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Starting on 2021-08-30, the driving speed limit within the Paris municipality was reduced from 50 km/h to 30 km/h. There is a debate on the impact that this will have on air pollution.

The Paris municipality justifies lists the benefits as better safety, noise reduction, and a fairer sharing of public space, all of which are out of scope of this question.

Le Point published an inflammatory article titled “30 km/h in town, a promise of worsened pollution”. Le Figaro has a more factual title “Cars pollute more at an average speed of 30 km/h than 50 km/h, according to a study”. Le Journal du Dimanche, on the other hand, quotes an expert stating that “from one study to the next, and within the same study, the results are highly variable”. A Libération article merely cites a 2014 study with “‘contrasted’ short term results”.

The claims that the speed reduction will increase air pollution are mostly based on 2021 report by Cerema (a French public agency) (in French). This report compared fuel consumption and NOx, PM and CO2 emissions depending on the average speed, ranging from 10 km/h to 130 km/h. This report shows that for cars and light utility vehicles, pollution per km traveled has a U shape: consumption is higher at low speeds and at high speeds than at medium speeds, and the optimum speed is over 50 km/h. (Other studies show somewhat different effects; see Does lowering speed limits by 10 km/h have any impact on air pollution? for a discussion of pollution at higher speeds.)

However, it is not clear to me that this report is applicable to urban driving, in a city that wasn't designed for cars and doesn't have long stretches where a car can drive at a sustained speed. Urban traffic is stop-and-go, with frequent intersections and pedestrian crossings. Streets don't follow much of a grid shape and synchronized traffic lights only concern a small number of streets (“The longest ‘green wave’ is (…) 3.2 km long. (…) In practice, traffic has to be fluid to drive the whole 3.2 km without stopping, which is rare (…) except at night or on Sunday morning.” — Le Parisien, 2018). The average vehicle speed in Paris during daytime was 11,6 km/h in 2019Q4, down from 13,6 km/h two years earlier (source). The relevant comparison is not between a smooth 30 km/h and a smooth 50 km/h, but between frequently speeding up from 0 to the limit (or less) and braking back to 0. A lower speed limit reduces the amount of energy spent accelerating and of braking (which emits particles).

The speeds considered in the report are average speeds based on COPERT version 5 methodology. It is not clear to me what this means: COPERT includes some speed variation, but does it correspond to typical urban traffic or to typical road traffic? And if it does model urban traffic, how does the average speed figure in the Cerema report relate to the speed limit?

Reducing the speed limit is likely to slightly reduce traffic as some drivers decide to reroute or use a different means of transportation. My primary question assumes the same amount of traffic, but if there's a relevant empirical data in places where the speed limit was reduced in a similar way and there was a reduction in traffic, I'm interested as well.

TL,DR: in dense urban driving conditions, for cars, does reducing the speed limit from 50 km/h to 30 km/h increase air pollution?

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    I don't think we have a situation in which this could be answered. The problem is that it's not what the speed limit is, but what speed cars are actually moving. Commented Aug 29, 2021 at 23:58
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    @LorenPechtel correct, and to complicate matters further, it would depend greatly on the vehicles involved as well.
    – jwenting
    Commented Aug 30, 2021 at 7:49
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    It might not be so much to do with the actual speed but on how smoothly the traffic flows. Some UK motorway speeds are reduced when congested to make all lanes move at the same speed with a more even flow, and less lane switching, bunching, braking or accelerating. Commented Aug 30, 2021 at 9:44
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    One of the explicit goals of this rule that will slow automobile traffic to a fast walking pace is to convince motorists to consider using alternative forms of transportation such as public transportation, cycling, or walking. Fewer cars on the road => less pollution from cars on the road. Commented Aug 30, 2021 at 10:55
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    @DenisS As I explain in my question, that earlier question only discusses higher speeds typical of road driving. My question here is about urban driving. Commented Aug 30, 2021 at 14:59

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It depends, but not in general

I found a report to the German Bundestag, which references several studies. I would summarize the results as follows:

1.) particulate matter from the Motor exhaust increases at lower speeds, this is however offset by less particulate matter due to brakes, tires etc. The net is lower for 30km/h than for 50 km/h.

