flashers are popular with the public, and especially with operators of large commercial vehicles, which take much longer to stop than passenger cars. However, while the flashers do tend to reduce red light running for truck traffic, traffic researchers have not seen a clear correlation between advance warning flashers and a lower accident rate.
Thomas Smith, a University researcher in the Department of Kinesiology, is investigating this issue in a project titled "Effects of Advance Warning Flashers at Signalized Intersections on Simulated Driving Performance." Studying drivers' reactions to advance warning flashers in the field is highly problematic because the devices are relatively uncommon and because it is difficult or impossible to establish a controlled experimental environment in which variable parameters can be tested individually, Smith says. The Human Factors Research Laboratory's wrap-around driving simulator has allowed Smith's research team to bypass these pitfalls and create a precisely regulated driving course that includes multiple flasher-equipped intersections. The virtual route is based on a set of roads in the southwest Twin Cities metro area in order to facilitate comparison between simulator results and real-life traffic observations.
Measures of driving performance, including speed, acceleration, deceleration, and braking, were recorded for research subjects as they navigated the virtual roadways. These data were then tabulated and analyzed to uncover trends and correlations indicating patterns of driver behavior under specific test conditions.
After analysis of the experimental data, the researchers concluded that advance warning flashers often improve stopping behavior at suitable intersections. But as is so often seen in human factors research, human response to a complex situation is not as simple as a linear relationship, Smith adds. In this case, variability in human response resulted in some drivers making a more aggressive—and risky—decision to proceed through the intersection. This finding has obvious implications for field implementation of advance warning flashers at dangerous intersections. Beverly Farraher of the Minnesota Department of Transportation, who served as the technical liaison on the project, agrees that the use of advanced warning flashers is a complex issue. "Their benefits are not rigorously quantified for standard signal system applications," she says, and so not all traffic engineers support their implementation.
One result that emerged from the study was that under a speed limit of 50 mph, advance warning flashers reduced unsafe stops and red-light running, but at a speed limit of 65 mph the positive effect disappeared. This may be due to differences in decision making depending on how much time is available to make the decision.
Humans, Smith says, make decisions in different ways depending on how much time they have to decide. If an immediate response is required, decisions are made on a psychomotor level; these are often characterized as "instinctive" or "reflex" responses. However, if more time is available, decisions are made on a cognitive level. In this type of decision making, the individual has sufficient time to consider a range of different possible responses to the situation.
In general, psychomotor decision making is focused on avoiding danger; cognitive decision making may favor more risky behavior under certain circumstances. By effectively starting the decision process sooner, advance warning flashers extend the time available for decision making at intersections, and this can result in a change from psychomotor to cognitive decision making as the driver approaches a signal. Smith recommends that traffic planners carefully examine the characteristics of any intersection where the installation of advance warning flashers is considered.