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Sunday, 8 June 2014

Does a better car result in safer driving ?

It is often assumed that better cars result in safer driving and fewer accidents. Although this seems to make sense, the reality is that people adapt their behaviour to vehicle characteristics. For example, if a car accelerates better, the driver often accepts smaller safety margins. If the brake system of the car is better, drivers may follow a lead vehicle with a smaller headway.

A number of studies have revealed effects of vehicle characteristics on tactical driver behaviour. Tactical driver behaviour consists of choice of speed, headway, gap between vehicles at an intersection. In contrast, operational behaviour consists of steering and handling of the pedals (for example braking). Rumar et al. (1976) studied the effects of studded tires on speed choice in curves. Drivers with studded tires drove faster compared to drivers with unstudded tires in icy road conditions. This did not result in lower safety, since the ‘safety margin’, defined as the difference between real and critical lateral acceleration, was larger with studded tires. Summala and Merisalo (1980) also found that drivers with studded tires chose higher speeds in curves in low-friction conditions and that the safety margin was greater for drivers with studded tires in slippery conditions. The higher speeds with studded tires in low friction conditions may be regarded as an adaptation of tactical behaviour to the increased friction coefficient induced by studded tires.

Also, the acceleration capability of cars has been shown to affect behaviour. Evans and Herman (1976) found that drivers accepted smaller gaps with oncoming cars while negotiating intersections if the acceleration capability of the car was higher. However, the physical safety margin was not negatively affected by acceleration capability. Also, newer cars used higher levels of deceleration compared to older cars when they stopped at signalized intersections (Evans and Rothery, 1976). This was explained as a possible adaptation of behaviour (on the tactical level) to compensate for reduced mechanical conditions in older vehicles.

Evans and Wasielewski (1983) found that drivers of newer cars and cars with intermediate mass followed with a smaller time-headway. This may also be the result of better deceleration capabilities of newer cars. Evans (1991) postulated that improved braking and vehicle handling characteristics result in increased speeds, closer following and higher speeds in curves.

When safety changes are invisible to the user, as may be the case with seat belts and increased crashworthiness, there is no evidence of any measurable human behaviour feedback. A similar point was made by Lund and O’Neill (1986). Design changes that reduce the likelihood of a crash do have an effect on behaviour. They stated that how a car is driven depends on feedback to the driver about the car’s handling characteristics. Vehicle-related factors may then affect both operational and tactical driver behaviour depending on the visibility of the feedback. This type of behavioural adaptation has been studied in a driving simulator.

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