Abstract
It is an offence to drive a car whilst impaired due to use of a psychoactive substance. The incidence of illegal drug use has escalated in recent years, and this escalation is also represented in the driving population. At present there is no objective tool which can be used at the roadside to assess the drug induced performance impairment of drivers suspected of illegal drug intoxication. A literature analysis highlighted that empirical investigation into the cognitive and psychomotor effects of illegal drug use on aspects of performance related to car driving was scarce. By drawing together empirical and epidemiological evidence it is possible to conclude that driver impairment, as a consequence of illegal drug use poses a significant threat to traffic safety. Psychopharmacology is based on the premise that drugs affect behaviour, and that this change in behaviour is measurable. Behaviour is assessed through the use of repeated psychometric assessment in the laboratory. The aim of this thesis was to demonstrate that by using a portable device it was possible to reliably assess drug induced performance impairment in field conditions, with just one set of assessments. The portable psychometric test battery was demonstrated to be a valid tool in discerning the performance decrement induced by an impairing dose of alcohol. The test battery also proved efficacious in assessing the cognitive and psychomotor performance of drugged subjects in field conditions. Using statistical modelling it was demonstrated that it was possible to make retrospective judgments regarding an individual's drug use based solely on their performance on the psychometric test battery. The field impairment test is a subjective assessment tool in which police officers assess a suspect's performance on certain physical tasks. Based on performance officers are required to make personal judgments regarding the individual's impairment. The portable psychometric battery was shown to be more reliable than the subjective measure currently endorsed by the Police Force.