On a freezing winter morning, when your windscreen is covered in a thick layer of ice, it can be tempting to start your car, and then head back into the warmth of your house for a few minutes while it defrosts. 

But aside from the obvious risk of inviting an opportunist car thief, leaving your car while it’s running could actually be breaking the law.

Fines for idling 

The problem isn’t just constrained to drivers idling to clear ice from their windscreens. Cars are often left idling for other reasons, such as waiting to pick someone up.

Leaving your car idling is looked upon dimly by the law. The Highway Code clearly states: "You must not leave a vehicle’s engine running unnecessarily while that vehicle is stationary on a public road."

Drivers can face a fine of upwards of £20 for leaving the engine on while in a stationary car; this can go up to £80 in some areas of London.

In fact, some councils have been pushing for tougher rules regarding idling, including instant on-the-spot fines. This was echoed by the government in June 2019, when the Transport Secretary at the time, Chris Grayling, announced a consultation regarding tightening up the law in this area.

Notably, idling could be banned outside schools and hospitals, and other areas where people are especially sensitive to air pollution.

Are there exceptions?

There are some exceptions and qualifications to the law regarding idling.

The law only applies on public roads, meaning that you can legally leave the engine on if you’re on a private driveway or supermarket car park, for instance.

What’s so bad about idling?

Running engines emit fumes. And in some cases, cars can pump out almost twice the emissions while stationary as they do while in motion.

Emissions such as nitrogen oxide, sulphur dioxide and fine particulate matter are major contributors to poor air quality.

And exposure to these emissions can have serious negative effects on health. It’s estimated that man-made air pollution causes between 28,000 and 36,000 deaths in the UK every year, contributing to asthma, lung cancer and heart disease.

Although it’s difficult to separate different types of pollutants for the purposes of these figures, traffic emissions are doubtless a huge factor. According to Transport for London, over half of the capital’s air pollution is caused by polluting vehicles.

Stuck in traffic

If you have a modern car, there’s a good chance it’s already fitted with a ‘stop-start’ system. As the engine cuts out when the car’s at a standstill, you need not worry about idling. Although manufacturers allow drivers to switch off these systems manually, it’s advisable not to.

It won’t run your battery flat either, even if you’re caught for a long time in stationary traffic. The system will automatically restart the engine before the battery runs out.

If you have an older car without this system, the RAC recommends that drivers switch off their engines if they think they’ll be waiting for two minutes or more.

It’s recommended that you don’t do this too often if you have an older vehicle, however, or a battery that’s five years old or more. Your car may struggle if you switch the engine off and on again too frequently, especially in cold weather.

For more handy motoring tips and guides, go to Solved.