Untersuchungen, die die Landesanstalt für Umwelt, Messungen und Naturschutz Baden-Württemberg aus dem Jahr 2012 kommt zu uneinheitlichen Ergebnissen: Die motorbedingten Belastungen durch Feinstaub (PM 10) stiegen bei Tempo 30. Doch es gibt noch einen gegenläufigen Effekt: Die Belastung durch Feinstaub, der durch Abrieb (Reifen, Bremsen, Straßen) und Verwirbelung entsteht, sinkt. Daher kann eine Geschwindigkeitsbeschränkung auch die Belastung mit Feinstaub verringern. Auch die Emissionen durch Stickstoffdioxid (NO2) muss man differenziert betrachten: Auf ebener Strecke bewirkt ein Tempolimit von 30 eher höhere NO2-Werte. An Steigungen und an Stellen, wo der Verkehrsfluss häufiger gestört ist, sinken die Stickstoffdioxid-Emissionen. Das hat eine Untersuchung des Landesumweltamtes von Baden-Württemberg ergeben. Warum ist es so kompliziert? „Es kommt immer auf den Vergleichsfall an“, erklärt Verkehrsökologe Falk Richter. Je mehr Beschleunigung, desto höher die Emissionen. Wenn man also schnell (Tempo 50) den Berg hochfährt, ist die Leistung des Fahrzeugs größer - das bedeutet mehr NO2 als bei Tempo 30. In der Ebene ist die Situation anders, da kann es Situationen geben, in denen bei Tempo 50 weniger NO2 entsteht.2

2.) traffic flow is much more important for the pollution, so the expectation is only less pollution at lower speeds if this keeps the traffic flowing as well or better than the higher speed

Die Berliner Senatsverwaltung kommt nach ihren Untersuchungen zu dem Schluss: „Tempo 30“ reduziert im Stadtverkehr nachweislich den Stickoxidausstoß, indem die besonders schadstofflastigen Beschleunigungsvorgänge deutlich verringert werden. Genau das ist mit Verstetigung des Verkehrs gemeint. Die Emissionsminderung von Tempo 30 beruht also auf einem gleichmäßigeren Verkehrsfluss mit einem höheren Anteil konstanter Fahrweise.“ 3 „Der Berliner Senat hat an drei Straßen über drei Jahre gemessen. Dort sanken die NO2-Werte nach Einführung von Tempo 30 zwischen 5,7 und 12,8 Prozent. Der elementare Kohlenstoff nahm ebenfalls ab (zwischen 0,3 und 2,2 Prozent) und geringfügig auch der Feinstaub (1,8 Prozent).

Das UBA und die Bundesanstalt für Straßenwesen (BASt) kommen zu dem Ergebnis, dass vor allem der Verkehrsfluss für die Schadstoffmengen entscheidend ist. „Dies bedeutet, dass das Ziel einer Verkehrsberuhigung nicht nur die Geschwindigkeitsreduktion sein sollte, sondern gleichermaßen eine Verstetigung des Geschwindigkeitsverlaufes über längere Strecken beinhalten muss“, so die BASt. Tempo 30 kann die Schadstoffbelastung reduzieren, wenn der Verkehrsfluss beibehalten oder verbessert wird, so das UBA

3.) Most studies come to the conclusion that the pollution is expected to be lower at lower speeds (assuming traffic flow stays as good, which is nowhere explicitely mentioned as expectation, but rather as a goal to be achieved). One ADAC study comes to the conclusion that 30 km/h is worse than 50km/h, but the link to that study is broken. Another study I found is Effects of low speed limits on freeway traffic flow which comes to the conclusion that low speed limits, while increasing the capacity of the road, can also hinder flow (which might be a sign this could increase pollution). But sadly, this one is not for urban environments.

Eine Studie des Allgemeinen Deutschen Automobil Clubs aus dem Jahr 2018 kam zu einem anderen Ergebnis: „Im Ergebnis führt Tempo 30 weder zur Reduzierung der NOx- noch zur Einsparung von CO2-Emissionen, sondern insgesamt sogar zu schlechteren Ergebnissen. Somit ist die Begrenzung der zulässigen Höchstgeschwindigkeit auf 30 km/h keine wirksame Maßnahme zur Senkung der Pkw-Emissionen. Zudem zeigen die Untersuchungsergebnisse, dass auch Dieselfahrzeuge schadstoffarm sein können.

Conclusion

As far as I can tell, it's not completely wrong to say at 30km/h the pollution may be worse than at 50 km/h, but this seems to be only true on a road with no elevation and where lowering the speed limit does not improve traffic flow. As such, a limit of 30km/h seems to be not effective to reduce pollution everywhere, but can be very effective if applied at the right places.

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    About (3.): the question is in an urban environment where traffic flow is already bad. Studies on freeways are unlikely to be relevant. Commented Nov 2, 2022 at 15:15

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