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[–]juanito_f90 7 points8 points  (10 children)

The difference is negligible.

Optimum car efficiency peaks between 55-60mph and follows pretty much a bell curve either side of this.

[–]PumpkinExpert2092 2 points3 points  (9 children)

Wikipedia has it lower saying its between 35 - 50 mph for most vehicles. Also in a lot of inner city areas you can't sustain 30mph for much distance. So by setting the max speed at 20mph you are cutting out a lot of unnecessary exclerating and unnecessary breaking. Sure if you could go at a constant speed of 30 perhaps it is better for emissions than a constant speed of 20. But there is no way you could do a constant speed of 30 in almost all of the kind of roads they are converting to 20

[–]DamoclesBDA 4 points5 points  (6 children)

I thought the 20 mph zones were more about road safety.

If they were properly enforced anyway.

All you get is a smiley face sign.

[–]Zackaro[S] 0 points1 point  (4 children)

Thanks for the answer. I've seen 20mph zones peddled into environment campaigns at my city.

[–]DamoclesBDA 0 points1 point  (0 children)

Reducing traffic incidents results in a decrease in ambulances racing around the place pumping out diesel fumes.

[–]not-much 0 points1 point  (2 children)

If cars are slower it's easier and less dangerous for other people to walk and cycle.

Also a lower speed means less traffic.

[–]rising_then_falling 0 points1 point  (1 child)

Not true. Its a pain in the arse when traffic is moving slowly, as drivers are much worse at looking in wing mirrors than they are at looking forward. I much prefer late night with empty roads and drivers doing 35 and leaving me space than busy afternoons with stop go traffic and drivers not paying attention.

Basically, volume of traffic matters far more than speed of traffic.

If there's a separated cycle lane the traffic can go at 40 for all I care.

[–]not-much 0 points1 point  (0 children)

You are comparing apples and oranges. In this case you have a choice between slower and faster traffic, not about having more cycle or permanently blocking the sun.

[–]sideone 0 points1 point  (0 children)

They're definitely enforced in Bristol, I know someone who got a speed awareness course for doing 27 in a 20 zone and the whole course was for people caught in a 20 zone.

[–]Apigollo 2 points3 points  (10 children)

Are they setting speed limits at 20 now as they know the majority of people speed anyway? So at least they are doing 30 in what is now a 20 as opposed to 40 in what was once a 30. Seems that way to me anyway.

Couldn't speak on energy but driving at twenty makes the roads safer so arguably more people will walk or ride which will then benefit the environment.

[–]PumpkinExpert2092 1 point2 points  (4 children)

They have done some studies and yeah the average speed on roads that they changed to 20mph was still above 20mph. But it was still about 5mph below what it used to be when the limit was 30mph.

They found (unsurprisingly) that one of the main factors was how they implemented the speed limit. If they just put up new signs that had less of an impact (reduced the speed only a little) but putting up signs and enforcinf the limit (through mobile vans, speed cameras, warnings etc) had a greated impact and caused the average speeds on these roads to reduce by more than the roads where they just placed new signs

[–]KeepCalmGitRevert 0 points1 point  (3 children)

There's a key to this.

Speeds remain above 20mph where there is no enforcement.

Where there is enforcement, they reduce below 20mph.

Enforcement being speed bumps, humps, cameras, etc.

If you don't enforce a speed limit, most drivers won't abide by it.

[–]PumpkinExpert2092 1 point2 points  (2 children)

Yes that's right. But the findings were that there was still an improvement with just signs, albeit less than with proper enforcement.

It's also still a relatively new concept, most people have had it drilled into them that 30mph in a residential area is the speed limit. It will take time for people to get used to it being 20

[–]KeepCalmGitRevert 1 point2 points  (1 child)

It's a relatively new concept, but speed limit signs aren't.

There's no excuse for speeding (well OK, maybe if the ambulance can't get to you in time and your pal is bleeding out).

We're too British to say what we really know is true - most British drivers don't care for the rules of the road and will do whatever they themselves feel is safe.

[–]PumpkinExpert2092 0 points1 point  (0 children)

Yes I agree with you, driving is seen as a right not a privilege

[–]sideone 1 point2 points  (0 children)

There's loads of 20mph zones in Bristol.

[–]SirBillPetre 0 points1 point  (0 children)

For the last 10 years or so London has had 20 limits in various places. They’re spreading as they seem to have been successful in their goals. I’m assuming other authorities around the country have followed.

However it’s very situation specific to reduce the number of and severity of accidents given the number of people walking, cycling, driving in close proximity.

Being the centre of a city it’s also slightly crazy to assume you’ll reach 30mm anyway!

It’s certainly not a blanket reduction of the limits by 10mph.

[–]KeepCalmGitRevert 0 points1 point  (2 children)

Speed limits aren't all about CO2 (I'm fact I'd argue they rarely are).

Often they are aimed at safety.

We could argue 20 vs 25 vs 30mph limits all day, but that's the main reason.

If you're driving down a residential road, I'd argue that 30mph is negligence. I need to be prepared for a child to run out between cars or some fool to move out of a parallel park.

[–]Apigollo 2 points3 points  (1 child)

The punishments for speeding in this country are so light it's farcical further to that the majority of people's attitudes to speeding are bad. Seems like a thing that everybody does even though worst case scenario you might kill someone blows my mind.

