Definition
In motorsports racing there is a concept called the racing line which is defined in lay terms as the optimal path around a race course that will allow the racer to complete a lap in the least possible time with the highest possible average speed. The racing line is a function of the track's layout and the combination of a particular type of racing vehicle's (such as a car versus a motorcycle) capabilities and the physics of motorsports racing.
Because the physics involved in a motorsports event generally propels the racing vehicles along a certain path, and since the racers tend to work with the forces acting on the vehicle and make course changes while not suddenly confronting the forces in play on the vehicle, their locations at certain points along the track can be predicted and their course of travel extrapolated. As a result, in areas where a vehicle is more likely to depart the course (i.e., immediately after a corner as opposed to alongside a straight-away), course designers will place a run-off area.
Design
Run-off areas generally consist of four things:
1. A large, open space.
2. A surface material that acts to rapidly bleed off energy from an out-of-control vehicle that has been forced into the run-off area. Gravel traps are the most commonly used run-off area surface material as they do a good job in slowing off-course vehicles to recoverable speeds, they do minimum damage to the race vehicles, and they act to "soften" the surface of the ground in that area, similar to sand on a beach. Their use during motorcycle races is less frequent because of the risk of gravel ending up on the race course itself, which could be tolerated by cars but could be catastrophic for bikes.
3. A safety device, such as a tire wall or an air fence that safely prevents the vehicle from going beyond the run-off area and into another area or a hard barrier. If a run-off area is sufficiently large, there sometimes will not be a safety barrier as the racer and vehicle will come to a complete stop before they run out of run-off room.
4. The conspicuous absence of any obstructions or dangerous objects in the run-off area, such as an abutment for a nearby bridge, spectator grandstands, billboards, etc., which an out-of-control vehicle might impact.