[Closed] 20mph speed limit vs pollution???

I can't imagine 20mph speed limits being much worse than 30 for air pollution. But speed bumps are notoriously bad for it, as it causes cars to speed up and slow down a lot, which is obviously an inefficient way of driving. I've heard there are schemes to remove a lot of them so they the government can hit their target to reduce emissions.

20mph in busy areas reduces the amount of accelerating and decelerating that occurs, reducing pollution. Far more effective than using street furniture to stop high speeds. It relies on us drivers sticking to 20 though, which all surveys show we do not.

How on earth do lower speeds create more pollution?

How on earth do lower speeds create more pollution?

I can see how folk could come to that conclusion*. At 20mph, it takes longer to drive along a street than at 30mph, so the car is there longer and is therefore polluting the air more. Simples!  😉

I guess there's also the thought that car engines might be "designed" to be more efficient at certain speeds/power levels.

The other factor involved in slowing down motor vehicles is that if it's safer/less stressful to cycle, more folk will choose that option, further reducing vehicle emissions.

*I'm not amongst them

my actual evidence ie increased use of medication when commuting or riding in slow moving traffic

That's evidence that you increase your use of medication, not that pollution is worse.

slow moving cars operating inefficiently creating increase levels of pollution

What evidence have you got that this is actually a thing? Why is driving at 20mph less efficient than driving at 30mph? Assuming competant use of gears.

The RAC tried to prove this point. Selecting the cars that would best show it and trying to fiddle the figures. Even so in the end they had to admit that in city conditions 20 mph reduces fuel use and thus pollution.

my actual evidence ie increased use of medication when commuting or riding in slow moving traffic

I imagine this is because there is more traffic rather than more pollution per car. Your understanding of the term "evidence" seems different to mine also.

My Zoé produces less pollution the slower it goes. That's the solution.

Petrol/diesel cars are most fuel efficient at around 70kmh at constant speed. But constantly accelerating to 70 and stopping over short distances uses more fuel than constantly accelerating to 30 and stopping over short distances. So it depends on the frequency of stopping.

Its much nicer to walk and cycle in 20mph zones. As a cyclist you can ride at a similar speed to everyone else and its easier to pull out at junctions. As a pedestrian its easier to cross roads as you don't need to wait for a big gap. Hopefully this will reduce car use and consequently pollution. As a like for like comparison I doubt there is much difference in car efficiency but the lower limit does allow for smoother driving.

Not sure why you put "save lives" in quotes OP, it seems like a good enough reason to be lowering speed limits, irrespective of any environmental benefits.

Our area has had a blanket 20 limit implemented not so long ago, personally I'm in favour a lot of the local "progress makers" are up in arms (although we were informed by letter Almost a year before). Several schools and lots of kids in the area...

Of course almost everyone exceeds 20 and none of the speed cameras have been adjusted, so it's really just window dressing...

They exceed it here too but now they are doing 25-30 so basically obeying the old 30 limit. Its only occasionally that you get someone doing 35 and its very noticeable, whereas before the 20 limit that was the norm.

I have found that 20 mph speed limits have certainly changed the behavior of drivers overall. More folk who wait behind me on my bike rather than pushing past and more folk not following to close. Its made zero difference to the idiots but the decent drivers have improved.

Didn’t that RAC survey also prove that slower moving traffic creates more slower moving traffic, hence more pollution ?

🤷‍♂️

All towns should be 20mph max, no question. I live in a town with one of the few busy main streets, loads of indy shops, bars, cafes etc. When the Glasgow Express bus rattles by at 35mph, a few feet from pedestrians, makes me wince a bit.

Stopping distance of such a beast must be a fair bit.

There are a lot of stupid people who claim that 20mph is the “wrong speed” for their car, forcing them to use a lower than appropriate gear.

It’s a load of rubbish. They see it as an attack on their civil liberties and will break the 20mph limit as a matter of principle.