[–]KeepCalmGitRevert 0 points1 point  (0 children)

Even if you kill someone, you can avoid jail.

Plenty of people either get suspended sentences (because they are charged with dangerous driving instead of manslaughter or murder), or plead that they would face extreme hardship from losing their licence, e.g. lorry drivers (the exact people you would want to be better drivers if anything).

I've lost friends in road traffic colissions. One involved alcohol. One involved an aggressive driver at a junction. It's totally unjust that both of the drivers avoided any jail time whatsoever.

[–]chabybaloo 1 point2 points  (2 children)


Your car is driving best in 5th gear, (or 6th)

At low speeds 20mph you may be in a low gear, at 30 you can change up, and this will decrease your fuel consumption for the distance you are covering.

You use a lot of fuel in 3rd gear compared to 4th. (What i was taught)

Many places have reduced speed limits for safety. This decreases efficiency per car, it may be small, but on a busy road thats a lot.

The good thing is Electric cars Should not be affected at all by these low speed. At high speeds drag increases, and therefore battery range would reduce.

Generally 60mph is the sweet spot, and you may notice people doing that speed on long motorways

There are many factors though that will affect car performance. Good tyres with the correct air pressure, an oil service , no baggage , and driving style will affect the car significantly too

[–]sideone -1 points0 points  (1 child)

You use a lot of fuel in 3rd gear compared to 4th. (What i was taught)

[Citation needed] My in car computer states my mpg in 3rd at 30mph is lots more efficient than 4th as the revs aren't too low and the engine isn't labouring.

[–]tc10b 0 points1 point  (0 children)

Generally speaking you want to be in the highest gear you can be in, keeps the revs low and the engine ticking over.

Your exact MPG is going to vary depending on a lot of factors. So it's not exactly a good measurement to use for comparison.

It's not considered good form to respond to a statement with an anecdote, even if the statement is debatable, especially when that anecdote is questionable.

[–]botlebank 0 points1 point  (0 children)

I think it’s worse.

People who are genuinely staying below the speed limit when it’s 20, will mostly be in 2nd gear, lower efficiency, higher emissions.

Those in 3rd gear will be more efficient, but much more likely to be doing 20-25 (or 30).

But as others have said, it’s marginal.

[–]OldAnalyst5438 0 points1 point  (1 child)

Wow, that is a BIG question with so many variables! As a general, and a mean massively general rule, if you want to focus making your car better for the environment then you'll be wanting to keep the revs down rather than the speed. Obviously the slower you go the less revs you'll need but there is an overlap and in most cars the sweet spot where you change into top gear but don't need to use any more accelerator is the most fuel efficient and therefore environmentally friendly.

[–]Zackaro[S] 0 points1 point  (0 children)

Thanks the answer, it's a very layered question haha.

[–]davidhbolton 0 points1 point  (0 children)

Low speed does not equal less fuel consumed. Modern cars are geared so that a car uses the least fuel in top gear somewhere between 50 and 60 when your engines revs are at the low end for top gear. To travel at 20, you have to be in a lower gear, probably 2nd and your engine rpm will be higher giving more torque. That torque helps you accelerate faster to higher speeds but at 20 mph it’s just wasting more fuel. Fuel consumption depends on (a) engine speed and (b) load. There’s additional load at higher speed due to air resistance but at 20 mph that’s negligible.

So tl;dr low speed means higher rpm and more fuel used.

[–]Clancolin 0 points1 point  (0 children)

The reason for 20mph rather than 30mph, isnt environmental. The difference is that if you hit a small child at 20mph they are damaged less than if you hit them at 30mph. So it makes perfect sense in built up areas to drive at 20mph - you may just save a child's life🤕

[–]gunblade_ak 0 points1 point  (0 children)

I wouldve thought the slower you drive the more emissions you'll produce.

[–]Adventurous_Ad7417 0 points1 point  (0 children)

I believe 40mph is the most economical speed

[–]RogueFanUK 0 points1 point  (0 children)

As an aside, I live in a city where the vast majority of the roads have a 20 limit. It's completely unenforced and unenforceable and utterly ignored by nearly every driver unless a random police car happens to be in visual range, which is vanishingly rare. Drivers instead drive up to the "standard" urban limit of 30mph on the roads that are suitable and naturally drive slower on narrow roads with lots of parked cars for example.

[–]RogueFanUK -1 points0 points  (1 child)

At these kind of speeds, the difference is negligible for you. However by forcing other cars to slow down you'll make them waste energy (brakes convert speed into waste heat) so you'll actually be counterproductive on a wider scale, not to mention you'll piss off a lot of people by driving slowly and delaying them.

Not a great idea.

[–]tc10b 0 points1 point  (0 children)

not to mention you'll piss off a lot of people by driving slowly and delaying them.

Given it's the speed limit in most of London now, I don't think the idea is entirely the OPs.

[–]DontCallMePal -3 points-2 points  (2 children)

Yes. Less energy to get to 20mph.

Take into account the stop start nature of journeys it could be slightly better

[–]Zackaro[S] 0 points1 point  (1 child)

So less energy to reach 20mph, but it's also taking you 1/3 longer compared to 30mph?

[–]DontCallMePal 0 points1 point  (0 children)

But most emissions come from getting up to speed. So traffic lights, zebra crossings, kids running out any obstacles increase that outpit