Fatal single vehicle crashes study: Summary report

Monash University Accident Research Centre - Report #122

Authors: N. Haworth, P. Vulcan, L. Bowland & N. Pronk

Full report in .pdf format [180KB]

Abstract:

This report summarises the findings of the Case-control study of fatal single-vehicle crashes. The cases in the study were fatal single-vehicle crashes (or crash trips) which occurred during the period from 1 December 1995 to 30 November 1996 within 200 km of Melbourne. The cases have location, driver/rider and vehicle characteristics. The controls were (non-crash) trips which also have location, driver/rider and vehicle characteristics.

The most important risk factors, in terms of magnitude of the odds ratio and fraction of the crashes involved were: alcohol and cannabis, younger drivers, and older drivers. The major factors contributing to the severity of fatal single vehicle crashes were: trees and poles, not wearing seat belts and pre-1978 vehicles.

A number of possible improvements to procedures for the investigation of serious injury crashes are recommended.

Executive Summary

This report summarises the findings of the Case-control study of fatal single-vehicle crashes. The cases in the study were fatal single-vehicle crashes (or crash trips) which occurred during the period from 1 December 1995 to 30 November 1996 within 200 km of Melbourne. The controls were (non-crash) trips during the same period.

The characteristics of the cases are described in the report, Characteristics of fatal single vehicle crashes. The comparisons of cases and controls to derive relative risk estimates are presented in the companion report entitled Estimation of risk factors for fatal single-vehicle crashes.

The aims of the Case-control study of fatal single-vehicle crashes study were to:

1. investigate single vehicle crashes to determine the circumstances and factors contributing to them
2. estimate the over-involvement (relative risk) of these factors
3. identify improvements in procedures for the investigation of road deaths and life threatening injuries
4. provide information from which countermeasures can be developed to the agencies responsible for road safety in Victoria

This report summarises work on the first three aims. A report has been provided to the funding bodies which deals with Aim 4.

SUMMARY OF FINDINGS

Driver age and experience

Drivers aged under 25 and those aged 60 and over were at higher risk of being involved in a fatal single vehicle crash than drivers aged 25 to 59. Among these groups the risks were greatest for drivers aged under 21 and 70 and over.

Compared with full licence holders, learner permit and probationary licence holders were at higher risk. Having driven for less than three years and having driven the current vehicle for less than 10,000 km were also associated with increased risk of being involved in a fatal single vehicle crash.

Among drivers with at least five years experience, those who had been involved in a previous crash in the last five years were at greater risk of crashing.

Alcohol and cannabis

Driver BAC levels exceeded .05 in 36% of crashes for which this was known, including 27% of crashes in which BAC exceeded .15. BAC>.05 was associated with significantly increased crash risk. The lack of control drivers with high BAC levels prevented estimation of relative risk for BAC>.15.

Alcohol or cannabis was present in some cases and some controls. Among the cases (but not the controls), cannabis was most commonly present at high levels of alcohol and very rarely present when there was no alcohol. The lack of alcohol and cannabis together in any controls meant that statistical tests of the effects of the interaction of the two drugs could not be performed. There is a need to focus further research and investigation on the combined effects of alcohol and cannabis.

Differences in data collection methods for cases and controls may have inflated the risk estimate for cannabis. The analysis showed that the presence of cannabis increased the risk of crashing when BAC<=.05. When all BAC data was combined, the presence of cannabis was associated with an increased risk of crashing but it is unclear the extent to which this resulted from an effect of cannabis alone or from an interaction with alcohol.

Passengers

Among the drivers who had passengers, having only male passengers rather than female or male and female passengers was associated with higher crash risk. Having passengers aged 15-24 was also identified as a risk factor.

Vehicle-related factors

Not wearing a seatbelt was associated with significantly increased risk, as was driving a vehicle manufactured before 1978. Driving someone else’s car (not the employer’s) was also associated with increased crash risk.

