The present study forms part of a larger, EU-wide investigation into the impact of drugs, medications and medical conditions on road safety. This research programme, known as the IMMORTAL project (Impaired Motorists, Methods of Roadside Testing and Assessment for Licensing) investigated the accident and/or injury risk associated with different types of driver impairment and examined the implications for licensing assessment and roadside impairment testing (including drug screening).
Eight drug groups were included in the Tilburg study: benzodiazepines, tricyclic antidepressants, methadone, opiates, amphetamines, cannabis, cocaine and alcohol. The methodology used was that of a case-control study, where the prevalence of the substances among injured drivers (a hospital sample) was compared with the prevalence in the general driving population (a random roadside sample), and risk ratios were calculated. Data on substance use by seriously injured drivers (in-patients) was collected in the St. Elisabeth Hospital in the city of Tilburg.
Data on substance use by the general driving population was collected in the Tilburg police district, which is the hospital's catchment area. A random roadside survey was conducted in close cooperation with the Tilburg police force. Among the general driving population, cannabis, benzodiazepines and alcohol were the prevailing substances. Out of the 3,799 stopped and tested drivers:
- 4.5% were positive for cannabis; 3.9% for cannabis only and 0.6% for cannabis in combination with other drugs and/or alcohol;
- 2.1% were positive for benzodiazepines; 2.0% for benzodiazepines only and 0.1% for benzodiazepines in combination with other drugs and/or alcohol;
- 2.1% were positive for alcohol (BAC 0.2 g/l); 1.8% for alcohol only and 0.3% for alcohol in combination with other psychoactive substances.
Drugs of abuse were strongly concentrated in male drivers aged 18-24. No less than 17.5% were positive for illegal drugs. Psychoactive prescription drugs were strongly concentrated in female drivers aged 50 and older: 11.3% were positive.
Comparison of the road and hospital samples showed that approx. 35% of serious injuries among drivers in the Tilburg police district were associated with self-administered alcohol and/or illegal drugs, and especially with drug-free BAC-levels 1.3 g/l, with drug/alcohol combinations at BAC-levels 0.8 g/l, and with drug/drug combinations. These three categories accounted for 12.7%, 8.3% and 7.2%, respectively, of the 184 seriously injured drivers included in the hospital sample. The corresponding odds ratios were 87, 179 and 24, respectively. In order to be effective, road safety policy should target these three categories with priority.
Considering the fact that in part of the alcohol and/or drug-related serious-injury crashes a sober driver was seriously injured, it can be assumed that alcohol and/or illegal drug use accounted for even more than 35% of serious road injuries in the Tilburg police district.
In addition to the case-control study, and combined with the roadside survey, an observational method of detecting drug impaired drivers was tested. The method consisted of a checklist of signs of impairment supplemented with two questions about recent drug use. Specificity (98.6%) and negative predictive value (95.8%) of the method were satisfactory, whereas sensitivity (61.1%) and positive predictive value (82.9%) were rather low. It is recommended to try and improve the method in future trials, since large-scale random analytical drug-screening at the roadside will probably not be feasible in the years to come. This is due partly to the high cost of the screening devices and partly to the time-consuming procedure.