"I have found that 20 mph speed limits have certainly changed the behavior of drivers overall. More folk who wait behind me on my bike rather than pushing past and more folk not following to close. Its made zero difference to the idiots but the decent drivers have improved."
I agree with all of that.
As for the pollution ,the 20 mph zone allows* the traffic to at least flow steadily rather than sit and idle. *usually.
But I don't think pollution is a valid reason to oppose a 20 zone.
Hopefully the road adjacent to ours will be a 20 soon. Sadly it has taken a tragically young death to hasten it.
🙁

There are a lot of stupid people who claim that 20mph is the “wrong speed” for their car, forcing them to use a lower than appropriate gear.

Well, the car does use a lower gear doing 20 vs 30. Main impact seems to be to slow the 40 in a 30 drivers to 30 in a 20. The main road safety improvement needed round here though would be for pedestrians to look before stepping off the pavement. So many just don't.

As for the pollution ,the 20 mph zone allows* the traffic to at least flow steadily rather than sit and idle. *usually.

^^this^^

Stationary idling is still the major contributor for urban pollution from cars innit? and driving faster won't actually solve the queues.

For those really concerned about the environmental impact of "slow driving" there is a nice straight 30 limit A-road parallel to our estate where drivers can (and most do) boot it up to 35-40 for 1/4 of a mile, sit and idle at some lights and then repeat the whole rewarding exercise.

So they don't have to use that 20 limit rat run through all the housing and past schools, and yet many do...

Of course if rush hour air quality is a genuine concern there's always walking, EVs, Hybrids, public transport and (gulp) bicycles as less fumey urban transport options, maybe we could start trying persuading people to start using these (at 20 mph) both to make the air breathable and limit the chances of RTIs...

All a bit moot really as in 10-15 years most cars will be electric and road side pollution will minimal.

I'd like to see e-bikes limited to 30kmh rather than 25kmh, then ther would be no reason to overtake e-bikes in 20mph limits (I know the units are mixed but 25kmh is what in mph)

I think a very good way of decreasing air pollution would be to implement much smarter ways of traffic lights working - for example, a set of traffic lights at a busy junction at rush hour makes sense. That same set of traffic lights at 2am on a Sunday morning still needlessly stopping cars just because of the cycle is daft. I think the American approach of allowing cars to turn right at many red lights is sensible - they are just filtering into a road. We could do that here very easily, then it could be extended to have lights at junctions at quieter periods to flash in a new sequence to alert drivers that they can proceed if safe to do so (and follow normal junction rules). In Harrogate there is a set of lights at the bottom of Parliament Street/Ripon Road/Kings Road which can cause some pretty bad delays at times but a couple of years ago they broke so everyone just approached with caution and the traffic moved much more smoothly.

A lot of particulate pollution is from brake and tyre dust, plus from wear and tear of the road surface/markings. These will be hugely reduced by 20 limits compared to 30.

Interesting take on the fuel efficiency element here:
https://www.quora.com/Which-consumes-more-fuel-driving-fast-or-driving-slow-and-why

Obvs fuel efficiency is only part of the story.

ie increased use of medication when commuting or riding in slow moving traffic

Is that actually traffic moving at 20mph or is it really traffic, nose to tail, in a 30mph zone moving in very short bursts (commonly of only a few mph) before becoming stationery again?

Lower limits reduce congestion when followed (hence the smart motorways reducing the speed limit). More congested traffic produces more pollution than less congested traffic for the same volume of vehicles, so because a 20mph limit reduces congestion, its less polluting than a 30 limit.

What's really needed is consistent and wide spread limits, having a 20mph limit over a small area causes bunching and encourages acceleration and braking so isn't great for pollution. A blanket 20mph limit in all built up areas would be pretty effective at pollution reduction.

I guess there’s also the thought that car engines might be “designed” to be more efficient at certain speeds/power levels.

Back in the days of carburetors, this was a much bigger thing because carburetors were optimized for specific flow volumes of air and the engines didn't run well at low revs, especially if you floored the throttle and dumped a load of gas into the engine. Modern electronically injected engines have much better control of the fuel-air mixtures, so I find it difficult to believe that 20 mph is going to cause any problems. 2 mph, maybe, but 20 seems unlikely.

