Road traffic accidents 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
  Head-on collision
  A head-on collision is one where the front ends of two ships, trains, planes or vehicles hit each other, as opposed to a side collision or rear-end collision.-Rail transport:...
  
   and run-off-road collision
  Run-off-road collision
  A run-off-road collision is a type of single-vehicle accident that occurs when a vehicle leaves the roadway. Contributing factors often include loss of control or mis-judging a curve, or attempting to avoid colliding with another road user or an animal...
  
  s.
• accidents at junctions include rear-end collision
  Rear-end collision
  A rear-end collision is a traffic accident wherein a vehicle crashes into the vehicle in front of it, usually caused by tailgating or panic stops...
  
   and angle or side impacts
  Side collision
  Side collisions are vehicle crashes where the side of one or more vehicles is impacted. These crashes often occur at intersections, in parking lots, when two vehicles pass on a multi-lane roadway, or when a vehicle hits a fixed object....
  
  .
• Accidents involving pedestrians and cyclists
• Collisions with animals

Although other types of accident do occur. Rollover

Rollover

A rollover is a type of vehicle accident in which a vehicle tips over onto its side or roof. The most common cause of a rollover is traveling too fast while turning.- Dynamics :Vehicles can roll over in several ways...

s 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.

Head-on collisions

Head-on collisions often have poor outcomes because of the speed involved when the collision takes place. The obvious 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.

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 Wire Rope Safety barrier

Wire Rope Safety barrier

Wire Rope Safety Barrier is a type of road safety barrier. It is also referred to as cable barrier, safety fence, or cable guardrail.-Common Specifications:-Common characteristics:...

s, Concrete step barrier

Concrete step barrier

A concrete step barrier is a safety barrier used on the central reservation of motorways and dual carriageways as an alternative to the standard steel crash barrier.The barrier has contained all vehicles up to 13.5 tonnes.-United Kingdom:...

s, Jersey barrier

Jersey barrier

A Jersey barrier or Jersey wall is a modular concrete barrier employed to separate lanes of traffic. It is designed to both minimize vehicle damage in cases of incidental contact while still preventing crossover in the case of head-on accidents....

s, 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 accidents in the order of 70%. Indeed both Ireland and Sweden have undertaken large programmes of safety fencing on 2+1 road

2+1 road

2+1 road is a specific category of three-lane road, consisting of two lanes in one direction and one lane in the other, alternating every few kilometres, and separated usually with a steel cable barrier. Traditional roads of at least width can be converted to 2+1 roads and reach near-motorway...

s.

However the installation of median barriers can cost in excess of $100,000. Much cheaper accident reduction methods are to improve road markings, reduce speeds and to separate traffic with wide central hatching.

Sealing of safety zones along the side of the road (also known as a hard-shoulder

Shoulder (road)

A hard shoulder, or simply shoulder, is a reserved area by the verge of a road or motorway. Generally it is kept clear of motor vehicle traffic...

) can also reduce the risk of head-on collisions caused by steering over correction when drivers accidentally veer off the side of the road.

Where a hard shoulder cannot be provided, a "safety edge" can reduce the chances of steering overcorrection. An attachement 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.

Single accidents

A Single accident is when a single road vehicle has an accident, without involving any other vehicle.

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 accidents of this type can happen on motorways, since speeds are extra high, increasing the severity.

Intersection accidents

Crashes at intersections (road junctions) are a very common type of road accident types. Accidents 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 accidents

The risk of intersection accidents 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 accident occurring is high as they typically have a higher density of junctions. On rural roads while the likelihood of an accident may be lower (because of fewer intersections) the outcome of the accident is often significantly worse because of the increased speeds involved.

Because intersection accidents 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.

Countermeasures

Although expensive to implement, roundabouts are an effective way of reducing the speed of traffic at intersections reducing the likelihood of high speed right-angle collisions. Clear road markings and signing are low cost methods of improving safety at intersections.