A car accident is a road traffic incident which usually involves at least one road vehicle being in collision with, either another vehicle, another road user, or a stationary roadside object, and which may result in injury or property damage. Phrases used to describe accidents include: auto accident, car crash, car smash, car wreck, fender bender, motor vehicle accident (MVA), personal injury collision (PIC), road accident, road traffic accident (RTA), road traffic collision (RTC), road traffic incident (RTI), smash-up, and traffic collision.

A well-designed and well-maintained vehicle, with good brakes, tires and well-adjusted suspension will be more controllable in an emergency and thus be better equipped to avoid collisions. Some mandatory vehicle inspection schemes include tests for some aspects of road worthiness, such as the UK's MOT test or German TÜV conformance inspection.

The design of vehicles has also evolved to improve protection after collision, both for vehicle occupants and for those outside of the vehicle. Much of this work was led by automotive industry competition and technological innovation, leading to measures such as Saab's safety cage and reinforced roof pillars of 1946, Ford´s 1956 Lifeguard safety package, and Saab and Volvo's introduction of standard fit seatbelts in 1959. Other initiatives were accelerated as a reaction to consumer pressure, after publications such as Ralph Nader's 1965 book Unsafe at Any Speed accused motor manufacturers of indifference towards safety.

The ability to deliver prompt medical attention has also improved through improvements in ambulance and rescue equipment, availability of air ambulances, rapid response units, and paramedic training, while design changes have made collisions more survivable. Thus injuries from a collision that once would have been fatal may now be averted, while remote locations may report few accidents but with more fatalities.

For this reason modern accident statistics often focus on reportable injury accidents (which include deaths) rather than reporting on deaths alone. It is also believed that serious accidents are often significantly under-reported, under-recorded and misclassified and that the completeness of reporting may vary over time and between sources.

Common types of collision
Statistics can be gathered on the direction of impact, or impacts. The most common collisions on the road where both parties are moving involve:

• frontal impacts (most common)

• side impacts

• rear impacts

• rollovers (most rare)

In the USA rollovers have contributed to more than 10,000 deaths a year even though they only account for about 3 percent of all crashes.

Sometimes the vehicles in the collision can suffer more than one type of impact, such as during a shunt or high-speed spin. This most often occurs when there is a "second harmful event," such as when a vehicle is redirected by the first crash into another vehicle or fixed object.