Road traffic safety refers to the methods and measures used to prevent road users from being killed or seriously injured. Typical road users include. For example, the chances of survival for an unprotected pedestrian hit by a vehicle diminish rapidly at speeds greater than 30 km/h, whereas for a properly restrained motor vehicle occupant the critical impact speed is 50 km/h and 70 km/h. The basic strategy of a Safe System approach is to ensure that in the event of a crash, the impact energies remain below the threshold likely to produce either death or serious injury. This threshold will vary from crash scenario to crash scenario, depending upon the level of protection offered to the road users involved. At the highest level is sustainable prevention of serious injury and death crashes, with sustainable requiring all key result areas to be considered.
At the second level is real time risk reduction, which involves providing users at severe risk with a specific warning to enable them to take mitigating action. However, crash reconstruction techniques can estimate vehicle speeds before a crash. Therefore, the change in speed is used as a surrogate for acceleration. The third level is about reducing the crash risk which involves applying the road design standards and guidelines, improving driver behavior and enforcement. It was not until the 1960s that safety testing ensured adequate protection, and even then only for vehicles of a limited weight class. As sustainable solutions for all classes of road safety have not been identified, particularly low-traffic rural and remote roads, a hierarchy of control should be applied, similar to classifications used to improve occupational safety and health.
Guardrails save a vehicle from a long fall c. 1920; but so said the original caption, guard rails were only sometimes effective at the time. The report also noted that the problem was most severe in developing countries and that simple prevention measures could halve the number of deaths. Road traffic crashes are one of the world's largest public health and injury prevention problems. The problem is all the more acute because the victims are overwhelmingly healthy before their crashes. According to the World Health Organization, more than 1 million people are killed on the world's roads each year. The standard measures used in assessing road safety interventions are fatalities and killed or seriously injured (KSI) rates, usually per billion (109) passenger kilometres. A report published by the WHO in 2004 estimated that some 1.2 million people were killed and 50 million injured in traffic collisions on the roads around the world each year and was the leading cause of death among children 10–19 years of age.
Countries caught in the old road safety paradigm, replace KSI rates with crash rates – for example, crashes per million vehicle miles. This enabled the Swedish Road Administration to identify the KSI risk curves using actual crash reconstruction data which led to the human tolerances for serious injury and death referenced above. Vehicle speed within the human tolerances for avoiding serious injury and death is a key goal of modern road design because impact speed affects the severity of injury to both occupants and pedestrians.