Road traffic collisions generally fall into one of five common types:
Lane departure crashes, which occur when a driver leaves the lane they are in and collides with another vehicle or a roadside object. These include head-on collisions and run-off-road collisions. Collisions at junctions, including rear-end collision and angle or side impacts. Collisions involving pedestrians and cyclists. Collisions with animals. Collisions with buildings.
Other types of collision may occur. Rollovers are not very common, but lead to greater rates of severe injury and death. Some of these are secondary events that occur after a collision with a run-off-road crash or a collision with another vehicle.
If several vehicles are involved, the term 'serial crash' may be used. If many vehicles are involved, the term 'major incident' may be used rather than 'pile up'.
Crash typology by country
In some countries, crash type classification exists for statistical purpose so that a crash is counted in one type or another.
California
California classify road collision by: Head-on, sideswipe, rear end, broadside, hit object, overturn, auto-pedestrian, other, not stated
France
French official statistics have 7 road collision types: – Two vehicles – frontal – Two vehicles – rear – Two vehicles – side – Three vehicles and more – in series – Three vehicles and more – multiple collisions – Other collision – Without collision
Head-on collisions
A head-on collision occurs when two vehicles travelling in opposite directions (more or less) collide frontally with each other. The typical cause of head-on collisions is when one vehicle inadvertently strays into the path of an oncoming vehicle. However, the root cause sometimes lies in a steering overcorrection after veering to the side of the road as opposed to the centre. Head-on collisions often have poor outcomes because of the speed involved when the collision takes place. In a head-on collision, the vehicle speeds are additive—the effective collision speed is the sum of the speeds of the two vehicles. This means that, for example, if two vehicles both moving at 45 MPH collide head-on, the effective crash speed is 90 MPH; if the weights of both vehicles are about the same, then the crash forces experienced by the occupants of both vehicles will be essentially equal to those forces that would be experienced if one of the vehicles were travelling at 90 MPH and the other vehicle was stopped (with its brakes released) when the collision occurred. (If one vehicle is heavier than the other, the crash physics are not symmetrical and the 90 MPH of total crash speed will be apportioned differently depending on the actual speed of each vehicle, as the kinetic energy of each vehicle depends on both its speed and its weight.)
Risk of head-on collisions
The likelihood of head-on collision is at its greatest on roads with narrow lanes, sharp curves, no separation of lanes of opposing traffic and high volumes of traffic. Crash severity, measured as risk of death and injury, and repair costs to vehicles, increases as speed increases.
Therefore, the roads with the greatest risk of head-on collision are busy single-carriageway roads outside urban areas where speeds are highest.
Contrast this with motorways, which rarely have a high risk of head-on collision in spite of the high speeds involved, because of the median separation treatments such as cable barriers, concrete step barriers, Jersey barriers, metal crash barriers, and wide medians.
Countermeasures
The greatest risk reduction in terms of head-on collision comes through the separation of oncoming traffic, also known as median separation or median treatment, which can reduce road collisions in the order of 70%. Indeed, both Ireland and Sweden have undertaken large programmes of safety fencing on 2+1 roads.