Effects of alcohol on driving
Ethanol is the most commonly encountered alcohol in terms of DUI. Consumption of ethanol is legal in many jurisdictions where it is generally considered socially acceptable if consumed in moderation. Other alcohols are occasionally encountered in terms of DUI. Methanol is a highly toxic substance. It is not generally consumed knowingly but may be consumed by accident, particularly by persons desperate to consume alcohol who are unaware of the toxicity of methanol. Other alcohols such as isopropanol (propan-2-ol) and ethylene glycol can cause impairment but, as for methanol, consumption of these substances is uncommon. Alcohol is readily absorbed through the stomach and, in particular, through the small intestine, with virtually 100% of an oral dose is absorbed in healthy persons. It transfers readily to the bloodstream owing to its high-water solubility and is then distributed rapidly throughout the body, portioning into all tissues, particularly those with a high-water content. Alcohol is metabolised (Fig. 1A) in the liver, although some is eliminated unchanged. Although the major pathway for metabolism is oxidation via acetaldehyde to acetic acid, a small percentage of ethanol is conjugated to give ethyl glucuronide and ethyl sulfate (Fig. 1B). Being volatile, alcohol can diffuse into free air and hence can pass from the pulmonary system into the lungs where it is expelled in breath. This excretion of alcohol in breath makes breath alcohol testing possible. Food present in the stomach can slow the absorption of ethanol. Ethanol (hereafter referred to as alcohol) is a central nervous system (CNS) depressant. The precise mechanism by which alcohol affects cognition and psychomotor control is not understood completely, but the fact that it does so is very well documented, and is well under stood by most members of the public. To understand the effects of alcohol on driving, the complexity of the driving task itself must be appreciated. Driving is a complex process that requires the simultaneous integration of psychomotor tasks, hand–eye coordination, muscle control and cognitive tasks. Moreover, since driving is a divided-attention task, the driver does not have the luxury of focusing exclusively on any one of these components, but rather must attend to all appropriately, monitor a number of sensory inputs, and prioritise each task depending on changing circumstances. Drug or alcohol use can affect the execution of individual tasks, and their effects are therefore more pronounced on more complex tasks, of which driving is a textbook example.
Alcohol causes a slowing of nerve conduction, which results in slower reaction times, difficulty in processing and integrating information, and consequently diminished performance in divided-attention tests. Individuals are overtly affected differently at different blood alcohol concentrations (BACs). This results from a variety of factors, which include prior exposure to alcohol, degree of both acute and chronic alcohol tolerance, innate physical condition and any associated limitations. Sometimes the effects of alcohol are not obvious to a casual or lay observer and must be assessed by tasks that challenge those abilities most subject to alcohol effects. An individual can be affected by alcohol to the extent that it can interfere with driving before some of these overt signs of intoxication (e.g. staggering and slurred speech) appear. The BAC at which all individuals are affected in their driving by alcohol consumption is a continuing topic of discussion among forensic toxicologists, although most toxicologists agree that any individual with a BAC above 1.5 g/L would display overt signs of being affected on close inspection, even by a lay witness.
Figure 1 Metabolism of ethanol. (A) Showing the major metabolic pathway. (B) Showing the minor conjugation reactions with sulfate and glucuronic acid.
