I have repeatedly heard from various car enthusiasts that start stop technology has no benefit in improving air quality (or the tiniest smallest negligble benefit). Does it have an effect and how much?

I live in a busy town with many traffic lights and junctions, there is a taxi rank where taxis idle for minutes at a time, there are high streets where cars wait outside shops with their engines running and there are bus stations where buses leave their engines running while loading up passengers.

Note: I'm talking about the effects here on air quality, not on fuel efficiency (although of course they are closely related). Also, I asked the question in the Earth Science stack exchange site but it was deemed more relevent here on Sustainable Living.

  • Where I am, idling for minutes (the taxi and shop scenarios as well as e.g. when waiting on railway crossings or warm up idling in winter) are actually forbidden and nowadays, noone does this any more. Hence, I'd tend to say that start/stop is not needed for those scenarios, a key to turn is sufficient. Start/stop would be good for the bus, maybe at traffic lights (but I've successfully been turning off my engine manually at traffic lights for years). I'm not sure about junctions: here, most junctions w/o traffic light either have low waiting time or require fast start. Commented Nov 5, 2019 at 20:37

3 Answers 3


Idling is particularly bad for NO2 emissions, which are a major concern for air quality. Here's an old paper showing that an idling engine emits more NO2 than one driving at 40 km/h, despite emitting less NO; NO2 is much worse for human health. More recent studies that I've found don't change the conclusion.

At a junction near me there's currently an experiment taking place to reduce idling. This isn't by encouraging people to switch off, but by improving traffic flow on the major routes, removing some turning options that cause a lot of waiting. This is projected to have a noticeable improvement on NO2 levels. There's quite a bit of background at that link.

  • Thanks for the link, info and answer. Really interesting about the NO2 being worse when idling, I did not know that. I strongly agree that managing traffic flow is a really great way to improve air quality. I have thought, if stop-start tech was mandatory and law (or hybrid) then cars could be stopped at lights for longer and then left to travel for longer. This would improve air quality by getting cars to drive at speeds more optimal for fuel efficiency and pollution, reducing acceleration (which is known to be costly especially the way drivers drive) and reducing idling entirely.
    – atreeon
    Commented Oct 10, 2019 at 10:02
  • Do you have a link or info on any more recent studies?
    – atreeon
    Commented Oct 13, 2019 at 10:29

A car stationary but left running (idling) will cause pollution in that specific location; start stop technology could avoid the pollution responsible for idling. Here I will concentrate not on the global impact of the pollution but at the localised affected area. The differing scenarios are important to understand how much of an effect start-stop will have on air quality.

Taxi ranks, junctions, city centres, high streets, spaces outside schools, hospitals and other areas where cars often stop for longer than a few seconds; the important part is that these are usually places with a large number of people. The longer you idle and the more cars doing it, the more pollution is created.

I present a few scenarios (a link at the bottom for my calculations)

A long journey with minimal stopping Here the savings on pollution would be negligible because the car would not be stopped idling for much time at all! Although of course the locations where the car does stop would be subject to fewer pollutants.

Major junction at 11:30am on a weekday (figures taken from Preston Circus Brighton) During a full cycle of all traffic lights going from red to green and back for all junctions, 69 cars passed during 112 seconds. The cars used an estimated 0.451L of petrol. During this time 32 cars waited for 30 seconds using 0.2L of petrol. Idling in this scenario is responsible for 44% of the fuel used and therefore I assume idling is responsible for 44% of the pollution.

Method: Cars waiting count - I counted all the cars stopped just before the lights turned red and timed the total for the red light for each traffic light. I then divided that number by two assuming the cars waited on average half the time.

32 cars waiting an average of 30s (adjusted 10s for startup)
69 cars passed this junction over 112s
Length of junction (with a bit either side) = 65m

Total fuel used whilst idling
0.00021L/s x 32cars x 30s = 0.2L

Total fuel used driving along junction
1/(10000m/L x (1m/65m)) x 69 cars = 0.451L

A major city centre junction like this is a place where pedestrians will wait at crossings, drivers will sit in their cars, cyclists will stop to catch their breath and where pedestrians sometimes have no alternative route. 44% here is a huge and I think a very significant number.

