I know some countries enforce having cars automatically shut down the engine when stopping at a traffic light.

  • How much resources are really saved by this process?
  • Does the car need to be designed differently to make this effective? For example, do they make the ignition system different?
  • Can we do the same manually (just remember to shut down when stopping, and start again when the light turns green) and get similar benefits?
  • 2
    Cars with auto start-stop will always have upgraded batteries (often marketed as 'start-stop' batteries) and a more powerful starter motor, often 'tandem solenoid' versions which cope better in the scenario where the engine has started to shut down but is still spinning, and the driver wishes to accelerate again.
    – John M
    Commented Feb 13, 2019 at 21:37

12 Answers 12


I’ve already touched on this topic on the sister SE site once or twice. To summarize what is told there (check for more detail), there would be several considerations as related to energy use and sustainability:

  • Idling in itself wastes fuel for no good reason, and that adds up. In US (country whose residents drive probably more than anyone else on Earth, to put it in perspective) the average cars idles 16 minutes a day (including warming up, waiting, and sitting in traffic). Even 2.0 L compact car would burn (at a rate of 1 L per hour) almost 100 L of fuel a year. Obviously, some idling is unavoidable, but even half of that saved and multiplied by the number of all the cars in the world would amount to significant impact.
  • Idling negatively affects the engine internals due to incomplete combustion (glazing of the combustion chamber, fouling of the spark plugs) which further decreases fuel efficiency for all other regimes of operation.
  • What’s more, excessive idling affects other components (burning up of the catalytic converter, increased corrosion of exhaust system, increased engine wear due to higher vibration etc.), whose premature wear and replacement lead to increased use of raw materials.

What one can do (and what I do) is to turn off engine during long traffic lights, railway crossings and so forth. Anything more than 10 to 15 seconds (very generously) would save fuel, because car that is already warm does not need that much effort to re-start, as it is at the proper operating temperature, and oil is distributed around the engine and at a high lubricity. Motor stays within the efficient range of temperatures for more than few minutes, and takes several hours to completely cool down (depending on the weather), so there is definitely no need to keep the car running during short errand (never mind someone could simply run off with the vehicle).

Any concerns about premature wear of the starting infrastructure are moot considering savings on fuel and other engine components (one could afford to replace starters every 3-4 years at the worst). See the linked posts for detailed arithmetic. And any energy losses that occur during starting, and keeping the electrical loads going during the time the engine is off are minuscule compared to energy used by petrol combustion while idling. See relevant question at Physics.SE.

  • 2
    @Flimzy, Average ICE efficiency outlays put maximum of 2 to 3 % for all accessory power take-off (including AC, electrical and electronics), which is order of magnitude less than friction and heat losses inside the motor itself. Never mind that when car idles it goes nowhere fast, so it wastes 100% of all energy input.
    – theUg
    Commented Mar 22, 2013 at 0:22
  • 1
    1) The starter is not included in the 2-3% for acceories, in the link you provided. 2) "It wastes 100% of all energy input" isn't accurate, at minimum, by considering the energy required to re-start the motor (and the loss associated with generating and storing that electric energy). It's also not true if running other accessories that depend directly on the engine (A/C seems the obvious example). And any accessories that can run off of the alternator rather than battery will benefit by an idle engine versus a stopped engine, due to efficiency lost in storing and retrieving electric energy.
    – Flimzy
    Commented Mar 22, 2013 at 21:13
  • 3
    Your answer provides a figure of 10-15 seconds, and maybe that's right (although it sure seems low to me). Can you provide a source for this figure?
    – Flimzy
    Commented Mar 22, 2013 at 21:24
  • 2
    @Flimzy, of course it takes energy to re-start and regenerate the battery. But it also more efficient to do it while car does useful work (that is, moving).
    – theUg
    Commented Mar 23, 2013 at 2:28
  • 3
    @Flimzy While the "100%" figure is easy to disprove, the conditions required to do so are far from typical driving conditions. This answer on the Physics SE site provides some helpful numbers. Based on some quick searches, it looks like the alternator for the example vehicle can push out up to 70A at the voltage required, so I will estimate (conservatively) that it takes 3-4x the time required to restart to replace the energy lost. That works out to about 9-12 seconds of engine-on time, so it might be a concern in stop-and-go traffic. Commented Sep 3, 2013 at 20:09

I have been using this method for years, on both my Toyota and my Ford Explorer. I save lots of gas, nothing has ever happened to my batteries, and have never had a mechanical problem. You need to use common sense. I typically shut it off when it's a long light and I'm in a long line, railroad crossings, and any other situation where it's going to be at least a minute wait.

