A British company makes a "drugalyzer" that can detect amphetamines, cocaine, opiates, cannabis and methadone in a saliva sample. That machine is already in use in police stations, but a field unit would need to be approved by Britain's Home Office.
Right now, those thought to be driving on drugs are given the same walk-the-line field used for perceived drunk drivers for decades before the Breathalyzer came along. Countries like Romania, Australia, and Italy already perform mobile drug testing. If enacted in Britain, the bill would likely help the authorities reign in the suspected 23% of drivers and 21% of riders in accidents who have "impairing" drugs in their systems.
[Source: The UK Telegraph]