Travelling Speed and the Risk of Crash Involvement

EXECUTIVE SUMMARY

The main aim of this project was to quantify the relationship between free travelling speed and the risk of involvement in a casualty crash, for sober drivers of cars in 60 km/h speed limit zones in the Adelaide metropolitan area.

The secondary aims of the project were to examine the effect of hypothetical speed reductions on the crashes in this study and to explore the relationship between travelling speed and driver blood alcohol concentration.

Using a case-control study design, the speeds of cars involved in casualty crashes were compared with the speeds of cars 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 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 car travelling at a free speed in a 60 km/h zone becoming involved in a casualty crash compared to the risk for a car travelling at 60 km/h. Data collection was concentrated during the hours of 9:30am-4:30pm, Monday to Friday as these times had the highest number of non-alcohol-related crashes in Adelaide. Some cases were also collected at nights and on weekends.

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 was obtained by calculating what the results of the crash would have been if the case vehicle had been travelling at a different speed.

A separate study was set up to measure the relationship between blood alcohol concentration and travelling speed. The speed of an approaching car was measured 200-300 metres before a signalised intersection using a laser speed meter. When the car stopped at this intersection for a red light, the driver was approached and asked to blow into a breath alcohol meter.

Results

Cars involved in casualty crashes were generally travelling faster than cars that were not involved in a crash: 68 per cent of casualty crash involved cars were exceeding 60 km/h compared to 42 per cent of those not involved in a crash. The difference was even greater at higher speeds: 14 per cent of casualty crash involved cars were travelling faster than 80 km/h in a 60 km/h speed zone compared to less than 1 per cent of those not involved in a crash.

None of the travelling speeds below 60 km/h was shown to be associated with a risk of involvement in a casualty crash that was statistically significantly different from the risk at 60 km/h. Above 60 km/h there is an exponential increase in risk of involvement in a casualty crash with increasing travelling speed such that the risk approximately doubles with each 5 km/h increase in travelling speed.

By working back from the risk estimates we have concluded that nearly half (46 per cent) of these free travelling speed casualty crashes probably would have been avoided, or reduced to non-casualty crashes, if none of the case vehicles had been travelling above the speed limit. A more conservative estimate, based on calculation of stopping distances and impact speeds, indicates that 29 per cent of crashes would have been avoided altogether, with a reduction of 22 per cent in the impact energy of the remaining cases.

Using the second, more conservative, method we also estimate that a 10 km/h reduction in the travelling speeds of the crash involved cars in this study would probably have resulted in a reduction of at least 42 per cent in the number of crashes. A 5 km/h reduction showed much less effect but would still have resulted in a reduction of at least 15 per cent in the number of crashes.

Again using the conservative method, we estimate that an urban area speed limit of 50 km/h on all roads, with the present level of compliance, would be likely to result in a reduction of at least 33 per cent in the number of free travelling speed casualty crashes. However, a speed limit of 50 km/h in local streets, while having a significant effect on local street crashes, would be likely to have only a small effect on free travelling speed casualty crashes as a whole (a 6 per cent reduction) due mainly to the very small proportion (14 per cent) of these crashes which occurred on local streets.

The study of the relationship between free travelling speed and the driver's blood alcohol concentration (BAC) showed that higher BAC levels are associated with slightly higher travelling speeds although the average difference in speed is less than three kilometres per hour.

Discussion

We found that the risk of involvement in a casualty crash, relative to the risk for a car travelling at 60 km/h, increased at an exponential rate for free travelling speeds above 60 km/h. 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. A detailed description of each crash and the methods that we used to estimated the travelling speed of the case vehicles is presented in Volume 2.

Our results show that the risk of involvement in a casualty crash is twice as great at 65 km/h as it is at 60 km/h, and four times as great at 70 km/h. Increases in risk of such magnitude would appear to be sufficient to justify the reduction or elimination of the enforcement tolerance that currently applies to the enforcement of speed limits.

Although the risk of involvement in a casualty crash increases rapidly with increasing speed, the overall contribution of speeding to crash causation is still considerable at speeds below, say, 75 km/h because the majority of speeding drivers are travelling in the speed range from 61 to 74 km/h.

A large proportion of the crashes in this study would have been avoided had the case vehicles been travelling at a slower speed. We have shown that even modest reductions in travelling speeds can have the potential to greatly reduce crash and injury frequency. Large though these potential safety benefits are, it is probable they are still considerable underestimates. This is because we have only considered the effect of reduced travelling speed on the collision configuration that we actually observed and not taken into account possibilities for crash avoidance and the lower potential for injury at lower speeds.

It is instructive to compare the extent to which the risk of involvement in a casualty crash varies with a driver's blood alcohol concentration (BAC) and with travelling at a speed above the speed limit. We are able to do this because a case-control study of crash risk and BAC was conducted by the Road Accident Research Unit in Adelaide in 1979. Comparable case control studies on speed and alcohol have not been conducted in the same city anywhere else in the world. The results of these two studies indicate that if the blood alcohol concentration is multiplied by 100, and the resulting number is added to 60 km/h, the risk of involvement in a casualty crash associated with that free travelling speed is almost the same as the risk associated with the blood alcohol concentration. Hence, the risk is similar for 0.05 and 65; for 0.08 and 68; for .12 and 72, and so on.

Given that the relative risk of involvement in a casualty crash at 72 km/h is similar to that for a BAC of 0.12, it is more than a little incongruous that the penalty for the BAC offence is a $500-$900 fine and automatic licence disqualification for at least six months while the penalty for the speeding offence is only a $110 fine.

Conclusions and Recommendations

In a 60 km/h speed limit area, the risk of involvement in a casualty crash doubles with each 5 km/h increase in travelling speed above 60 km/h.

Speeding in an urban area is as dangerous as driving with an illegal blood alcohol concentration. Even travelling at 5 km/h above the 60 km/h limit increases the risk of crash involvement as much as driving with a blood alcohol concentration of 0.05.

In this study the free speed casualty crashes occurred almost entirely on main roads. There is a compelling case for a lower speed limit throughout urban areas, particularly on arterial roads. Most motorised countries have an urban area speed limit of 50 km/h, as did Victoria and NSW until the early 1960s.

We therefore recommend that:

  1. The tolerance allowed in the enforcement of the 60 km/h speed limit be reduced or removed.

  2. The level of enforcement of the 60 km/h speed limit be increased.

  3. The penalties for speeding and illegal drink driving be reviewed to align them more closely to the risk of being involved in a casualty crash.

  4. 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, similar to that which has developed in relation to compliance with blood alcohol limits during the past 15 years.

  5. To assist with the preceding recommendation, we also recommend that the results of this study be widely publicised, emphasising the risks associated with speeding in relation to the risks associated with illegal drink driving.

  6. After a period with stricter enforcement of the 60 km/h urban area speed limit, consideration be given to changing the urban area speed limit to 50 km/h on all roads, as in most other highly motorised countries.


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