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.

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    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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    Car manufacturers aren't known to make their wind tunnel data public, so I can't give a Skeptics-worthy answer, but wind resistance is widely accepted to be the dominant variable in fuel economy at speeds above 90 kph. The data isn't public, but Tesla made a plot showing this (in this SE.physics answer, the image is deprecated on tesla.com). The Tesla plot is especially useful because it separates total efficiency from the dependence of a combustion engine on RPMs. – Sam Mar 10 '20 at 18:25

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 60km/h. In this study the authors calculate societal costs of different speeds (not only, but including pollution cost). They also show a speed of about 80km/h is optimal. In their paper you can see that pollution costs are higher for 20km/h than for 30km/h.

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 20km/h compared to 30km/h, 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.

  • The question mentions urban areas. In urban driving the amount of acceleration and deceleration is an important component. Lower top speeds will therefore lead to less pollution. Also, braking causes wear on brake blocks and tyres, which also contribute to pollution. – Paul Johnson Apr 24 '20 at 20:14

The most popular emission models (Caline in the US, Copert in the EU) provide emission functions for average trip speeds. This should no be confused with changes to immediate speed (for which there are very few models available, e.g. Modelling instantaneous traffic emission and the influence of traffic speed limits).

The answer therefore depends on the type of road and the speed reduction envisaged. Average speed models will inevitably result in higher emissions for e.g. 30km/h zones because they 'assume' higher dynamics (braking & accelaration) which may not be correct for some urban traffic calming schemes (see e.g.: Impact of 30 km/h zone introduction on vehicle exhaust emissions in urban areas & Effect of speed reduction on emissions of heavy duty lorries).

For modern cars, much depends on the gear changing behaviour of the driver which has hardly been studied (Influence of gear-changing behaviour on fuel use and vehicular exhaust emissions). For PI-hybrids, the chances are higher that they will switch to electric mode.

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