There's no need to panic about a mechanical issue or the battery going bad; it isn't any different than shutting the car off at the mall or the grocery store. If the battery went bad in a parking lot somewhere or if a mechanical problem developed, it's just normal wear and tear. Starting a car in the roadway is not different than starting it in your driveway or the parking lot of a grocery store.


I'm trying this out just now, car in question 1.2 carburetor astra mk 4/ astra g estate/caravan/station. Thus far I seem to be making a saving, (as long as I allow the engine to warm up a little) but it's entirely possible my general fuel conscious driving is making more of a difference than turning off the engine.

*related though another matter... Coasting down hills is probably saving more fuel than shutting off at traffic lights, and though not very safe certainly avoids premature starter wear by virtue of leaving the ignition on and bump starting when I reach the bottom of the hill.

  • Re the second paragraph: I'm not wishing to tell the answerer what to do or not to do, but for others, bear in mind that if coasting down hills with the engine off your braking will be severely impaired after the first use. Also, don't even think about this if you have power steering!!!
    – Flyto
    Commented Feb 27, 2014 at 8:59
  • 5
    Modern fuel-injected cars shut off fuel delivery in over-run conditions (when RPM is high relative to idle, and throttle is released), so if you are going down hill with no throttle and with gear engaged, you would actually not use any petrol.
    – theUg
    Commented Mar 4, 2014 at 14:12

Nobody who turns off their engine ever mentions the fact that restarting their engine causes a slight delay in getting away and must result in increased congestion. This will increase the fuel consumption of everyone in the queue who should have got away but didn't and those who had to wait a few moments longer.

  • Welcome to Sustainability.SE! If you are new to Stack Exchange we recommend taking the tour first to get familiar with how everything works.
    – THelper
    Commented Jul 23, 2016 at 17:24
  • 2
    Welcome to sustainability.SE! This doesn't really answer the question, since it specifically asks about changes in technology to automatically shut engines off at traffic lights: your answer addresses only the second part of the question, in the case that drivers are manually shutting off their engines. Simple technologies, like the redlight countdown timers which are prevalent in India, can address this issue..
    – LShaver
    Commented Jul 23, 2016 at 17:40
  • 1
    I have heard this before but I think it is around 0.5 seconds at worse, probably the same amount of time it would take to put the car in first gear and find the biting point...the amber light should allow for that :-)
    – atreeon
    Commented Oct 8, 2019 at 13:43
  • Another way to address this with manually shutting off the engine (and one I use in filter lanes that don't have an advacne warning that the light will turn green) is to shut off only if you're not the first car in the queue. The time it takes for the first car to get moving gives you time to restart
    – Chris H
    Commented Aug 11, 2021 at 9:48

I tried this method on my old 323 manual early last year for about 3 months with no issues but then that car went in for a normal service and I was forced to borrow my mother's smaller Jazz auto for a few days. It broke down the 2nd day of this use, the battery had gone dead. This car had a good history and the battery wasn't that old. In fact the battery was able to be recharged and is still in use today.

My uncle who's a mechanic said this method was bad for the car (no specifics). I also stopped using this method with my old 323 manual as I wanted to sell it soon in good working order (although it did gain an unknown motor issue about this time that fast forwarded my selling it, a mechanic bought it). I won't using this method with my new auto. I can't risk breaking down on my way to work again and I don't want to damage my nice new i30. I would like to be convinced otherwise if you have good evidence, I want to save fuel and reduce my carbon footprint.

  • 1
    To be fair, you do not provide good evidence to the contrary a as well. ;)
    – theUg
    Commented Mar 4, 2014 at 14:15

My 95 Accord MT went from ~28.5mpg to ~32mpg easily from mostly shutting off at stoplights, even as little as 7sec or more. About 10%-12% increase in mpg. I also use engine breaking and conserve momentum in general, but I feel the bulk of the saving is from shutting off when idle.

