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the time difference is negligible
Yes, at these speeds we can safely ignore relativistic effects.
No negatives at all despite what the gammon predicted
Are you coining a new collective noun here? I like it!
That’s not what I’ve read – it applies to residential streets where the streetlights are less than 200 metres apart.
Those are the roads where 30mph is the current UK-wide default legal speed limit, ie local authorities do not need to pass by-laws to set 30 as the speed limit but they would to make it 20 or 40. They still put up signs though. I assume the proposal is to change that default to 20 in Wales.
guidance for motorists:
https://www.gov.uk/speed-limits
and guidance for local authorities:
https://www.gov.uk/government/publications/setting-local-speed-limits/setting-local-speed-limits
I assume the latter is still valid as it is on the .gov.uk, probably if you looked up the legislation referred to you would find the 200m bit.
As above it’s targeted at urban areas, and the idea is not to slow journey times by preserving higher speed limits where it’s appropriate. It’s not ALL speed limits that are changing – it’s only the DEFAULT limit where there are no signs in towns and cities.
Yeah, that much was obvious from my brief perusal of the BBC website, which is why I'd already pointed it out in this thread 😉
I see a lot of STWers saying “oh I live in X city, it’s fine” please bear in mind, we’re not talking about a busy city or town centre, we’re talking about a whole Country.
Nope wrong
Edit, multiple responses already
I see a lot of STWers saying “oh I live in X city, it’s fine” please bear in mind, we’re not talking about a busy city or town centre, we’re talking about a whole Country (I’m not here to debate Wales is or isn’t a Country).
Ah yes, the Scottish borders... the thriving metropolis wall to wall cars and no country villages.
its got a 6th of the population density of wales and is still absolutely fine.
Link to a page linking to the current *Welsh* guidance here - they are updating it.
Basically, local authorities will need evidence-based reasons for going above 20, currently they need that for going below 30. They can't just pass by-laws to maintain the status quo.
less polluting at 30 than at 20. So it would seem they’re losing that right.
Simply untrue. This idea comes from the RAC who did a bit of very slanted fake research and when you dug into it was obvious cherry picking to get the results they wanted
Basic physics tells you it is less energy needed to accelerate to 20 rather than 30
cars are more ecoonomical and less polluting at 30 than at 20. So it would seem they’re losing that right.
Could you show your working out for this please?
Also what about safety are kids in small villages not worth trying to protect?
It’s completely arbitray, it applies to every 30 mph road
To be fair, so's the 30 limit, I presume?
Basic physics tells you it is less energy needed to accelerate to 20 rather than 30
However slightly more advanced physics tells you that it may well be the case that because you might need a lower gear 20mph may be less efficient due to the way the engine is valved, and the fact there'll be more revs of the engine per wheel rev and hence the engine friction plays a bigger part in overall consumption.
I no longer have a car that gives me meaningful on-the-spot fuel economy figures (there is a digital readout on the Merc for it but I think it's the same one from the AMG C63 because it only goes up to 40mpg!) I can't test that theory out.
That "might" is doing some heavy lifting... 😉
The bottom line is, with the speed limit reduced, less people will die. How on earth can you argue against that?
Basic physics tells you it is less energy needed to accelerate to 20 rather than 30
You're forgetting to divide by time taken for the journeys.
Also, most cars efficiency is along a reverse bathtub curve dependent on speed.
That “might” is doing some heavy lifting
It's doing what it needs to do which is to say that it's a bit more complicated than slower = more efficient when it comes to ICE cars.
Feel free to wrap up the debate with some real numbers, I'm quite keen to know. I would be testing this out if I could.
Less speed differential between different urban road users can't be a bad thing, it will be even better when we start using technology to limit speed rather than self administered speeds when not approaching speed cameras. It might well make fossil fuel vehicles a bit less efficient, but I'm presuming it will have no efficiency loss for electric vehicles?
