The vast majority of car crashes are caused solely or primarily by driver-related factors, including distraction, fatigue, intoxication and driving at unsafe speeds. Each of these factors is under the control of the driver, which makes them difficult or impossible to remedy with legislation or changes to road systems.
Speeding is a major factor, not just in the number of collisions but in lethality. Speeding works to make road travel less safe on several levels. It increases the chances of a collision occurring, reduces the time available to stop or swerve safely, and increases the net energy available to the colliding vehicles, which thereby increases the chance that a collision results in fatality.
The dangers of driving while intoxicated are so well known that the practice is almost universally illegal, and it is severely punished in most jurisdictions. Impairment can be caused by drug or alcohol use, certain medical conditions and various prescription medications.
Driver fatigue is a common cause of collisions. It is a known hazard among professional drivers, such as truck drivers and taxi operators. Even a low level of drowsiness can dramatically slow reaction times and impair judgment.
Distraction is the leading cause of automobile accidents, according to Mark Edwards of the American Automobile Association. Texting, phone calls, side conversations and in-car activities unrelated to operating the vehicle are all factors in distracted driving.