New research on emissions strengthens case for a 20mph default urban speed limit

New research[1] from engineering consultants, Future Transport, models the impact of capping speeds at 20mph vs. 30mph. This “real life” modelling that takes account of the stop/start nature of urban traffic yields a very different result from traditional steady-state models. It shows significant and substantial reductions in emissions: CO2 lower by 26% and NOx 28% lower. With UK hosting COP26, campaigners are calling on governments to set 20mph or 30km/h limits as national urban/village defaults.

Although the auto industry is fully aware of the impact of acceleration on vehicle emissions, it does not publish the results. Basic physics means that 2.25 times more energy is required to reach 30mph than 20mph. When this is repeated in the real-world environment, where we slow down at junctions, crossings, congestion points and other hazards, acceleration becomes the dominant factor in overall journey emissions.

Future Transport modelled the CO2 and NOx emissions for accelerating from stationary to between 5 and 50 mph for a number of vehicles, with the following results for a petrol Ford Focus.


Repeated acceleration and braking, rather than steady-state, represents a far better modelling of real-world emissions in our congested cities and towns.

Wales is already planning to change the national urban default limit of 30mph to 20mph by 2023. Besides “liveability” and casualty reduction, transport carbon reduction[2] is also cited as a reason to change. The Scottish Government[3] has announced its plans to set 20mph as a norm across the country from 2025.

Speed limit reductions are a key initiative to reduce climate-warming CO2 emissions and harmful NOx. They also have a significant effect on public health through air quality improvements and active travel.

Rod King MBE, Founder and Campaign Director for 20’s Plenty for Us commented

“It is clear that repeated acceleration dominates emissions in town driving. This research quantifies the effect and shows how reducing maximum speeds can have a significant beneficial effect on emissions. It’s time for all governments to say 20’s Plenty for the planet and for our health. With COP26 approaching it is an effective step towards transport carbon reduction”




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  • Rod King
    commented 2022-10-03 19:51:52 +0100
    Thanks for your comment Tim. Its always useful to have a different perspective.
  • Tim Coote
    commented 2022-09-22 11:30:30 +0100
    The paper isn’t really research, it’s an exercise in modelling/simulation – something that I used to do professionally. Although the paper describes some model validation, this is limited to vehicle performance, rather than actual driving patterns. As a modelling exercise, it is intended to try to simulate ‘typical’ London traffic, although it is not clear whether that’s a typical period of time, or driving experience. I would expect any modelling that elaborates a ‘typical’ scenario to also provide a credible interval (to use a Bayesian term), as do the IPCC climate change scenarios. It’s very debateable as to how typical typical London traffic conditions are for most urban driving. As there is no relevant validation, it is not possible to know how representative the results would be for actual driving.

    The results are not validated at all against actual driver behaviour. Some of the discussion is wrong, and, rather typically, shrouded in unnecessarily complex maths that is simultaneously over-simplistic (e.g. the discussion on how far a vehicle would have to accelerate to a higher speed to be more efficient than a car limited to 20mph.

    There is value in educating drivers on how to drive efficiently. However, it’s debatable whether the best overall/ethical outcome arises from imposing regulations on peripheral characteristics (the speed limit, all the time), rather than, say appealing to the drivers’ pocket.

    For the most part, the environmental and health impacts of driving result from specific locations where road layouts are not suitable, rather than large areas (e.g. look at the locations of STATS19 incidents to make this very evident from an accident point of view). And technology continues to improve both the impact in both dimensions.
  • Rod King
    commented 2022-08-22 10:52:35 +0100
    Thank you Chris

    I will bypass your first two “firstlies”, but if I can comments on your “secondly” (or is it “thirdly”), the emissions in the article are measured in grms/km. Hence a reduction of 25% per km is also a reduction of 25% per trip.
  • Chris Bridgland
    commented 2022-08-15 13:43:55 +0100
    Firstly, I do understand the need for speed limits where is makes sense and Cities and commercial towns need to consider this carefully, but there are 2 items that aren’t addressed.

    Firstly, much of this is about education. I grew up with the Green Cross Code Man, Charley Says series and films about things such as playing in a farm. The lack of such teachings like these on our school curriculums, televisions and radios, to be replaced by beauty products, bladder control and erectile dysfunction advertising is such a missed opportunity for our government.

    Secondly, this research misses the effect on increased travel times and associated stress levels that can impact some people in a services environment, such as carers who need their vehicles to get around, or Commercial/HGV delivery drivers who need to complete their schedules. A 25% reduction in emissions but a 33.3% increase in travel times. The public transport in this country is still too inconvenient for many to consider as an alternative, as some councils, such as Kingston-upon-Thames, use this argument as one of their reasons for the breadth of their 20MPH limits that they have applied.
  • Rod King
    commented 2022-07-14 20:52:42 +0100
    Thank you Andrew. I assure you that the researchers are very well acquainted with the design of modern cars, but would challenge the idea that they are designed to run in top gear at 30mph. I am not sure where you obtained that information. And of course the whole point of gears is that you can run at a slower speed and maintain the same revs. That is what gearboxes do. And engine management systems meter fuel to the engine based on load and revs in order to burn as leanly as possible.
  • Andrew Maskell
    commented 2022-07-12 13:30:05 +0100
    Does your research take account of the fact that modern cars are deigned to run in top gear at 30 miles perhour but have to change to a much lower gear at 20 mph, thereby using more fuel and therefore producing more pollution? I think your conclusion is too simplistic and wrong.
  • Rod King
    published this page in Briefings 2021-10-05 22:08:13 +0100
  • Rod King
    published this page in Briefings 2021-10-04 19:58:59 +0100