A charge based on this type of offense is when the person who is in care and control of a motor vehicle, boat, or aircraft has their ability to operate the vehicle compromised by alcohol or drugs.
This type of charge is usually made through observation of how a person is driving a vehicle. If they are driving erratically and different then normal a police officer has the right to pull them over. Other methods police use to determine this type of offense are: the smell of alcohol on the driver and the appearance of the driver (example: red eyes, instability, etc). At the trial, the judge will review all the evidence to determine whether the charge can be justified beyond a reasonable doubt.
In Canada it is an offense to operaate a vehicle with a blood alcohol level of more then 80 milligrams of alcohol per 100 milliliters of blood (0.08%). This is also known as driving "over 80". With reasonable cause a police officer can take a breath sample to prove that you are over the legal limit of 0.08%. Even if the person feels fine or sober they can still be charged if the reading shows above 0.08%.
A person can be charged with impaired driving if they refuse to provide a breath or blood sample to a police officer when they request it. It is against the law to deny a blood alcohol test from a police officer. This type of test includes a roadside sample or a sample into a breathalyzer machine at the police station. This type of offense is considered as serious as driving "over 80" and should be dealt with in the same type of manner.