Identifying the drug- or alcohol-impaired driver
Before the drug- or alcohol-impaired driver can be subjected to chemical tests, he or she must be caught. Studies carried out in the USA by the National Highway Traffic Safety Administration have identified several characteristics which may indicate that a person is driving under the influence of alcohol (or drugs) (see Table 11.2). The final column in the table indicates the probability of the driver being impaired. Once a driver is stopped by the police, the officer looks for other evidence of intoxication, such as:
• smell – e.g. the smell of alcohol or burning cannabis
• bloodshot eyes
• unusual behaviour or responses by the driver to an officer’s questions (including slow or slurred speech and physical responses), inappropriate answers to simple questions
• flat, dull, or excited demeanour
• unusually large or small pupils, unusual eye movements
• impairment of fine motor skills (e.g. difficulties with removing documents from wallet, dropping documents).
The presence of alcohol containers or drug paraphernalia which could be readily accessed by the driver (e.g. on the passenger seat), while not evidence of drug or alcohol use per se, should not be overlooked. Generally, after noting the presence of some of these indicators or clues, the officer asks the subject to get out of the vehicle. This provides an opportunity to observe some gross motor skills. Table 11.3 documents clinical signs and symptoms and associated blood alcohol levels taken from a study by Dubowski (1994). The police may ask the subject to carry out some tasks which are designed to test psychomotor and cognitive abilities, and divided-attention skills. If drug influence is suspected, documenting the physiological parameters influenced by the use of these drugs (pulse, blood pressure, muscle tone, eye movement, pupil size and reaction to light, etc.) can assist in identifying the class or classes of drugs that might account for the observed impairment.
It is quite straightforward to identify a subject under the influence of a CNS depressant, since these signs are well documented for alcohol. Often noted are problems with fine or coarse motor control, staggering gait (ataxia), loss of balance, impairment in divided-attention tests, slurred speech and glassy and/or bloodshot eyes. Readily observable horizontal gaze nystagmus (HGN) is a consistent and common feature of CNS depressant intoxication (see Fig. 11.2). Subjects often have a sleepy or dazed appearance, have difficulty understanding questions or responding appropriately and may be disoriented to time and place. Driving behaviours commonly noted are a direct consequence of the CNS depressant effects on coordination and attention, such as weaving within or between lanes, failure to notice and obey traffic signals, failure to obey posted speed limits, wide turns, no use of turn signals and driving at night without lights.