Yep it's idling and sharp accelerations that cause most of the pollutants that will agitate asthma. Nice video highlighting NOx spikes because of a speed bump. So yes the best approach is a consistent lower speed limit with good traffic modelling and signal control (and removal of speed bumps) to have a smooth flow of traffic.

Nice video highlighting NOx spikes because of a speed bump.

Small world & rather pointless fact....I went for a job interview there about 15 yrs ago. Didn't get the job though....

cookeaa

Subscriber

Not sure why you put “save lives” in quotes OP, it seems like a good enough reason to be lowering speed limits, irrespective of any environmental benefits.

I recall there were a couple of statistics showing that 20mph limits had increased deaths/serious injuries. Not sure if that's a widespread thing or just specific to those schemes, or whether it was a sustained thing or a one-off.

But speed bumps are notoriously bad for it, as it causes cars to speed up and slow down a lot,

I have a speed table outside of my house and another 100 m down the road. The number of shagwits down my road who tear between the two and brake hard is amazing. They don’t seem to be clever enough to work out that’s its not only bad for the environment, but can’t be doing their car much good either.

and removal of speed bumps) to have a smooth flow of traffic.

Of course, there’s no need to brake at all for a speed bump if you’re actually driving below or at least close to the 20mph speed limit and the bumps are properly profiled. The problem is that most people are such poor drivers. You can drive smoothly over speed bumps simply by easing your foot off the accelerator and letting the car slow itself rather than braking (except perhaps downhill).

Of course, there’s no need to brake at all for a speed bump if you’re actually driving below or at least close to the 20mph speed limit and the bumps are properly profiled

That is the problem - they often are not - around us there are roads where every one of the bumps is fine to take at the speed limit but one will happily rip your exhaust off. For anyone in Harrogate, the one I specifically dislike is Stonefall Avenue.

Surely, the slower you go, the less power is required? And less power equates to less pollution?

I'm willing to accept that some cars may have an optimum range, which may or may not exceed this. However, the difference between 20mph and 30mph has got to be small. It's the acceleration involved in urban driving that increases fuel consumption and pollution at these speeds, so even if it was marginally worse (and it will be marginal at best), any increase at 20mph is likely to be offset by the lack of acceleration to reach this speed.

And of course, as mentioned, it makes for a nicer, safer environment, which then encourages people to walk, cycle, etc... So the end result is less cars.

It's an odd world that we've created. Or rather, it's odd that we're so resistant to moving away from a model that is so hostile. We've lived with it for so long, and become so used to it, we no longer see the negatives in the constant droning background noise, the pollution, the lack of safe places for our kids to play...

Yes butcher the logic is sound - lower speed requires less energy. Saw an infographic recently that pointed out that 90ish% of the energy burnt by a car is to move itself due to the weight of the car. But its not the same energy requirement from 10 to 20 as 20 to 30, it's double.

also the applying of brakes is polluting (brake dust is mega-noxious) so lower speeds help with this as well, a well timed foot off the accelerator without touching the brakes to go over a speedbump is possible at 20mph but less possible at 30mph.

lowering the speed limit increases road capacity as well because the braking distances reduce.

Subscriber
Didn’t that RAC survey also prove that slower moving traffic creates more slower moving traffic, hence more pollution ?

It may have done but its bollox. Slower speed limits allow more cars per hour past a point so traffic flows increase and also the concertina effect is minimised

Petrol/diesel cars are most fuel efficient at around 70kmh at constant speed

Just to be clear that's not true.

Back in the day Leyland got into trouble for saying the metro would do 100mpg. Trouble was it was at a constant and therefore considered unrealistic

The slower you go the less the resistance so the less energy needed. There must be a speed where the efficiency stars to offset this in my experience is less than 20 mph

Hybrids and fully electric cars produce less air pollution at lower speeds.

Just to be clear that’s not true.

It is and you should really type something like "most fuel efficent speed" into Google before rubbishing people.

https://www.mpgforspeed.com/

And many many other sites.

Or maybe we should all drive around with a person walking in front of us waving a red flag.

