Canterbury expert appeals for caution in the holiday road toll debate

January 22, 2015

A University of Canterbury expert has appealed for caution in finding answers to the jump in the holiday road toll.

Seventeen people died on the roads during the official Christmas holiday period, more than double those of the last two years. Transport engineering professor Alan Nicholson says commentators must be extremely careful when interpreting fatal collision counts for holiday periods.

Collision counts are inherently random and unpredictable, especially for fatal collisions during short periods, Professor Nicholson says.

The University of Canterbury’s Department of Civil and Natural Resources Engineering has been teaching a specialist course on road safety for over 20 years, as part of its postgraduate transportation engineering programme.

The course covers the analysis of road collision data, to identify the factors involved in collisions, and the interpretation of collision counts. It also covers the identification and selection of options for improving road safety. During the last three years, 12 research theses or reports on road safety topics have been completed by transport engineering postgraduate students.

“Due to the number of factors involved in collisions and the randomness of fatal collision counts, a thorough analysis should be done before jumping to conclusions regarding the cause of the recent holiday road toll. The police should involve people with the appropriate expertise in that analysis,” Professor Nicholson says.

“There are many factors involved in road collisions. Those factors are commonly associated with the user (such as driver tiredness), the vehicle (such as faulty brakes) or the road, including the road alignment and the activities next to the road.

“When a collision occurs, it is frequently difficult to clearly identify which factors are involved and how they contributed. Expert knowledge about a wide range of factors is frequently required to identify the role of any particular factor.

“Speed has been the focus of much attention recently, and there is a high level of agreement among experts that speed is a major factor, with both the probability and severity of a collision increasing as vehicle speed increases. However, other factors need to be considered.

“Some people argue that their perception-reaction time is lower when driving fast, because they are more alert. However, research has shown that the decrease in perception-reaction time is insufficient to outweigh the increase in the stopping distance, so the probability and severity of a collision increases.

“Each driver decides on their speed, whether to overtake or whether to cut a corner, based on their perception of the probability and consequence of a collision. Reducing speed limits will not always result in reduced speeds, and it has been observed to result in higher speeds in some circumstances.

“A thorough review of the tolerance for driving faster than the speed limit should consider how the probability and severity of a collision are affected. It should also take account of road design issues.

“Rather than penalising just those drivers who might briefly exceed the speed limit at particular points on the road, consideration should be given to penalising drivers who persistently exceed the speed limit. This has been done in the United Kingdom for over a decade, by measuring average speeds over a long length of road.

“It is important to note that in 2013, New Zealand had only about six fatal collisions per billion kilometres of vehicle travel, so the probability of a collision during a particular trip is very low. This means that many drivers feel relatively safe on the roads and this affects their driving behaviour,” Professor Nicholson says.

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