Trip factors and activities in the previous 24 hours

Unfamiliarity with the road and reason for the trip did not affect crash risk after adjusting for potentially confounding variables.

Assessment of the crash risk related to excessive speed was restricted by the unavailability of speed data for more than half of the crashes. It is recommended that a future project involve estimation of pre-collision speeds from the scale plans available for most of the crashes followed by a reanalysis of the speed data.

Drivers who had slept for more than ten hours in the previous 24 hours were found to be at higher risk of being involved in a fatal single vehicle crash.

Site factors

The analyses of the site variables were complicated by a number of spurious relationships and the interrelationships of the site variables. In addition, the data set included only 127 crash sites and 100 control sites which meant that an effect would have to be large before it could be detected. Taking into consideration the problems of insufficient statistical power, the following conclusions can be drawn.

The risk of occurrence of a single vehicle crash at a site was found to increase by 3% with every extra 1000 vehicles per day. Relative risk was also higher at sites on curves. The reductions in risk observed with the presence of side drains and traffic controls may have resulted from biases in data collection procedures.

Trees and poles were present in about three-quarters of the crashes. However, the difficulties in identification of control objects prevented the relative risks for trees and poles from being calculated.

While the inability of this study to identify road factors which contribute strongly to the risk of occurrence of fatal single vehicle crashes is somewhat disappointing, it is a similar finding to that of earlier studies.

SUMMARY OF FINDINGS

The adjusted odds ratios for the significant risk factors are summarised in the tables which follow.

Table 1. Matrix of risk factors and their magnitude.
Within each category, risk factors are ranked in order of the size of the odds ratio.

1 Considerable missing data for cases
2 Percent in drivers with BAC<=.05
3 Percent in drivers without cannabis

Table 2. Matrix of risk factors for which countermeasures are less likely.
Within each category, risk factors are ranked in order of the size of the odds ratio.

POSSIBLE IMPROVEMENTS IN INVESTIGATION PROCEDURES

In order to improve the availability of information about severe crashes, it was recommended that:

1. The availability of toxicological information for drivers who may be charged be investigated and, if necessary, a procedure be developed to incorporate the BAC data into the State Traffic Accident Record at an appropriate time (perhaps after completion of criminal procedures).
2. Given the lack of information on drugs in nonfatal crashes, perhaps testing of all (or a given proportion) of blood samples for drugs should be undertaken for a specified period to increase our knowledge in this area.
3. Blood samples of all drivers in fatal crashes (whether or not injured) be taken and analysed for both alcohol and other drugs. An alternative would be to have blood samples taken and analysed whenever the breath shows alcohol (building on the strong relationship between alcohol and cannabis).
4. The family’s right to object to autopsy be maintained but that the authorities have the right to take and analyse a blood sample from the deceased.
5. Testing for the active ingredient, rather than the metabolite, of cannabis be undertaken, at least for fatal crashes, in the short term. Improvements in technology may reduce costs etc and allow this to be extended to nonfatal crashes in the future.
6. Vehicle Identification Numbers (VINs) be recorded to allow better identification of makes and models of vehicles and judgements of the presence of safety features. Currently, VINs could be added to the State Traffic Accident Record routinely by interrogating the registration data, using the vehicle registration number. In the future, automatic capture of VINs at the crash site may replace this method.
7. In addition, for the next couple of years, it would be desirable to record deployment of airbags.
8. It is probably preferable for items regarding the state of the vehicle immediately prior to the crash (e.g. heating and ventilation) to be collected in special studies, rather than to burden the Police with additional workload for every reported crash.
9. While resource limitations are an important issue which needs to be considered, it may be useful from the point of view of training Accident Investigation Section staff and developing a high quality knowledge base for prevention, to have a special focus on particular crash types (or perhaps even locations) for a particular period.

Sponsoring Organisation: Baseline Research Program - Department of Justice, Transport Accident Commission, Royal Automobile Club of Victoria (RACV) Ltd, VicRoads