Roadside testing is part of a plan to reduce the number of drivers getting behind the wheel while under the influence of drugs - and new technology may help to combat this problem.
Government scientists have been working on a device that can detect a number of drugs in saliva, and it could be in use in two years' time.
Not only would it test for illegal drugs such as cocaine and amphetamine, but also for legal drugs that can impair driving, such as sleeping pills.
Currently, police officers can stop a motorist that they believe may be 'drug-driving' and ask them to carry out a number of field impairment tests at the side of the road.
These include walking in a straight line, standing on one leg and counting out 30 seconds. If these aren't carried out to a satisfactory level, the driver may the face prosecution.
The detainee is then taken back to a police station, where blood tests are carried out to determine if their driving was impaired due to drugs.
The Home Office says that this procedure is crude and that the new 'drugalyser' will provide more accurate results.
This would mean that a police officer could stop a motorist and take a saliva sample at the side of the road. If they fail this, they will automatically be arrested and taken to a police station for blood tests.
The extent of drug-driving in the UK is difficult to gauge. In 2005, however, the latest year for which figures are available, 363 drivers were convicted for driving under the influence of drugs, with a further 28 found guilty of being in charge of a vehicle while impaired through drugs.
However, it is believed that the true number is much larger.