Rule 123: The driver and the environment
The driver and the environment. You MUST NOT leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road. Generally, if the vehicle is stationary and is likely to remain so for more than a couple of minutes, you should apply the parking brake and switch off the engine to reduce emissions and noise pollution. However it is permissible to leave the engine running if the vehicle is stationary in traffic or for diagnosing faults.
Law CUR regs 98 & 107
Why the capitals matter. The Highway Code writes a rule in capitals — MUST or MUST NOT — when it is backed by law, so breaking it is a criminal offence. The rules that do not use capitals are advice.
5 theory test questions on this rule
From the DVSA revision question bank. These are not the questions asked in the real theory test. Each one shows the answer, the explanation, and our own note on why every other option is not it.