The widely quoted UWE study on the Bristol 20mph limit is shonky research, yet is repeated by the media as if it's been brought down from Mount Sinai by Moses. I quote:

'Because of the study design employed, it is not possible to state with certainty that the reductions in casualties are due to the introduction of the 20mph limits.'

I now await the rabidly anti-car faction of STW's responses.

XX
JP

It is and you should really type something like “most fuel efficent speed” into Google before rubbishing people.

Surely that is going to be very dependent on the vehicle?

A van has a much higher CdA than say a Prius, and a Mondeo somewhere in between? So I bet the Van's graph is skewed much more to the lower speeds simply because the point at which air resistance becomes the dominant factor over engine efficiency and rolling resistance will come much earlier, and the Prius to the higher end for the opposite reason.

But that graph shows it pretty much flat from 20-50mph, which even if it's true in the lab isn't true in the real world. As the energy spent accelerating and braking is much higher the faster you go (E=0.5mv^2) and you never manage to drive at an exactly constant speed.

Surely that is going to be very dependent on the vehicle

Those calculations rely on the fact that most fuel loss is down to things that don't change when the car is moving. Around 60% of the energy in petrol is lost to heat, engine resistance etc. So if you are doing 0 your mileage is terrible. If you go 1mph your mileage is 90% better and so on. Eventually you hit a speed where your fixed losses are spread over enough mph as to be negligible per mile, at which point the variable losses (Tyre/air resistance etc) become more important. I think that is the basis of that graph. Although it doesnt appear to be on the website it is attributed to.
The rest is a bit meaningless. In the US, the national speed limit on highways was 55mph. The CAFE legislation forced manufacturers to achieve certain levels so cars were geared to be as efficient as possible at 55mph to offset the terrible efficiency the 5 and a bit litre performance cars turned out. Pick a different speed and design the gears for it and you'll get different results.

I posted a link, thisisnotaspoon, read down and you'll find graphs for various cars including the Prius but only for over 50mph, the speed beyond which nearly all cars get les economical.

I've tried myself in cars with instantaneous consumption readouts in different gears and different speeds. The lowest consumption has been around 70kmh in every car I've tried, then each time I have to change down because the engine will no longer pull the fuel consumption gets worse. For example in a TCE Renault 50kmh (30mph) requires 4th and consumption is much worse than 55 kmh in 5th, 30kmh (20mph) requires 3rd and consumption is worse again. Try it.

That's only at constant speed, other posters have given good explanations of why in stop-start situations much lower speeds are more economical.

I'm very much in favour of 20mph limits and the fuel economy argument against is false in the stop-start urban areas where 20mph limits are applied.

Or maybe we should all drive around with a person walking in front of us waving a red flag.

...

I now await the rabidly anti-car faction of STW’s responses.

The type of comments opposing these schemes are not too dissimilar to those that generally support Brexit... i.e. Attack thy enemy with petty insults and omit any meaningful dialogue.

I've said this on here before, but I've been an active member of several car clubs, attending meets, shows, charity events and all the rest of it. I was even admin on one of the forums for a while. Spend plenty of time in my car as well as under it. Cars have been a big part of my life and I have a lot of love for them, as well as a lot of reliance living fairly rural.

Supporting schemes that have the potential to improve peoples lives does not make you the 'rabid anti-car faction' of anywhere. The evidence you supply yourself against, suggests that safety has improved. Not only that, people will feel safer and more relaxed around slower moving vehicles. They will be subject to less pollution and less noise.

What are the actual downsides?

Ming the Merciless

Subscriber
All a bit moot really as in 10-15 years most cars will be electric and road side pollution will minimal.

no they wont. You will find a large proportion of gasoline vehicles will be a form of hybrid, but the ICE is not going away. There are some very sensible predictions about this that are accepted by the EU (and other larger organisations). As the population increases, the number of cars produced will continue to rise, but only a moderate % of those will be fully electric, the prediction is effectively a net effect based on today's outputs.

Diesel will also continue, and is important in keeping emissions controlled based on how we transport today (it's more efficient and less polluting). New technologies are actively being looked at, unfortunately most are cost prohibitive at the moment and still in lab/pilot stages, which will take many years to fruit.

