Does lowering speed limits by 10 km/h have any impact on air pollution?

Question

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.

Answer 1

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.

Answer 2

First, as far as I remember, there is a major factual mistake (possibly a mistranslation) in the article you quote. What happened is that speeds were reduced by 20 km/h in non-urban areas. I do not recall, and cannot find historical records of, speed reductions to 20 km/h, or in urban areas during the 2014 winter pollution peaks. Specifically, the rule was:

The maximum permitted speed for motor vehicles is reduced by 20 km/h:

• On motorways: [a decree] limits the speed to 110 km/h on motorways instead of 130 km/h.
• […] limits the speed to 70 km/h on road sections limited to 90 km/h and to 90 km/h on sections limited to 110 km/h.

Source: Oise prefecture, 13 March 2014 announcement. (Oise is a district (département) just outside the Paris region.) My translation.

Maximum speeds are legally reduced to 110 km/h on motorway segments where the normal maximum is 130 km/h, to 70 km/h on [roads] where the normal maximum is 90 km/h, and to 60 km/h on the [Paris] ring road [where the normal maximum is 80 km/h].

Source: 12 March 2014 announcement from the Paris police administation, 4th body paragraph, my translation. The 110-to-90 reduction applied in the Île-de-France region too.

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The influence of speed on air pollution depends on the type of vehicle and the type of pollutant, but the studies I've found consistently find that for cars, the fuel consumption curve has a U shape: low speeds and high speeds result in more pollution per distance traveled than medium speeds. However, pollution doesn't always directly follow consumption because the efficiency of after-treatment can also vary.

A 2020 study by Emisia relayed by the European Environment Agency evaluated fuel consumption and emissions for speeds above 90 km/h. It found that at these speeds, a higher speed means more fuel consumption and more CO₂ per distance traveled, as well as more particulate matter for diesel engines (apparently the study didn't record PM for gasoline engines). For NO_x, emissions increase with speeds for diesel engines, but for gasoline there is an optimum around 115 km/h. Gasoline cars emit more CO at higher speeds, but diesel cars emit less (due to a catalyst which is more efficient at higher speeds due to the higher temperature).

The environmental effects of changing speed limits: A quantile regression approach by Germà Bel et al., Transportation Research Part D, vol. 36 p. 76–85, 2015 studied actual data for NO_x and PM pollution before and after policy changes in Barcelona. I haven't read the full article (which is paywalled). Here's a passage from the abstract:

In 2008, the maximum speed limit was reduced to 80 km/h and, in 2009, a variable speed system was introduced on some metropolitan motorways. (…) We find that the variable speed system improves air quality with regard to the two pollutants considered here. (…) However, reducing the maximum speed limit from 120/100 km/h to 80 km/h has no effect – or even a slightly increasing effect – on the two pollutants, depending on the pollution scenario.

A 2021 report by Cerema (a French public agency) compared fuel consumption and NO_x, PM and CO₂ emissions depending on the average speed, ranging from 10 km/h to 130 km/h. They used averages over different types of vehicles based on the typical fleet composition in 2020 and anticipated fleet composition in the future. They found that for an average car, fuel consumption, CO₂ NO_x and PM emissions all have a U shape and are lowest per km traveled around 70 km/h. On the other hand, for a truck, slower is always better, even down to 10 km/h.

Answer 3

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.