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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?

Gilles 'SO- stop being evil'
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  • Apparently this limit reduction was done in Belgium a decade ago https://trid.trb.org/view/846788 so there should be some data by now. Whether the traffic jams/patterns are as atrocious as in Paris is another matter. And https://gov.wales/sites/default/files/publications/2019-08/the-state-of-the-evidence-on-20mph-speed-limits-with-regards-to-road-safety-active-travel-and-air-pollution-impacts-august-2018.pdf is probably more useful as it summarizes a bunch of studies. – Fizz Aug 29 '21 at 23:16
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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. – Loren Pechtel Aug 29 '21 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 Aug 30 '21 at 07:49
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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. – Weather Vane Aug 30 '21 at 09:44
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    One of the [explicit goals](https://news.yahoo.com/paris-extend-30-kph-speed-171137419.html) 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. – David Hammen Aug 30 '21 at 10:55
  • @DavidHammen In the article you quoted I cannot find this explicit goal formulated in the way you rephrase it in your comment. Which passage are you referring to? – Hartmut Braun Aug 30 '21 at 11:22
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    @HartmutBraun For example, the vice mayor in charge of transportation saying "the point is to reduce the space taken by cars, which involves lowering their speeds." The mayor, vice mayor, and several others in charge of Paris have passed multiple rules (e.g., closing some streets to automobile traffic, reducing lanes available to automobile traffic, reducing public parking spaces for automobiles, and reducing the speed limit) that are intended to make driving an automobile in Paris an extreme pain in the rear. – David Hammen Aug 30 '21 at 11:39
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    Does this answer your question? [Does lowering speed limits by 10 km/h have any impact on air pollution?](https://skeptics.stackexchange.com/questions/19914/does-lowering-speed-limits-by-10-km-h-have-any-impact-on-air-pollution) – DenisS Aug 30 '21 at 13:04
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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. – Gilles 'SO- stop being evil' Aug 30 '21 at 14:59
  • @DavidHammen it seems that way: in my city the speed limits have been reduced to 20mph, bays for bus stops have been removed which prevents cars from travelling faster than buses, and some routes have been made bus-only with others closed to all through traffic where no buses run. – Weather Vane Sep 01 '21 at 12:39
  • @WeatherVane At face value, making cars drive much slower than their optimal speed with regard to fuel consumption (50 to 80 km/hour, with no stops) increases rather than decreases pollution. However, if the rules of the road make driving a complete PITA, maybe, just maybe some people will switch to alternative modes of transportation. That is the intent of draconian rules such as this. – David Hammen Sep 01 '21 at 13:01
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    @DavidHammen Here the comparison is not between 50km/h with no stops and 30km/h with no stops, but between 50km/h (if you managed to reach it) with frequent stops and 30km/h with frequent stops. Few streets go more than ~200m without an intersection which more often than not requires at least slowing down if not stopping due to traffic lights, a need to yield to cross roads or to pedestrians, poor visibility on cross roads, etc. – Gilles 'SO- stop being evil' Sep 01 '21 at 13:23
  • @Gilles'SO-stopbeingevil' It's pretty expletively deleted obvious that if the number of cars on the road stays the same, reducing the speed limit to well under the optimal speed with regard to fuel consumption will increase pollution from automotive traffic. But if that ridiculously low speed limit makes some people reconsider their use of an internal combustion engine for transportation then maybe, just maybe, air pollution from traffic might go down. – David Hammen Sep 01 '21 at 13:39
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    @DavidHammen If it's so obvious, please post a reference to a serious study that validates it. It's not at all obvious to me that reduced emissions during the small fraction of the time that the car is driving at the speed limit compensates for the increased emissions during the acceleration and braking phases. – Gilles 'SO- stop being evil' Sep 01 '21 at 13:52
  • I doubt this can be answered generally. Even if you do real-world measurements, you will get opposite results for similar settings, based on boundary conditions. For example, traffic light control is typically optimized over years for best throughput on main routes; on a typical day you can get from green light to green light with actual 50 km/h flow. It could take months to years to achieve the same degree of optimization with a different speed limit, because any change is followed by an adaption process of car drivers. Thus, there will likely be more stop-and-go for a long time. – Philippos Sep 03 '21 at 11:44
  • @Philippos I don't think traffic light control would be impacted that much. Paris doesn't have many long streets where synchronization makes sense, and has dynamic control [article in French](https://www.leparisien.fr/info-paris-ile-de-france-oise/transports/c-est-ici-que-sont-geres-les-feux-tricolores-de-paris-15-04-2018-7665432.php). – Gilles 'SO- stop being evil' Sep 03 '21 at 12:07
  • So your question is specific to Paris, not as general as the title suggests, with Paris just being the example of the current debate? – Philippos Sep 03 '21 at 13:11
