CR 204: Travelling Speed and Risk of Crash Involvement on Rural Roads (2001)

The main aim of this project was to quantify the relationship between free travelling speed and the relative risk of involvement in a casualty crash, for sober drivers of passenger vehicles in rural out of town 80 km/h and above speed limit zones in South Australia.

The secondary aim of the project was to examine the effect of various hypothetical speed reductions on rural casualty crash frequency.

Using a case control study design and logistic regression modelling, the speeds of passenger vehicles involved in casualty crashes (the cases) were compared with the speeds of passenger vehicles not involved in crashes but travelling in the same direction, at the same location, time of day, day of week, and time of year (the controls). The conditions imposed on the selection of case vehicles were designed to ensure that the study would yield valid estimates of the relative risk of a passenger vehicle travelling at a free speed on a rural road becoming involved in a casualty crash compared to the risk for a passenger vehicle travelling at the average speed of the control vehicles.

The pre-crash travelling speeds of the case vehicles were determined using computer-aided accident reconstruction techniques. This was made possible by the detailed investigation of each crash at the scene which provided the physical evidence needed for input to the computer reconstruction program (M-SMAC).

Additional information about the effects of travelling speed on casualty crash involvement was obtained by calculating the expected reduction in rural crashes due to various hypothetical reductions in vehicle travelling speeds in rural areas.

We found that the risk of a free travelling speed passenger vehicle being involved in a casualty crash, relative to the risk for a passenger vehicle travelling at an average speed, increased at greater than an exponential rate. No evidence was found of a U-shaped risk curve whereby slower vehicles were also at greater risk. We are aware of a number of matters which could have affected the validity of the risk estimates and they are discussed in the report. However, we are not aware of any consistent bias which would be likely to invalidate the general relationship between free travelling speed and the risk of involvement in a casualty crash that we present in this report.

Our results show that the risk of involvement in a casualty crash is more than twice as great when travelling 10 km/h above the average speed of non-crash involved vehicles and nearly six times as great when travelling 20 km/h above that average speed. The mechanisms explored for this increase in risk (where higher speeds are associated with longer stopping distances, increased crash energy and more likely loss of control) also suggest that a reduction in the absolute speed of traffic is much more important in reducing crash frequency than a reduction in traffic speed differences.

In order to explore the possible effect of changing vehicle travelling speeds on rural casualty crash frequency the risk curve was applied to the crashes investigated in this study under a number of hypothetical scenarios. It was found that a large proportion of the casualty crashes attended in this study would have been avoided had the free travelling speed vehicles been travelling at a slower speed. It was shown that even small reductions in travelling speeds have the potential to greatly reduce crash and injury frequency. For example, it is estimated that even a 5 km/h reduction in the speed of all the rural free travelling speed vehicles in this study would have led to a 31 per cent reduction in casualty crashes. This percentage applies to the total sample of casualty crashes investigated, including those for which the hypothetical speed reduction was deemed to be irrelevant (for example, crashes where no vehicle had a free travelling speed). It was also estimated that 24 per cent of all the casualty crashes investigated would have been avoided if none of the vehicles had been travelling above the speed limit and that lowering the maximum speed limit on undivided roads to 80 km/h could be expected to lower casualty crash frequency by 32 per cent.

Conclusions and recommendations

In rural out of town areas, the risk of involvement in a casualty crash increases greater than exponentially with increasing free travel speed. Even travelling just 10 km/h faster than the average speed of other traffic was found to double the risk of crash involvement.

It was also found that small reductions in travelling speed in rural areas have the potential to greatly reduce casualty crashes in those areas; that illegal speeding is responsible for a significant proportion of rural crashes; and that reducing the maximum speed limit on undivided roads to 80 km/h could be expected to have a marked effect on casualty crash frequency.

We therefore recommend that:

  1. The level of enforcement of speed limits in rural areas be increased.
  2. The tolerance allowed in the enforcement of rural speed limits be reduced or eliminated.
  3. All currently zoned 110 km/h undivided roads be rezoned to no more than 100 km/h.
  4. Speed limits be reduced where current limits are considerably greater than average travelling speeds and where there are frequently occurring Advisory Speed signs.
  5. After a period with stricter enforcement of rural area speed limits, consideration be given to changing the maximum speed limit to 80 km/h on all two lane rural roads, as is the practice on two lane rural roads in many States in the USA.
  6. The level of public awareness of the risk of involvement in a casualty crash associated with speeding be increased with the aim of developing a culture of compliance with speed limits, and support for strict limits, similar to that which has developed in relation to compliance with blood alcohol limits during recent decades.
  7. To assist with the preceding recommendation, we also recommend that the results of this study be widely publicised.

Type: Research and Analysis Report

Sub Type: Consultant Report

Author(s): CN Kloeden, G Ponte & AJ McLean

Topics: Risk, Rural, Speed

Publication Date: 07/07/01