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Road safety

Methods and measures for reducing the risk of death and injury on roads

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This picture shows a street corner on Broadway near downtown Hagerstown. As typical for rural areas in the United States the lights are rather trivially mounted here on the opposite of the crossroads
Original uploader was JoSePh at en.wikipedia · Public domain

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Best practices in modern road safety strategy:

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. 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 (for side impact crashes) and 70 km/h (for head-on crashes).

— International Transport Forum, Towards Zero, Ambitious Road Safety Targets and the Safe System Approach, Executive Summary page 19

As sustainable solutions for 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. 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. The third level is about reducing the crash risk which involves applying the road-design standards and guidelines (such as from AASHTO), improving driver behavior and enforcement. Drivers' traffic behaviors are significantly influenced by their perceptions and attitudes.

Traffic safety has been studied as a science for more than 75 years.

Background

Traffic collisions have become one of the world's largest public-health and injury-prevention problems. The issue is all the more acute because the victims are overwhelmingly healthy before their crashes. According to the World Health Organization (WHO), more than 1 million people are killed on the world's roads each year. 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 that traffic accidents were the leading cause of death among children 10–19 years of age. The report also noted that the problem was most severe in developing countries and that simple prevention-measures could halve the number of deaths.

The standard measures used in assessing road safety interventions are fatalities and killed-or-seriously-injured (KSI) rates, usually expressed per billion (109) passenger kilometres. Countries using older road-safety paradigms replace KSI rates with crash rates – for example, crashes per million vehicle-miles.

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 both to vehicle occupants and to pedestrians. For occupants, Joksch (1993) found the probability of death for drivers in multi-vehicle collisions increased as the fourth power of impact speed (often referred to by the mathematical term δv ("delta V"), meaning change in velocity). Injuries are caused by sudden, severe acceleration (or deceleration); this is difficult to measure. However, crash reconstruction techniques can estimate vehicle speeds before a crash. Therefore, the change in speed is used as a surrogate for acceleration. 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.

Interventions are generally much easier to identify in the modern road-safety paradigm, which focusses on the human tolerances for serious injury and death. For example, the elimination of head-on KSI crashes simply required the installation of an appropriate median crash-barrier. Also, roundabouts, often with speed-reducing approaches, feature very few KSI crashes.

The old road-safety paradigm of purely crash risk is a far more complex matter. Contributing factors to highway crashes may be related to the driver (such as driver error, illness, or fatigue), the vehicle (brake, steering, or throttle failures), or the road itself (lack of sight distance, poor roadside clear- zones, etc.). Interventions may seek to reduce or compensate for these factors, or to reduce the severity of crashes. A comprehensive outline of interventions areas can be seen in management systems for road safety. Study conducted in Finland revealed that the fatality risk is increased most when a collision type is either pedestrian or meeting of the vehicles.

In addition to management systems, which apply predominantly to networks in built-up areas, another class of interventions relates to the design of roadway networks for new districts. Such interventions explore the configurations of a network that will inherently reduce the probability of collisions.

Interventions for the prevention of road traffic injuries are often evaluated; the Cochrane Library has published a wide variety of reviews of interventions for the prevention of road-traffic injuries.

For the purposes of road-traffic safety it can be helpful to classify roads into three usages:

  • built-up urban streets with slower speeds, greater densities, and more diversity among road users
  • non built-up rural roads with higher speeds
  • major highways (motorways/ Interstates/ freeways/ Autobahns, autostrade, etc.) reserved for motor-vehicles, and which are often designed to minimize and attenuate crashes

Most injuries occur on urban streets but most fatalities on rural roads, while motorways are the safest in relation to distance traveled. For example, in 2013, German autobahns carried 31% of motorized road traffic (in travel-kilometres) while accounting for 13% of Germany's traffic deaths. The autobahn fatality-rate of 1.9 deaths per billion-travel-kilometres compared favorably with the 4.7 rate on urban streets and 6.6 rate on rural roads.

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