If you were to plot your mpg on a graph, turning off the engine while idle cuts out all the time when your mpg is 0, therefore increasing overall mpg.

I imagine the gas used while idling is about the same as if you feather it at 60mph. So every cumulative minute you shut off, you add 1 mile of range, which is about the result I have experienced.

I don't think starting up uses any more fuel. The fuel pump is either on or off and puts out the same psi whether you start the car or driving. I think it possibly insignificantly shortens the lifespan of the starter, because you are just adding starts to it's lifetime. I think it could possibly kill the battery if you are starting too frequently (very excessive) without much driving in between for the alternator to charge it back up. If people had battery problems doing this, I imagine their battery health was poor to start with.

I don't know if I would suggest this for the typical person though. I do my own repair work; I have back up starter, I have upgraded grounds, starter wires, newer battery cable, and I usually trickle charge my battery every other weekend. I also drive with voltmeter for good measure.

Use common sense, be aware of your surroundings and learn to read the lights.

  • "I imagine the gas used while idling is about the same as if you feather it at 60mph." - I don't see how it could be possible that maintaining a speed of 60mph for 1 minute takes the same amount of gas as maintaining a speed of 0mph for 1 minute. If that were true, you could idle your way down the highway at 60mph without touching the gas pedal, since idle power would be enough to maintain your speed. Commented Mar 7, 2023 at 19:28

How much resources are really saved by this process?

Very little. In fact, so little that for example Toyota that used to have a stop/start system in they Yaris small car got rid of the system (probably because it requires beefied up battery and starter motor that are more expensive than in non-stop/start systems).

My journey to work is driving at about 50 km/h average speed but 20 km journey takes half hours. So I assume a commute has about 6 minutes of idling.

A car uses 0.7 liters per hour idling. So 0.07 liters of fuel is saved for my commute. At fuel consumption of 6.5 liters / 100 km, the commute takes 1.3 liters of fuel. That's only 5% savings. In contrast, we have technologies like hybrid vehicles that allow far greater reductions in fuel consumption.

Does the car need to be designed differently to make this effective? For example, do they make the ignition system different?


There are several differences in normal cars and stop/start cars.

Firstly, a stop/start car has a beefied up battery. In higher end vehicles it may be expensive absorbed glass mat battery, but cheaper stop/start vehicles use enhanced flooded batteries that have plates withstanding more use at slightly lower states of charge than 100% full and withstand cycling better. Also the stop/start system has a beefier starter motor that is rated for more cycles than on non-stop/start cars. Also usually the system has a counter that counts how many times the stop/start motor has been operated and tells the driver when to replace the stop/start motor to resume the stop/start functionality (when the lifetime is over, it starts to work like an ordinary non-stop/start car).

Furthermore, usually stop/start systems have engines with variable valve timing in intake and exhaust sides that allow minimal effective compression ratio at the time the engine is restarted. This makes for a very smooth restart process by minimizing the amount of energy needed to restart the engine.

The HVAC system also has improvements. For example, in stop/start cars the blower motor is often automatically controlled to have minimum possible speed during engine stop (this may be omitted in manual HVAC systems), and also there is automatic engine restart if the the temperature inside the car becomes too hot (requiring AC which requires engine to be on) or cold (requiring heat from engine).

Furthermore, the battery is saved by a low voltage detection circuit that automatically turns the engine on if the battery voltage becomes too low.

Can we do the same manually (just remember to shut down when stopping, and start again when the light turns green) and get similar benefits?

No. You'll wear your starter motor and your battery if doing that. Normal batteries aren't designed to continuously cyclically produce power for the lighting system when the engine is off, and normal starter motors will be worn out in no time if doing that. Also you lack the conveniences like auto-restart on too hot or cold temperature, and the essential feature which restarts the engine automatically if battery voltage becomes too low.

Also it's less convenient to not have the engine automatically restart when pressing the clutch or releasing the brake. Furthermore restart will be more rough because the car may not have dual variable valve timing minimizing compression ratio of restart.

The battery could obviously be replaced with a battery that withstands abuse, the type of battery used in stop/start cars, but you probably can't replace the starter motor with a stop/start variant because you may not find a compatible stop/start starter motor for your car that originally had non-stop/start starter motor.

