General rules, techniques and advice for all drivers and riders
This section should be read by all drivers, motorcyclists, cyclists and horse riders. The rules in The Highway Code do not give you the right of way in any circumstance, but they advise you when you should give way to others. Always give way if it can help to avoid an incident.
Control of the vehicle
Braking
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