The author of this article claims that

Cars produce more emissions when idling than when in motion.

This seems like a patently false statement. How could cars produce more emissions when idling than when in motion? The engine is hardly doing any work. Perhaps different kinds of emissions are produced when idling than when working the engine harder, but the total amount I would guess is smaller when idling.

Is the quoted statement correct?

  • 3
    This statement could be true if they're talking about emissions per mile, which is a very generous interpretation of the text - obviously, idling cars don't move, and produce an infinite amount of emissions per mile. Otherwise, it's highly suspect. A decently fuel efficient car might burn 2 gallons fuel in an hour on the highway. The same car would burn around 0.2 gallons if idling for an hour. I find it extremely unlikely that idling would produce 10 times the emissions per gallon, which would be required for that statement to make any sense. Commented Oct 30, 2018 at 21:10
  • this depends greatly on how you measure it. If you're monitoring air quality at a fixed location, you're going to get a very different result than if you're monitoring emissions at the tailpipe.
    – De Novo
    Commented Oct 30, 2018 at 22:57
  • It's hopelessly vague. Commented Oct 30, 2018 at 23:42
  • 3
    I wonder if they are trying to say "Cars spend such a high percentage of their time stationary, that over their lifetimes, there are more emissions from them idling than while moving."? (That is also a claim that needs evidence, and seems like it would be hugely regional.)
    – Oddthinking
    Commented Oct 31, 2018 at 0:12
  • 1
    So far, we have had three completely different interpretations of what the statement might mean: Emissions per km (undefined for idling), emissions per car lifetime, and that "idling" means "idling + motion over a standard section of road versus just slower motion". It seems unlikely that they meant emissions per hour. Is there any point us making up excuses and debating which was intended?
    – Oddthinking
    Commented Oct 31, 2018 at 12:30

1 Answer 1


Correct - only as used in the article

The article is comparing 2 cases, each involving driving along "a section of road with 10 traffic signal systems"

1) as today, when drivers have no information, leading to "unnecessary stopping and starting" - i.e. cars accelerate to a cruising speed, reach a red light, stop, idle, wait for green, accelerate again etc

2) in future, when drivers\self-driving cars are informed when lights will change, hence avoiding stopping and starting - i.e. cars change speed so that they will cover the distance to the lights in time for the lights to be green, therefore minimising acceleration, braking and preventing idling completely

In the second case fuel consumption is reduced - hence "Cars produce more emissions when idling [case 1] than when in motion [case 2]."

  • Why should we believe this particular interpretation, that doesn't seem to match the words. [Case 1 involves both moving and idling.]
    – Oddthinking
    Commented Oct 31, 2018 at 12:27
  • 2
    @Oddthinking Mobeer's interpretation of the article is also mine (as a former transport modeller). Mobeer's produced a good answer to a very poor question: the question has taken one sentence completely out of context, removing all its meaning. Mobeer has reintroduced the context, returning the meaning.
    – 410 gone
    Commented Oct 31, 2018 at 13:38
  • To be fair, it's the article's fault rather than the OPs, as the article does indeed make that claim. Although the prior sentence, "The system will help cars avoid unnecessary stopping and starting, reducing the amount of gasses they emit" clears it up a little, the sentence quoted by the OP as written is somewhat confusing.
    – komodosp
    Commented Oct 31, 2018 at 14:45
  • @Oddthinking, since when a personal interpretation requires any "additional references"? Commented Oct 31, 2018 at 17:01

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