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Leaving an engine idling is an offence
"Stationary idling is an offence under section 42 of the Road Traffic Act 1988," says Jeanette Miller, managing director of Geoffrey Miller Solicitors.
The Act enforces rule 123 of the Highway Code, which states: "You must not leave a vehicle’s engine running unnecessarily while that vehicle is stationary on a public road."
"Doing this can incur a £20 fixed-penalty fine under the Road Traffic (Vehicle Emissions) Regulations 2002. This goes up to £40 if unpaid within a given time frame.
But this is only imposed if the driver fails to turn off their engine when asked to do so by a marshal.
Of course, it doesn't mean you've got to cut your engine at every red light. You’re allowed to leave your engine running if you're stationary in traffic, diagnosing faults or defrosting your windshield."