One-third of all intersection crashes in the United States, and more than 40 percent of the fatal ones, occur at intersections controlled by stop signs. This amounts to about 700,000 crashes at stop signs each year.
About 70 percent of all crashes at one- and two-way stop signs involve the same basic pattern — a vehicle that's required to stop doesn't, or it stops and fails to yield, and then it collides at an angle with another vehicle going across the intersection. This is the main finding of new Institute research that looks at the specific patterns of crashes at one- and two-way stop signs to determine what's causing the collisions and what can be done to prevent them.
About two-thirds of the stop sign violation crashes involved drivers who said they stopped before proceeding. Only 17 percent of the crashes involving violations (12 percent of all stop sign crashes studied) involved drivers who ran through the signs. Another 12 percent of the crashes studied were rear-end collisions.
For the study, the Institute and Preusser Research Group analyzed about 1,800 police-reported crashes at stop signs in four U.S. cities — Oxnard, California; Westfield, New Jersey; Springfield, Missouri; and Germantown, Tennessee. The study excluded intersections where traffic from all directions is required to stop, because these intersections tend to have different traffic flow characteristics and fewer crashes than at one- or two-way stops.
Drivers didn't see conflicting vehicles
At stop signs drivers are required not only to stop but also to look for vehicle conflicts and judge whether it's safe to proceed. The findings of this study indicate that drivers don't always judge correctly.
"The most common situation we found was that a driver just didn't see the other vehicle coming," explains Richard Retting, the Institute's senior transportation engineer and lead author of the study. This is how 44 percent of the crash-involved drivers who stopped explained what happened. Another 16 percent said their views were obstructed. Only 6 percent saw the other vehicle but failed to avoid the collision.
In some cases, the shape or design of a roadway can make it hard for drivers to see approaching traffic. Parked vehicles, shrubbery, or even glare can obstruct drivers' views. However, the extent to which these environmental factors contribute to crashes isn't clear, in part because such information typically isn't noted in police reports.
A relatively small proportion of the crashes (12 percent) involved drivers who failed to stop, but these collisions were more likely to result in injury. They were twice as likely to happen at night, and they occurred more often at cross-type intersections than at t-intersections. Young drivers, particularly young men, were more often found to be at fault in these crashes.
Both older (65 and older) and younger drivers were disproportionately at fault in stop sign crashes. This finding is consistent with previous studies showing that older drivers generally are overrepresented in intersection crashes (see Status Report special issue: older drivers, Sept. 8, 2001). Problems include age-related visual impairments and loss of flexibility, which can make it hard to turn to look both ways when crossing intersections.
Ways to reduce the crashes
Retting says the focus needs to be on improving the designs of intersections or replacing them with safer forms of traffic control. "To the extent that failure to see stop signs is a problem, the solutions can be fairly simple. Intersections can be checked periodically to make sure obstructions aren't blocking drivers' views. The signs themselves can be checked for luminosity. Pavement markings that warn drivers of stop signs ahead can help, and even adding extra signs at problem locations can get drivers' attention.
Installing all-way stops at appropriate locations can be beneficial. Compared with one- or two-way stops, the all-way signs can reduce overall crashes by 40 to 60 percent and injury crashes by 50 to 80 percent (see Status Report special issue: urban crashes, May 2, 1998).
Another effective measure is to convert stop sign-controlled intersections to roundabouts. This can reduce crashes by 40 percent and improve traffic flow at the same time (see "Roundabouts reduce traffic backups and crashes, too," July 28, 2001).
"A main benefit of both roundabouts and four-way stops is that they slow the traffic," Retting says. "With the traffic moving more slowly, crashes are less likely to occur, and when they do occur they're less likely to be serious."