  • @Philippos I'm primarily interested in conditions that are similar to Paris. Other European cities would probably qualify. Suburban areas of US cities designed for cars might not qualify. I'll edit my question to reflect the fact that what I mean by “urban” may not be what a North American means by “urban”. – Gilles 'SO- stop being evil' Sep 03 '21 at 13:38
  • My knowledge of "green flow" traffic light control is based on a couple of historically grown, but smaller (100k–300k population) German towns. According to rumors the Germans could put more effort into keeping their beloved vehicles moving. (-; Plus, main street crossing almost always have traffic lights, unlike Paris with it's big roundabouts. – Philippos Sep 03 '21 at 14:20
  • https://www.radlobby.org/noe/wp-content/uploads/Tempo_30_40_Sammer_Meschik.pdf: See table 4.3-1 for standard deviations of moving speed for 30 and 50 km/h and table 4.4-1 for various emissions. I'm afraid the original publication (which is cited here) is not available online. The tables seems to be based on measurements in Graz/Austria before and after reducing speed limit from 50 to 30 km/h. – Philippos Sep 03 '21 at 14:36
  • gearing is going to have a large impact on fuel consumption: while consumption is influenced by drag, I think gearing ratios will have a bigger impact at the speeds in question. In my vehicle, at about 30kph, 1st gear would be about 4k rpm; 2nd about 2.2 k rpm; 3rd about 1k – Yorik Sep 03 '21 at 18:40
  • @DavidHammen says "*if the number of cars on the road stays the same …*". But it wouldn't. Restrict the situation to a specific section of road. If everyone drives at 30 km/h rather than at 50 km/h, it will take each car longer to traverse the section, so at any instant, there will be 50 cars on a section of road where previously there were only 30 cars. That's a 67% increase in the density of cars and running engines. Efficiency isn't changed by nearly that much, so there is a definite increase in the amount of fuel burned. (If that's not obvious, calculate it for a drop to 1 km/h.) – Ray Butterworth Sep 06 '21 at 07:48
  • @RayButterworth “if the number of cars on the road stays the same”… then the same number of cars still traverse the section every day. It might take longer for them to go through, but there aren't more cars producing more pollution. “Efficiency isn't changed by nearly that much” actually it is. Pollution figures are given per km travelled, not per time, for this reason. – Gilles 'SO- stop being evil' Sep 06 '21 at 08:10
  • @RayButterworth Perhaps I wasn't clear. By *if the number of cars on the road stays the same* I meant assuming that the number of car trips and distance traveled by those car doesn't change. Those cars will of course be spending more time on the road if the speed limit is dropped. – David Hammen Sep 06 '21 at 10:20
  • @DavidHammen, right, I'm the one that wasn't clear. We were using "same number of cars on the road" differently. You meant that if a thousand cars use that road *every day* now, a thousand cars will use that road after the speed change. I meant that at any *instant in time* there would be many more cars on the road. – Ray Butterworth Sep 06 '21 at 12:44
  • @Gilles'SO-stopbeingevil' said "*Pollution figures are given per km travelled, not per time*". But slower doesn't necessarily mean more efficient. As I said, imagine the speed is changed to 1 km/h. And imagine only one car uses the road each day. That car will spend 50 times as long on the road each trip. While traversing a 1 km section of road, the overhead of its continuously running internal combustion engine will use far far more fuel in 50 minutes than it would have in 1 minute going 50 times faster. – Ray Butterworth Sep 06 '21 at 12:54
  • @RayButterworth This is very likely true for such extreme figures as 1km/h vs 50km/h, but it's not true, for example, [for 70km/h vs 140km/h](https://skeptics.stackexchange.com/questions/19914/does-lowering-speed-limits-by-10-km-h-have-any-impact-on-air-pollution): 140km/h uses more fuel per distance travelled, meaning that it uses more than twice as much fuel per unit of time. – Gilles 'SO- stop being evil' Sep 06 '21 at 13:14
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    @Gilles'SO-stopbeingevil', at 140 km/h most of the excess fuel is used to overcome wind resistance. This is a non-linear factor though, and the effect at 30 or 50 km/h isn't very significant. At slow speeds, much of the fuel is spent simply keeping the engine turning over. That's one reason why, when driven properly, the Prius was able to be so fuel efficient, with significant intervals where the engine is shut off and the car glides (as if in neutral). – Ray Butterworth Sep 06 '21 at 13:47
  • A Welsh government assessment of the evidence - https://gov.wales/sites/default/files/publications/2019-08/the-state-of-the-evidence-on-20mph-speed-limits-with-regards-to-road-safety-active-travel-and-air-pollution-impacts-august-2018.pdf - concluded "air quality is likely to be improved as a result of 20mph speed limits but the evidence is weak." Someone more motivated may find useful information in the studies cited in that report. (30mph-20mph is similar to the 50kph->30kph in the question) – Jack Aidley Sep 08 '21 at 12:52
  • Are French drivers more conscientious of the posted speed limit? Why I ask is that in the US, in the urban/sub-urban environment in which I live, the posted speed limits have _almost_ no relationship to the speeds at which traffic flows. Traffic flows at the crowd-sourced speed limit of just under safe. (i.e. a highway may be posted 45, or even 55, but if it's a clear day, _traffic_ will invariably accelerate to a rate that is _much_ higher. This is true even for those stretches of road that are not constrained by block-spaced lights. – CGCampbell Sep 22 '21 at 15:23
  • Hahaha no, but many will do 50 in a 30 instead of 70 in a 50. And when there's enough traffic (i.e. most of the time on busy streets) they'll often be held behind one of the minority of conscientious drivers. – Gilles 'SO- stop being evil' Sep 22 '21 at 16:35

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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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