Single car waiting in a single spot In a car you would use around 0.75L of petrol per hour idling and you would change that single location from being pollution free to a place equal to a single lane road with regular traffic (1 car every 12 seconds)

A 1.5km stretch of road with 4 major junctions with traffic lights A single 1.5km stretch of road could have four major junctions in a busy city. A car stops twice for a combined total of 2 minutes. The car will use around 0.025L of petrol idling. Driving along this stretch of road would use around 0.15L of petrol, assuming the amount of fuel is equal to the pollution then start stop could reduce pollution by 6%. This is a fairly small amount compared to the total but it is still significant difference in the air quality along a large stretch of road. Streets like these in city centres will have many more people than in other areas waiting at the lights, walking along pavements, people working in shops, people living in flats above the roads, cyclists and drivers will be in their cars taking in the pollution. These areas are some of the most dangerous generally for people because of the number of people so 6% could make a significant difference.

A taxi rank A taxi rank with 10 cars idling in a space of around 35m. That 35m spot could have the air quality equal to a reasonably busy dual carriage way (a car every 1.68 seconds). Some taxi ranks are covered (such as those near train stations) which could make those spaces much much worse as the pollution would not be dispersed easily. There will be people waiting for taxis next to the taxi rank, people waiting nearby as taxi ranks are usually in busy locations and the taxi drivers themselves will be sitting directly in these toxic fumes so there will be many people exposed to this pollution. With start-stop, this pollution would be completely eradicated.

A school The cars could be spread out more so you could say it would be a half of the above, so a busy single lane A road maybe. The affects of pollution are greater on children because their lungs are smaller, more sensitive and growing. Children and babies in prams are also more affected because they are shorter and closer to the pollution.

Amount of fuel used per hour idling Cost of engine restart

Fuel used idling = 0.165 gallons/h = 0.75L/h = 0.00021L/s 
I assume 23.5mg = 10km/L = 10,000m/L 
I assume each restart is equal to 10 seconds running (more accurately 3s)


  • Welcome. Citations would help strengthen this answer. For example, do you have any material that supports the assumption that the duty cycle of the car engine is 100% correlated to emissions? Commented Oct 8, 2019 at 15:33
  • Thank you! Do you mean the cost in fuel to restart the engine?
    – atreeon
    Commented Oct 8, 2019 at 15:38
  • In the link at the bottom I included some references and calculations. Do you think it would be better if I included them in the answer? (I thought the answer was getting a bit long!)
    – atreeon
    Commented Oct 8, 2019 at 15:40
  • It is better to include all information in the answer on the site, yes. External links may go away over time and the information will be lost. Commented Oct 8, 2019 at 17:23
  • Fuel cost to restart the engine would indeed be interesting. Emissions during startup and shutdown seem like they might also matter. I don't know if they differ from emissions during normal operation but it seems possible. Commented Oct 8, 2019 at 17:24

Cars idling use about 1 litre per hour of petrol (link). Imagine 30 cars sitting together waiting in a traffic jam with their engines running for just one minute; they would burn 0.5L of petrol and create the associated pollution.

Here is a video of a 0.5L bottle of petrol being burnt.

The difference here between using stop-start and no stop-start would be like taking that bottle of petrol and burning it all within a minute or...not burning it at all.

Yes, there are various filters to catch some of the particulates but the gases have to go somewhere.

Other Info: You save fuel if the wait is longer than 0.2 seconds (link). Idling can damage your vehicle more than restarting (link).

  • But a car uses a bit more petrol when restarting the engine, so if the wait time is (very) short then idling is more efficient.
    – THelper
    Commented Oct 9, 2019 at 7:29
  • Thanks, I added to the answer above with a couple of links at the bottom
    – atreeon
    Commented Oct 9, 2019 at 8:34

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