Road collision types
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Road traffic collisions generally fall into one of four common types:
- Lane departure crashes, which occur when a driver leaves the lane they are in and collide with another vehicle or a roadside object. These include head-on collisions and run-off-road collisions.
- Collisions at junctions include rear-end collision and angle or side impacts.
- Collisions involving pedestrians and cyclists
- Collisions with animals
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'.
Head-on collisions often have poor outcomes because of the speed involved when the collision takes place. 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.
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.
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.
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.
Median barriers can be divided into three basic categories: rigid barrier systems, semi-rigid barrier systems, and flexible barrier systems. Rigid barrier systems are made up of concrete and are the most common barrier type in use today (e.g. Jersey barrier or concrete step barrier). They are the most costly to install, but have relatively low life-cycle costs, making them economically viable over time. The second barrier type, semi-rigid, is commonly known as guardrail or guiderail barriers. The initial installation of this type can reach as much as $100,000 per mile. These more forgiving barriers are meant to absorb the impact of an accident, and as a result, increase the cost of their life-cycle with each crash and each repair. The third median barrier type is the flexible barrier systems (e.g. cable barriers). Cable barriers are the most forgiving and the least expensive to install, but have high life-cycle costs due to repair needs after crashes. On the other hand, they have been shown to have calculated cost benefits calculating to as much as $420,000 per mile annually. Much cheaper collision reduction methods are to improve road markings, to reduce speeds and to separate traffic with wide central hatching.
Where a hard shoulder cannot be provided, a "safety edge" can reduce the chances of steering overcorrection. An attachment is added to the paving machine to provide a beveled edge at 30 to 35-degree angle to horizontal, rather than the usual near-vertical edge. This works by reducing the steering angle needed for the tire to climb up the pavement edge. For a vertical edge, the steering angle needed to mount the pavement edge is sharp enough to cause loss of control once the vehicle is back on top of the pavement. If the driver cannot correct this in time, the vehicle may veer into oncoming traffic, or off the opposite side of the road.
They usually have similar root causes as head-on collisions, but no other vehicle happened to be in the path of the vehicle leaving its lane. Severe collisions of this type can happen on motorways, since speeds are extra high, increasing the severity.
Crashes at intersections (road junctions) are a very common type of road collision types. Collisions may involve head-on impact when one vehicle crosses an opposing lane of traffic to turn at an intersection, or side impacts when one vehicle crosses the path of an adjoining vehicle at an intersection.
Risk of intersection collisions
The risk of intersection collisions differs on rural and urban roads, with around 50% of urban crashes and 30% of rural crashes occurring at junctions. In urban areas the likelihood of an intersection collision occurring is high as they typically have a higher density of junctions. On rural roads while the likelihood of a collision may be lower (because of fewer intersections) the outcome of the collision is often significantly worse because of the increased speeds involved.
Because intersection collisions often result in side-impacts they are therefore often fatal because people are seated close to the part of the car that provides little protection.
Although expensive to implement, roundabouts are an effective way of reducing the speed of traffic at intersections and dramatically reducing the likelihood of high speed right-angle collisions. Clear road markings and signing are low cost methods of improving safety at intersections.
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