Sirens (Drug Drive campaign)

It is illegal to drive if your driving is impaired by drugs or if you have certain drugs above a specified level in your blood.

If the police stop you and think you’re on drugs they will either test you at the roadside using a drug screening device or a "Field Impairment" test to assess your ability to drive.

If drugs are detected in your system or you are deemed impaired by drugs, you’ll be arrested and taken to a police station for blood or urine tests. If the tests show you've taken drugs or show a specified drug above the specified blood limits you could be charged with drug driving.

See: http://think.direct.gov.uk/drug-driving.html