Roadway departure collisions involve vehicles leaving the travel lane and encroaching into the opposite lanes or onto the shoulder and roadside environment. The result of this maneuver is that the vehicle hits an oncoming vehicle or fixed object(s) such as trees, poles, bridge walls, piers, or columns, embankments, or guardrails. Some of the top contributing factors for roadway departure fatal- or severe- injury collisions include driver distraction or inattention, excessive speed, driving under the influence, and driving on the wrong side or the wrong way on a road.
Maintaining a proper clear zone is the first priority for engineering improvements. Clear zones allow enough area for drivers to recover when departing from the travel lane. Additional improvements, such as installing edge line and centerline rumble strips, improving shoulders, and removing or shielding hazards may prevent roadway departure collisions or lessen their severity.
Roadway departure collisions accounted for approximately 43% of all fatal and severe injury collisions in South Carolina from 2008-2012, resulting in more than 2,100 fatalities and 6,400 severely injured persons. Nearly one in two roadway deaths and one in three severe injuries occurred in a roadway departure collision. While the number of severe injuries sustained in these types of collisions decreased 9.9% from 2008 to 2012, the number of fatalities declined at a slower rate, a 7.5% reduction.
More than half of the roadway departure collisions occurred on rural roads (59%), compared to 26% that occurred on urban roads.
When a vehicle is involved in a roadway departure, a sequence of events occurred prior to that vehicle leaving the roadway. Top events, or actions, made by a vehicle after it departed the roadway are collision with a fixed object, vehicle versus vehicle, and vehicle overturn. A collision with a fixed object far exceeds any other event, at 63% of the total fatal and severe injury collisions.
Fixed objects include items such as trees, ditches, fences, bridge rails, guardrails, and curbs. From 2008 to 2012, hitting trees accounted for nearly 40% of all fatal and severe injury collisions that involved hitting fixed objects.