To improve safety at often-dangerous rural road intersections, University of Minnesota researchers investigated drivers’ ability to judge traffic speed and gaps between cars at varying sight distances. Using a state-of-the-art driving simulator, the researchers showed that 1,000 feet of sight distance allows drivers to make better crossing decisions.
“We’ve known that sight distances were factors in rural intersection crashes,” says Tracey von Bargen, county engineer for Grant County. “This study gave us the hard data showing how long sight distances should be to allow drivers sufficient time and space at rural thru-stop intersections.”
The need for this project was pressing because fourway intersections carry inherent safety risks for drivers. From 2008 to 2012, nearly 42 percent of severe crashes occurring at Minnesota intersections resulted in serious injuries and fatalities, and crashes are often most severe at unsignalized rural intersections.
“Drivers on minor roads seeking to cross must determine when traffic gaps are sufficient to cross and can misjudge the time-to-collision,” says Nichole Morris, director of the U of M’s HumanFIRST Laboratory. “Limited visibility has been associated with drivers choosing insufficient gaps, so we wanted to determine the optimal sight distance at four-way intersections. This will maximize driver safety, reduce injuries, and save lives while potentially reducing unnecessary clearing of trees and brush at intersections.”
Study results show that 400- and 600-foot sight distances are insufficient for drivers to make good crossing decisions. The 1,000-foot sight distance and slower speeds (55 mph) allowed drivers to judge time-tocollision more effectively and respond to cars at minor thru-stops intruding into the mainline. “This means that drivers could effectively respond to a car running a stop sign—often a catastrophic crash scenario,” Morris says.
“Agencies could use the 1,000-foot sight distance and our other findings for guidance,” Morris says. The project was funded by the LRRB.
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