  • 2
    5% multiplied across millions of cars is a lot of savings!
    – LShaver
    Commented Aug 9, 2021 at 14:57

I turn off my engine at longer traffic lights, especially where I know the light cycle has me in for a wait. I appreciate the math at https://physics.stackexchange.com/a/75572, which puts the "break-even" cost of recharging the battery after restarting the engine at around 3-4 seconds. Another factor that has been on my mind is the Carbon impact of replacing the starter motor.

Per https://www.nerdwallet.com/article/loans/auto-loans/car-starter-cost:

A car starter can give you around 80,000 starts before you should expect it to struggle, regardless of the mileage you drive. This is around 150,000 miles for some cars, while some starters can last the vehicle's life.

That is about 22 starts per day over ten years to kill the starter. I drive a lot less than that. I wonder if the battery degrades a tiny bit with each start, but I can not find anything. As a solid-state process, I assume this isn't a big factor.


It has it been proven that it takes more fuel to start a car than it does to let it sit and idle at a traffic light nevertheless the extra wear and tear on the charging system and the starter

  • 2
    Citations that verify this would improve this answer.
    – Fred
    Commented Apr 17, 2022 at 17:55

I tried this for three months (turning off the engine at traffic lights). The result is that my car battery stopped working and there was smoke coming from under the bonnet. I then had to arrange to have the car towed to a garage. I was told that I would need a new starter motor at a cost of £150. It certainly doesn't represent a saving.

  • 9
    If shutting off the engine at traffic lights actually caused this problem, please provide evidence. Otherwise it could just as easily have been a faulty part or lack of maintenance. Commented Sep 5, 2013 at 15:27
  • 4
    It would also be valuable to describe what kind (make/model/year) of car this was (and maybe the battery age, too). Anecdotal evidence is difficult to go off of, especially if the anecdote is incomplete. It may also depend on your driving habits. If you're only in stop and go city traffic, and the car never gets a chance to recharge at high rpms on the highway, I could see that being harder on the battery.
    – Nate
    Commented Sep 6, 2013 at 4:36
  • 2
    Yea, Anecdotally, I do it all the time, on my moto, and in my cars. Done it for years — nothing broke so far. Certainly, not in three months.
    – theUg
    Commented Sep 6, 2013 at 15:42

No, you won't save any fuel. You should remember that it takes a lot of energy to start the engine. This intially comes from the battery. However as soon as the engine starts this energy must be replenished by the engine and this is more than is required to crank the engine (because the battery is only 70% efficient).

Also all electrical loads are supplied by the battery when the engine is stopped. When the engine is running these loads are from the alternator (less losses). When you start your engine, you have to drive something like 5-8 miles to recharge. So if you are stopped at traffic lights for 5 minutes you have to add this to the 5 mile equivalent. So in reality it can cost more fuel! People are wrong thinking they can get something for nothing, like the high MPG of hybrid cars where the diesel or petrol engine is driving an alternator, loss 70%. rectifier, controller and motor loss 30%, instead of the engine driving the wheels directly.

  • 3
    Welcome to Sustainable Living! Do you have any references for your claim that it takes 5-8 miles to recharge the energy you lost when starting?
    – THelper
    Commented Apr 16, 2014 at 19:39
  • Having to drive x km to recharge does not necessarily cancel out savings; one can only conclude that if one has the figures. In the daytime without air-conditioning I suspect that the electricity consumption is minimal.
    – PJTraill
    Commented Nov 21, 2018 at 20:33

I have been a great fan with this technology, however after some thought, I find that there are some drawbacks with this tech.

The part that made this work is a special starter that starts faster. So I assume it draws more power than normsl starter. So, I think this will eventually kill a battery faster unless thr battery is more powerful, which in turn becomes more expensive... meh...

Then there's the problem when starter reliability fails. It would cost more to replace this powerful starter.. meh..

Then there is doubt on the actual savings once you take in the recharging cost of fuel to power this start stop. Conventional cars only charge after the start of a car once in it's journey.. how much fuel used? I'm not sure.. but if you keep restarting the engine surly it will need to recharge more.. so, probably the savings wont be as much...

  • 1
    Welcome to Sustainable Living! As you can read in the theUg's answer estimates are that turing your engine off becomes more efficient if you stop for more than 10 seconds. If you have references to other numbers, please share them.
    – THelper
    Commented Jul 1, 2014 at 6:13

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.