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Europe has been under exceptional sunlight for the past ten days, cf. Eumetsat's picture. According to French media, this prevents smog from escaping major hubs. In an attempt to reduce this air pollution, some cities have lowered their speed limits, as RFI, Radio France Internationale, reports:

Paris police lowered the speed limit for cars to 20 km/h in some areas and banned trucks weighing over 3.5 tonnes from entering the city after a peak in air pollution in December 2013.

The question is meant to focus specifically on the 10 km/h arbitrary choice: it makes sense that lowering speed limits by 50 km/h will result in much less pollution. But is 10 km/h enough to have any substantial change? How likely is it that some drivers may be confused at to which gear to be in at 60 km/h instead of their usual 70: if they're driving with a higher or lower RPM than the optimal operating RPM then wouldn't these drivers actually pollute more? (Cf. this answer which specifies that at least the highest gear coincides with getting the maximum mileage -- and therefore lower consumption -- at the maximum allowed speeds https://skeptics.stackexchange.com/a/3355/2086 .)

What is the scientific basis of this claim? I did not find any relevant results on Google Scholar nor on Sparrho.

  • 3
    they assume less fuel usage and thus less pollution – ratchet freak Mar 14 '14 at 22:29
  • @ratchetfreak, I have focused my question slightly. – ChrisR Mar 15 '14 at 1:26
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Generally, every car has a specific speed where fuel efficiency (i.e. miles traveled per gallon) is highest. This is mostly around 40 - 60mph (~65 - 95 km/h). Above and below this speed fuel efficiency decreases. So if you travel the same distance, you will use more fuel and therefore produce more pollution.

In this paper the authors use a pretty exhaustive model to show fuel effieciency at different speeds for an electric and gasoline car. The most fuel efficient speed in their model is 60kph. In this study the authors calculate societal costs of different speeds (not only, but including pollution cost). They also show a speed of about 80kph is optimal. In their paper you can see that pollution costs are higher for 20kph than for 30kph.

What you have to keep in mind is that these are optimal speeds, which do not include reactions to speed limits. While pollution is slightly higher for a speed limit of 20kph compared to 30kph, a change in speed limit may result in less people driving, because of increased travel time. If these people take public transport/bike/walk instead, the overall effect of the lower speed limit on pollution may be a reduction.

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Technically no.

The best speed is the speed with a better ratio of consumption/distance. Usually (it depends on the vehicle) it's around from 50km/h to 80km/h. Reducing the speed will decrease the emission but increases the time the vehicle is polluting. For example, a parked car with the engine running still pollutes.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    On skeptics.SE, it's expected that you provide reliable references in your answers. In particular, there should be a reference that supports your implication that while reducing speed decreases emission rate, this effect is smaller than the increase due to the longer emission duration. – Schmuddi Apr 20 at 6:21

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