| 2. INTRODUCTION |
| In this paper I will examine briefly the studies which have sought an
understanding of the effect of cannabis and of alcohol on driving skills and their role in
road crashes. This information has been based upon scientific data which have been
collected from several scientific disciplines. I have outlined these in earlier papers and
will only mention them briefly here. |
| The major purpose of this paper is to compare the two drugs, alcohol and
cannabis and the status of the evidence as to their role in road crashes. |
| The determination of the legal limit for alcohol has been achieved in a
scientific manner. There are pharmacological reasons why it has not been possible to
follow these same techniques with drugs other than alcohol, including cannabis. This paper
will draw attention to these problems. |
| First, we might briefly outline the nature of the evidence which has been
generated to examine the effects of cannabis on driving skills and as a causative factor
in road crashes. This information has been derived from the employment of three scientific
disciplines: |
| 2.1 Pharmacology and pharmacokinetics |
| Pharmacology is the study of the way a drug exerts its action in the body.
This involves an understanding of the sites and the body systems where the drug acts and
the consequences of this drug-system interaction. Information obtained from these studies
can help to formulate an hypothesis as to how the drug may influence driving behaviour. |
| The pharmacological discipline known as pharmacokinetics studies the fate
of the drug after it has been taken. It provides information as to the rate of absorption
from the site of administration; the manner of its distribution in the body up to the
delivery to its site of action (eg. the brain). Pharmacokinetics also studies the way the
body eliminates the drug from the body and includes the understanding of the metabolism
and excretion of the drug. |
| 2..2 Behavioural pharmacology and psychology |
| These involve studies of the effects of the drug on human behaviour. The
behaviour of relevance to this discussion concerns those skills which are (or are related
to) those necessary for the safe control of a motor vehicle or other items of machinery.
Psychological studies also involve the effects of the drug on mood and cognition. |
| The three classifications of these studies are: |
| (i) Those performed on specific tests of behaviour or psychological
functioning (for example, tests of reaction times of various degrees of complexity;
tracking; divided attention or vigilance); |
| (ii) Those performed in a driving simulator; and |
| (iii) Those performed in a real car, either in a closed course or in
real traffic. |
