CHAPTER 1 RAISON D'ETRE AND OVERVIEW

Every psychoactive drug exerts its action within the central nervous system (cNs). Psychoactive drugs taken for medicinal purposes not only produce desired therapeutic effects but also unwanted side effects, such as sedation and memory impairment, due to the complex interaction of numerous neurotransmitter systems within the brain and the drugs' lack of selectivity for one apart from the others. It is in the interest of road safety to determine which drugs seriously interfere with brain functions to identify those that are incompatible with safe driving performance. It is estimated that about 10% of the more than 6500 registered medicinal drugs in the Netherlands have the potential to impair driving performance (Weseman et al., 1989).

Alcohol is the cardinal example of a licit, nonmedicinal psychoactive drug that adversely affects driving performance and causes many fatal road crashes. The relationship between dose and effect is unequivocal: rising blood concentrations produce exponential deterioration in experimentally measured driving performance (Louwerens et al., 1987) and the risk of causing a fatal traffic accident as determined by epidemiological surveys (Borkenstein et al., 1974) . Therefore, nearly all countries have adopted a legal limit of the blood alcohol concentration (BAG) for driving-under-the-influence prosecution.

Cannabis (e.g. marijuana and hashish) is the best-known example of an illicit, nonmedicinal psychoactive drug. Since the 1960s, when cannabis smoking became commonplace in Western societies, much research has been executed to determine cannabis' adverse and also beneficial effects on safety, health, personality, behavior, etc. This drug's effects on driving performance have also been investigated, both by epidemiological and by experimental studies. Both had their limitations as will be described in Chapter 3. Until now, epidemiological studies failed to show a reliable relationship between cannabis use and risk of a motor vehicle accident. These failures are mainly due to the absence of proper control groups and the high incidence of combined alcohol and cannabis use among drivers responsible for accidents. Experimental studies on cannabis and driving performance have mainly been executed in driving simulators and on closed circuits. Only one study reported previously in the literature measured the effects of marijuana smoking on actual driving performance (Klonoff, 1974). The results of these studies are not as unequivocal as those involving alcohol. Nonetheless, they generally failed to show drastic driving impairments after cannabis smoking.

The question if cannabis adversely affects driving performance is obviously highly relevant but the answer is subject to controversy. Some hold that cannabis is a relatively minor factor in traffic accidents (e.g. Mason and McBay, 1984; Gieringer, 1988) or that the magnitude of accident risk following cannabis smoking remains obscure (e.g. Peck et al., 1986). Others, including scientists and politicians, remain convinced that cannabis is a serious public safety hazard (e.g. Milner, 1977; Moskowitz, 1985). Their opinion is mainly based upon results from laboratory experiments showing adverse effects of the drug on cognitive and psychomotor performance, but also on anecdotal reports of accidents. Given the societal relevance of the issue and the persistence of the controversy, it was decided to investigate cannabis' effects on driving performance in the most valid ecologically environment that is available, i.e. in actual traffic on the public road.

This dissertation describes the results of one laboratory and three actual driving studies. In all driving studies, subjects operated a specially instrumented vehicle on a public road after smoking marijuana. In the first study, they drove on a primary highway closed for other traffic; in the second, on a primary highway in the presence of other traffic; and, in the third, through the city of Maastricht, again in the presence of other traffic. Together, these studies should provide an answer to the questions addressed by this dissertation: does marijuana smoking impair driving performance, and if so, to what magnitude in comparison with other psychoactive drugs.

Chapter 2 provides a general overview of cannabis. It starts with the history of cannabis use and goes on to describe its current usage prevalence. Then a short description of the plant and its preparations is given, followed by a more extensive part on the drug's chemistry and pharmacology. The concluding part of the chapter is dedicated to an overview of the acute and chronic effects of cannabis. After this general introduction, Chapter 3 concentrates on the literature pertinent to the effects of cannabis on driving. Both epidemiological surveys of users' involvement in traffic accidents and empirical studies measuring the drug's influence on skills related to driving are reviewed. This provides the justification for the experimental research reported in the dissertation. Chapter 4 describes methods that were common to all studies. These are successively reported in Chapters 5-8. The dissertation concludes in Chapter 9 with a general discussion, conclusions from the studies' results, and recommendations concerning further research requirements.