Roadway departure

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.

Current guidance adjusts the desired clear zone width for curvature, roadside slope, speed and volume.

They fall into three main categories: preventing excursion 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.

This helps any driver that runs off the edge of the roadway to maintain control while trying to steer back onto the pavement.

Examples include tree removal, using forgiving road infrastructure or extending cross culverts out of the clear zone.

Guard rails are used to reduce the severity of roadway departure crashes by interposing a barrier that is more forgiving to vehicle occupants.

One study found that installing guard rail above an embankment would only reduce run-off-road crashes by only 7%.

On low-speed, low-volume local roads, expensive improvements are likely to produce less in savings than they cost, and thus divert scarce resources from locations where they could be better used.