The impaired driving laws in Canada are amongst the toughest in the world. These laws are based on the known effects of alcohol on driving ability and the statistics on alcohol-related crashes and fatalities. In varying degrees—depending on body size, food and non-alcoholic beverage consumption while also consuming alcohol, combination with medications whether prescriptive or over-the counter— alcohol affects the ability to see, to react and to reason. Alcohol reduces the brain’s ability to process optical information. It’s as if the eyes can’t take in what they are seeing as quickly as they normally do. Glare can be blinding. Focus is reduced to the point that images double-up. Depth perception is reduced so that estimating the distance and speed of other vehicles is difficult if not impossible. Side vision is reduced so that hazards not approaching head-on may not register at all.           

 

Alcohol interferes with reasoning, alertness and the ability to control emotions. The ability to think logically, to focus and to concentrate is diminished in direct proportion to the amounts consumed. Too much alcohol consumption often leads to the “I’m-perfectly-fine” wrong conclusion.

 

Excessive alcohol consumption reduces muscle control, which reduces coordination and slows reaction time. It’s harder to steer, to brake properly and at the right time, and to identify and react to any potential road hazards. Many crashes are caused by alcohol consumption and excessive speed.     

 

Alcohol consumption and drug use are a particularly toxic combination for drivers. On their own, quite apart from when they are mixed with alcohol—most should never be—

many common prescription medicines and over-the-counter drugs have a significant negative impact on driving ability. The effect of these drugs on driving ability will be described in a future article.

 

Alcohol combined with marijuana is another increasingly common but toxic combination for drivers. Marijuana contains a psychoactive chemical called tetrahydrocannabinol or THC, some effects of which are very different from the effect of alcohol. Pot users, for example are more likely to be aware of their impairment and to compensate for it by driving more cautiously. But, at the same time, THC diminishes psychomotor skills and attention span, reduces the ability to perform tracking tasks and to steer properly. Add alcohol and cognitive functioning, including risk perception, decision-making, and planning is impaired. Increased aggressiveness caused by alcohol consumption may override THC’s counter-effects.

 

Under the Criminal Code of Canada, a person who operates a motor vehicle while his or her ability to do so is impaired by alcohol or a drug commits an offence. For alcohol the law defines a measure of concentration in a driver’s blood—in excess of 80 milligrams in 100 millilitres, or .08—as a criminal offence. This is based on statistical projections that the likelihood of unsafe driving by a driver with this amount of alcohol in his or her bloodstream is substantially increased. In most Canadian provinces an even lower measure of blood alcohol concentration, .05, results in penalties under provincial administrative legislation. No such measures under criminal or provincial administrative legislation have yet been set for drug impairment.

 

Safe driving requires skill and unwavering attention. Alcohol and drug consumption impair the ability to exercise these motor and mental skills. Drivers who are impaired put themselves and others at risk. Our laws seek to control this behaviour because its prevalence (thankfully declining) and danger to others causes so much harm.