Anyway, back at hand and speed vs pollution - there are sweet spots with any vehicle, and all vehicles are different, even within the same class, so you cannot make a broad brush assumption. the problem with lower speeds, is that most aftertreatment systems will simply not work, they need heat and load which you don't get at low speed, this is the reason why diesels are c8p at small intermittent journeys, likewise, gasoline particulate filters are experiencing similar issues due to the typical driving cycles associated with slow driving/school runs. Not an issue, if come in from a Aroad/Mway run, and then back out, but continued slow driving will cause higher particulates.

Or maybe we should all drive around with a person walking in front of us waving a red flag.

An aside but that's pretty much what happens on construction sites when vehicles are around workers, yet those same 4 axle tipper trucks then make their way through crowded city streets frequently blocking pedestrian crossings etc etc. Also think of the enhanced visibility, low, cabs that have been used for years on Bin lorries (because they're in close proximity to people working), while other pedestrians.

cars with instantaneous consumption readouts in different gears and different speeds.

i've not looked at the instantaneous consumption (i took that off the display as it's pretty useless) but our new petrol 7speed DSG volkswagen is presuambely programmed for optimum fuel consumption/emissions and heads straight for the higher gears. I'm pretty sure it can be in 4th at 20mph (but of course it can shift down in an instant when you call for more speed).

cars will be electric and road side pollution will minimal.

Not really - not least because about 50% of particulate pollution comes from brakes and tyres. Regenerative braking helps a little but electric and hybrid cars are heavier than their ICE equivalents at the moment (and that's not likely to change) which offsets the benefit (as i understand it).

typical driving cycles associated with slow driving/school runs.

and you get back to the answer being fewer cars, not slightly less polluting cars. 80% of UK car trips are under 5 miles, 23% are less than 1 mile, average occupancy is below 1.5 people.

...typical driving cycles associated with slow driving/school runs.

The reason for many school runs is that parents are terrified to let their children out alone because of the danger created by the traffic.

Regenerative braking helps a lot, Simon. Take a look at the front wheels of the next Zoé you see and compare with other vehicles for brake dust. At the current wear rate I doubt I'll ever need to change the pads, and if I do it'll be because the discs have rusted. At the point the road brakes start working, < 7-10kmh, there's not much kinetic energy left to dissipate. However, max torque at low speed means the front tyres have a hard life, again when you see a Zoé have a look, the front tyres will probably look feathered due to rapid starts.

and you get back to the answer being fewer cars, not slightly less polluting cars.

Yep, the same answer for most environmental issues. Reduce flights to one per person per year and the pollution caused by aircraft is a fraction of what it was without making any other changes
Reduce the worlds population by 50% and a whole load of environmental issues will be massively reduced. Of course nobody is ever going to do that as it is too hard, goes against teh wishes of capitalism and so on and will never happen.

Could we make it two flights per year for those that don't want to travel over Hogmanay?

Regenerative braking helps a lot, Simon.

Hmmm. https://eandt.theiet.org/content/articles/2017/03/particle-pollution-from-electric-cars-could-be-worse-than-from-diesel-ones/

Several readers of E&T have questioned Roy Harrison’s comments, based on the fact that they do not appear to take into account the use by electric vehicles of regenerative braking. Professor Harrison responds:

So, the answer, as ever, is "its a bit more complex than that". I suspect your Zoe is really good but a Tesla, or Hybrid Range Rover or Porsche not so much.

I'm intrigued by our new (petrol) VW Caddy braking "Regenerative braking is based upon the idea that vehicles aiming to save energy should attempt to recover as much of it as possible. Regenerative braking means that the energy which is usually lost as a result of braking is, instead, stored. This can then be used in starting or accelerating the van" and to know if this has any material impact on brake wear

I'm all for 20mph in cities. You rarely end up travelling faster than 20mph overall anyway.

Ok I'll accept that most efficient speed is more than I thought. But it is less than 70 km/h

https://www.telegraph.co.uk/news/2109539/Fuel-prices-Slower-driving-can-save-motorists-500-a-year.html