Drivers who veer off the road and into the shoulder often can be caught by surprise when suddenly one or two of their wheels drop off the pavement, and even if the drop-off is only a couple inches, the results can be deadly.

A recent study sponsored by the AAA Foundation for Traffic Safety has found that pavement edge drop-off related crashes are substantially more likely than other crashes on similar roadways to result in serious injuries, and are more than twice as likely to be fatal. Such drop-offs are hazardous because the significant vertical differences between surfaces can affect vehicle stability and reduce a driver’s ability to handle his or her car.

Characterized by a shoulder that is lower than the edge of the paved road surface, pavement edge drop-off can be caused by pavement breaking, erosion and wear of unpaved shoulders, inadequate maintenance, or resurfacing projects that neglect to keep shoulders level with the paved road surface. Although they can occur on any roadway, they are more common on rural two-lane roads with narrow lanes and shoulders.

A driver can accidentally drive off the road or be forced off the road for any number of reasons and pavement edge drop-offs, especially if they are 2 inches or deeper, can increase the risk of a crash and make it difficult for a driver to return to the road safely, the study found.

A typical pavement edge drop-off crash occurs when the driver attempts an immediate return to the roadway and tire scrubbing occurs, a condition in which the tire sidewall is forced into the pavement edge, resulting in friction between the tire and pavement. When this happens, some drivers overreact and increase their steering angle, which can cause them to lose control and cross into the oncoming traffic or can result in a rollover.

While crashes related to pavement edge drop-off are relatively infrequent, the study found they are often severe.

The research, conducted by the Iowa State University Center for Research and Education and Midwest Research Institute, included an assessment of the prevalence of pavement edge drop-offs on rural two-lane roads in Missouri and Iowa, as well as the crashes related to drop-offs. Included in the study were results from a federal study of crashes related to edge drop-offs in Illinois and North Carolina.

While several agencies at the national level provide guidance and recommendations related to edge drop-off for highway design and construction, no national standards exist. The study also found that maintenance practices related to this problem vary considerably from state to state.

The report recommends that states and agencies responsible for highways adopt a policy of providing paved shoulders, with a minimum width of two feet or wider if conditions warrant wherever practical, and endorsed the incorporation of a beveled edge in all roadway resurfacing projects to prevent the occurrence of severe drop-offs at pavement edges.

To promote safe driving when encountering a pavement edge drop-off, the AAA Foundation developed “Over the Edge and Back,” a brochure that explains what drivers should do if they drive off the road.

“It’s not about returning to the road quickly, it’s about returning to the road safely,” said Mike Right, a spokesman for AAA. “Don’t panic or immediately attempt to steer back on the road, especially at high speed, as this can easily cause you to lose control.”

With step-by-step instructions, the brochure advises motorists to steer straight and slow down gradually, not abruptly; braking hard while the tires are on different surfaces can cause the vehicle to skid. If it’s safe, drive all the way onto the shoulder and wait until there is a gap in traffic. Then you can drive back onto the road safely.

For a free copy of the brochure, send a self-addressed, stamped business-size envelope to: AAA, Over the Edge and Back, 12901 N. Forty Drive, St. Louis, MO, 63141. To see an online version, visit www.AAA.com.