Pavement/shoulder drop-offs are the result of poorly constructed shoulders, inadequate maintenance programs, and construction activity. They create serious hazards for motorcycles and become a problem for the normal passenger car when the depth (h1) of the vertical faced drop-off is greater than the height (h2) of the “bulge” in the tire sidewall.
In the classic pavement/shoulder drop-off accident, the problem begins when, drifting out of the travel lane, the right side tires fall off the pavement edge. An appropriate response to this situation is to slow down until recovery is possible, or move further to the right and return at a steeper, more manageable angle. Unfortunately, this unexpected jolt will usually prompt an immediate left-steering input that does not initially yield a change in direction. Additional steering input takes up any slack in the steering linkage, twists the tire and, if the drop-off is not too deep, eventually produces a left turn.
Similar to releasing a rubber band, the right front tire mounts the pavement lip, successfully reenters the travel lane, but at an angle much greater than expected, causing a sharp swerve to the left. This often causes an excursion into the opposing travel lane, resulting in a head-on collision, a roadway departure to the left, or a counter-steer to the right. This steering effort is frequently an over-correction, leading to a spinout or a second right side departure with a possible off-road impact or rollover. Like most emergency situations, the drop-off is unexpected, the driver is unaware of the proper solution and, unlike airline pilots, automobile drivers are not required to know or practice proper recovery procedures from the many potential emergency situations they will encounter.
In the investigation of a head-on collision or a single vehicle roadway departure, an investigation of the roadside upstream from the crash site is a must. If a drop-off is found, a careful examination may show a tire track in the rut or tire scuff marks or rim marks on the pavement edge of the reentry point. An examination of the inside of the right front tire may reveal scrub marks on the sidewall and/or scraping of the rim.
Further discussion of single vehicle collisions can be found in Highway Accidents: Investigation, Reconstruction and Causation available at: Amazon.com. Information about the book and author is available at: www.bmorrow.com.