Drug and alcohol tests vary - testing is required for most driver's license applicants and before a driver's license renewal. Nevertheless, it depends on the state a driver resides in really.

Some states have passed laws that empower the police to randomly test drivers for the presence of cannabis (tetrahydrocannabinol), commonly known as marijuana, and methamphetamines, commonly known as speed ice or crystal meth. Even a driver's saliva can be tested with an "absorbent collector" - whereby an individual presses their tongue to a "collector" and then chews on it. If a driver tests positive for any of these drugs, their license is immediately cancelled.

"Drug driving" is the term used for those who operate a motor vehicle while under the influence of drugs and/or alcohol. There are different consequences for those guilty of "drug driving" that depend on precisely what substances were in their system at the time.

Also, "drugs" has a different connotation to authorities than they would to let's say, a pharmacist. When a police officer thinks of drugs, he means depressants like cannabis, methadone and heroin, stimulants like speed, cocaine and ecstasy, and hallucinogens such as LSD. Even so, there are some prescription drugs that affect driving, like tranquilizing drugs (rohypnol and oxazepam), which cause drowsiness and render a person unfit to operate a vehicle.

Surveys show that the number of road accidents is directly related to the amount of drivers under the influence of illegal drugs and alcohol on the road. And sadly, a large number of those who are involved in accidents due to "drug driving," die. Some studies show that because the prevalence of different drugs has increased over the decade, so has the number of car accident casualties.

While it is easy to correlate drug use with motorist deaths, there is not a cause-effect relationship that can be proven as of yet but it is still interesting to note. And clearly, a diver who is free of drugs and alcohol is a safer driver than a driver under the influence. Legislators have the responsibility of carefully designing laws that limit "drug driving" and ultimately, the number of dangerous and fatal car accidents.

A person's capacity to handle the effects of drugs and alcohol certainly play a role in determining the degree of impairment they will experience while on the road. But this is very difficult to determine, as a person's tolerance of various substances is based on many different factors, like genes, past exposure, and even their diet that day. Likewise, the specific combination of drugs and alcohol can have unpredictable effects on a person. Essentially, it's all a precariously inexact science.

Hence, police officers must make general observations in order to determine if a driver is a potential threat on the road. Those abusing depressants tend to have slow reactions and reduced concentration - for example, cannabis users usually have trouble keeping their car in the correct lane and encounter difficulty maneuvering on busy roads and intersections.