Stop-start engines and engine idling – the law and common myths revealed

Stop-start engines and engine idling – the law and common myths revealed
Is it start-stop or stop-start? All we know is that the misinformation around this technology needs to stop!

Does it use more fuel? Does it cause more pollution? We decided to unearth the facts and find conclusive answers.

We're also looking at the harmful activity that partly inspired start-stop: engine idling.

What is stop-start?

A start-stop or stop-start system automatically switches off and restarts the internal combustion engine of your car. 

The idea is to reduce the amount of time your engine spends idling. Hopefully it will lead to cleaner roadside air, especially outside schools. 

Stop-start should also see a reduction in your fuel consumption and vehicle emissions, and it’s particularly helpful for vehicles that spend a lot of time stationary in heavy traffic.

How does it work?

Your car can detect when it has stopped moving. Once your brake pedal is depressed, or on manual cars, your clutch is depressed and the car is out of gear, the engine control unit (ECU) cuts the fuelling and ignition to turn off the engine.

When you go to move off again, your engine is triggered to restart in any one of three ways: releasing the brake pedal, engaging the clutch or pressing on the accelerator. You can then continue with your journey without pressing dashboard buttons or turning a key.

The automatic system can be turned off – usually through a button showing an ‘A’.

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Myth #1: Stop-start engines use more fuel

Research from the American Automobile Association found that stop-start technology resulted in a 5% to 7% improvement in fuel economy1

This saving may seem small but would certainly add up over the lifecycle of your car. In any case, the data shows that they certainly don’t use more fuel.

Myth #2: Stop-start systems damage your engine

The argument that stop-start technology is damaging mostly comes from the fear of a cold engine – your engine must heat up lubricants when it starts. However, the technology doesn’t come into use until your engine has warmed to an operational temperature.

If you’re stuck in traffic for a minute and your engine switches off, the lubricants will not cool down to damaging levels. 

If you find yourself stationary for a longer period of time, the system is designed to start the engine automatically before lubricant temperatures drop too dramatically.

Another feature that protects your engine prevents oil from returning to the oil pan. As a result, engines restart with a significant amount of fluid in the engine chamber. 

Whilst most engine parts such as your starter motor, ring gear, cambelt and flywheel have to work more frequently than a vehicle without stop start, manufacturers ensure longevity and ability to operate more frequently through higher manufacturing specs.

Turbos aren’t affected and build up of soot (such as around the EGR valve) is minimised through precise engine management control features.

Further advances, such as dry lubricants on engine bearings and improvements to the durability of engine bearings, mean today’s cars are designed to withstand these frequent engine restarts.

Myth#3: Stop-start systems wear out your battery

Many stop-start systems are found on newer cars that have modern, robust batteries with a high capacity.  In some cars, a separate battery is used to restart the engine as well.

Cleverest of all, your car’s computer checks charge levels in your battery before attempting to shut it down. If it decides there isn’t enough charge to restart the engine, it’s left running2.

Myth#4: Stop-start engines cause more pollution


Regardless of how somebody drives there will always be moments when their car is stationary. As a result, a journey using a stop-start engine will give off fewer emissions than the same journey without one. 

A  study from the Polytechnic University of Madrid3 measured the CO2 emissions of two four-wheel drive vehicles. A reduction of more than 20% was recorded for the car equipped with the stop-start system.

Researchers concluded that despite variability in driving style, grade and type of streets, traffic congestion and engine operating temperature: “the car equipped with the stop/start system has intrinsically a lower CO2 emission factor.”

Are stop-start engines better for the environment overall?

There’s no doubt that stop-start technology reduces CO2 emissions. The real question is whether the pollution caused by the manufacture of extra parts is offset by the reduction in emissions over the lifecycle of a vehicle. 

Similarly, how much waste is created at the end of a stop-start vehicle’s life? 

Unfortunately there don’t appear to be any studies that address these questions, so for the moment they remain unanswered. This is one potential myth which will have to go un-busted today.

Engine idling

Idling means leaving a vehicle's engine running while it is stationary.

While this is often because of traffic, there are some instances, such as waiting for children outside schools and in total gridlock, when idling is not necessary and should be avoided.

Idling increases the amount of exhaust fumes in the air – fumes that contain harmful gasses linked to asthma and other lung diseases. 

The Royal College of Physicians estimate 40,000 deaths a year in the UK are linked to air pollution4, with engine idling as a contributing factor.

The Government announced plans in June 2019 to introduce higher fines for idling drivers5.

Engine idling law


Is it illegal to let your engine idle when parked?

Rule 123 of The Highway Code looks at ‘The Driver and the Environment’. It states that drivers must not leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while the vehicle is stationary on a public road.

Local authorities have the power to issue £20 fixed penalties for emission offences and stationary idling under The Road Traffic (Vehicle Emissions) (Fixed Penalty) (England) Regulations 2002. The Government even plan to introduce higher fines for the offence.

However, it is important to note that fines are imposed only if a motorist refuses to switch off their engine when asked to do so by an authorised person. According to RAC research, 44% of drivers support the measure

Is it illegal to engine idle outside schools?

Local authorities have the power to issue £20 fixed penalties for engine idling if a motorist refuses to switch off their engine, regardless of the location. 

The penalty is given as a result of rule 123 of The Highway Code, stating that drivers must not leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while the vehicle is stationary on a public road.

RAC research found that 26% of drivers are spotted engine idling outside schools. An RAC scheme inviting schools to buy School Clean Air Zone banners launched in 2019.

Is it illegal to engine idle in a driveway?

The legality of idling on a private driveway depends on its classification as a road. 

The rules aren’t entirely clear. Annex 4 of The Highway Code states that references to ‘road’ “generally include footpaths, bridleways and cycle tracks, and many roadways and driveways on private land (including many car parks)”. 

What can I do about engine idling?


While it may be some time before more councils start getting tough with drivers who idle their engines, schools can now draw attention to the issue themselves by purchasing an RAC School Clean Air Zone banner which urges drivers to ‘show they care about our air’ by turning off their engines.

The banners, which cost £60 including delivery and VAT, are produced for the RAC by Ottimo Digital. The RAC makes no profit from the sale of the banners and Ottimo Digital has agreed to print them at a reduced margin.

Schools interested in buying banners can do so here.


Buy a banner



Hopefully the popularity of start-stop engines will make idling less of an issue in future, but what’s your take on engine idling now? 

Do you think it should be enforced more widely? Let us know your thoughts in the comments.



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