The main offences are operating a motor vehicle while the ability to do so is impaired by alcohol or a drug, contrary to section 253(a) of the Criminal Code, and operating a motor vehicle while having a blood-alcohol concentration of greater than 80 milligrams of alcohol in 100 millilitres of blood, contrary to section 253(b) of the Criminal Code.

The offences are usually investigated by the police coming across a driver with either an erratic driving pattern or who has been pulled over. The police make a demand that the driver give a sample of his breath into an approved screening device, which will determine the driver's blood-alcohol concentration on a preliminary, non-evidentiary basis. If the police believe on reasonable and probable grounds athat the driver is commiting an offence under section 253 of the Criminal Code, the police can demand that the driver go to the police station to give samples of his breath for an approved instrument test, which would be used to prosecute the driver.