Run-off-road collision

A run-off-road collision (or roadway departure) is a US term for a type of single-vehicle collision that occurs when a vehicle leaves the roadway. Contributing factors often include loss of control or mis-judging a curve, or attempting to avoid colliding with another road user or an animal.

If the vehicle strikes a fixed object (an object that will move very little when struck, such as a tree, bridge structure or utility pole) or rolls over, the crash is likely to result in injuries or fatalities. 2005 statistics from the US show that run off-road crashes resulted in 31% of fatal crashes, but were only 16% of all crashes. Run-off-road collisions where the vehicle is sliding or spinning and runs broadside into a fixed obstacle are particularly dangerous, since the vehicle doors and sides provide less protection to occupants than the front of the car.

An important concept in understanding run-off-road crashes is the clear zone. This is the roadside area that is free from obstacles and dangerous slopes. Early research at the General Motors Proving Grounds found that 80% of their test drivers stopped or regained vehicle control within 30 feet (9 m) of the edge of the travel lane. As a result, civil engineers began to try to provide thirty feet of clear, flat ground next to rural highways. The result was fewer crashes. Current guidance adjusts the desired clear zone width for curvature, roadside slope, speed and volume. More width is recommended on the outsides of curves, where the ground slopes down away from the road, and on high speed, high volume roads.

There are several ways to reduce consequences of run-off-road collisions. They fall into three main categories: preventing run-off-road incidents, minimizing the likelihood of a crash or roll-over if the vehicle travels off the shoulder, and reducing the severity of those that do occur.

Roadway cross section improvements include high friction overlays, improving curve banking, and widening shoulders or travel lanes. The intention is to help the driver to keep the car on the roadway. They are usually expensive unless included in a highway reconstruction project.

A relatively inexpensive countermeasure is the placement of a Safety Edge — a 30° compacted taper on the edge of the pavement. This helps any driver that runs off the edge of the roadway to maintain control while trying to steer back onto the pavement. A vertical edge dropoff often results in overcorrection, leading to a head-on collision, rollover, or a run-off-road collision on the far side of the road. Pavement edge dropoffs are problematic on roads where the hard shoulder is narrow or nonexistent. The safety edge adds about 1% to the pavement costs while building or resurfacing a road.

...
Wikipedia