This turned up on a thread on this topic elsewhere (admittedly from a lobby group, but there are citations at the end):
Myth:
A 30 km/h urban speed limit creates more emissionsFact: The opposite is true. Studies have shown that reducing the speed limit from 50 km/h to 30km/h results in a clear reduction in polluting emissions and energy-use. Simply reducing speed results in large reductions in CO2 (about 15%), NOx (about 40%) and carbon monoxide (about 45%). The only exception is hydrocarbons, which rise very slightly (about 4%). Changes to driving style (from aggressive to defensive driving) also makes a big difference. Ideally, drivers would both slow down and change their driving habits, but legislation can only supply the former.
Just to clarify I'm pro 20mph limit and I think it probably won't increase emissions, but I have this knee-jerk reaction whenever someone tries to win an argument with the words 'basic physics'* because physics isn't basic 🙂
* or 'it's a simple fact'. You cannot make something a simple fact just by declaring it so.
The bottom line is, with the speed limit reduced, less people will die. How on earth can you argue against that?
By that logic reduce all speeds limits to 20mph. And because it's a blunt tool so the Welsh government don't have to work out which roads can safely be driven at 30mph.
But there is no logic to these things. Village where I used to live 2 people have been killed in recent years in a 40mph zone (an old lady and young boy). It's still 40mph - it should be 20mph. They won't even put a crossing in.
Yet the wide open A-Road just outside the village has been reduced from national speed limit to 40mph when it can easily cope with 60mph. £1000s spent on new road markings and signage. 🤷♂️
...and then I could turn off the A-road and in theory rag-around on twisty country lanes at 60mph!
I think the variables for the claim that 20mph produces less pollution are too many. You'd have to take into account the infrastructure of each road, how many sets of lights, crossing, road humps, roundabouts, acceleration, braking, junctions, traffic flow, numbers of cars, whether it was on a hill, how steep the hill is, what kind of traffic uses it...I don't think you could make a blanket assumption.
For me, 20mph is all about saving lives and making vulnerable road users more safe. and it certainly does that.
At the risk of adding more "mights", "probably and "Simple facts":
At a constant 30mph i could probably believe that a lot of ICE cars are more efficient at 30mph than at 20mph. But urban driving isn't about driving at a constant speed, its accelerating and braking all the time and accelerating to a higher speed definitely uses more energy than accelerating to a lower one. Its not just exhaust emissions that need to be considered, there is tyre and brake particles and crucially noise too.
Plus the whole dead children thing is fairly important too
Reduced speed limits also reduce fuel consumption and so are better for everyone (especially with the cost of fuel being what it is).
How can that be for an ICE vehicle?
My (automatic) car:
20mph = 1400rpm in 3rd gear
30mph = 1400rpm in 4th gear
..... same rpm/emisions but for 30% longer and higher fuel consumption (less distance travelled for the same consumption).
How can that be for an ICE vehicle?
My (automatic) car:
20mph = 1400rpm in 3rd gear
30mph = 1400rpm in 4th gear….. same rpm/emisions but for 30% longer and higher fuel consumption (less distance travelled for the same consumption).
Get on your bike and do a cadence of 100 in the lowest gear. Then do the same in the highest gear. They are not the same thing at all. Less speed will always require less power. If (and its a big if) a car is less efficient at lower speeds, then that's a design problem, which can be fixed.
You can ignore all of that though and there are still huge safety and quality of life benefits.
There are defitive and quantifiable advantages. What are the actual downsides other than a few minutes lost on journey times? (which is also debatable, as already discussed)
How can that be for an ICE vehicle?
My (automatic) car:
20mph = 1400rpm in 3rd gear
30mph = 1400rpm in 4th gear….. same rpm/emisions but for 30% longer and higher fuel consumption (less distance travelled for the same consumption).
But fuel consumption and emissions are not the same as they do not directly scale to engine RPM. It takes less energy to maintain speed at 20 than it does at 30, so the engine is not working as hard, therefore not using as much fuel.
Many surveys have proved that a lower speed limit does not scale directly to an increase in journy time too - particularly in urban areas. On a road with no junctions or other reasons to stop then yes, journey time will increase. But even then other factors come into play - for example... slowing down from whatever the speed limit is before to 20 means a longer period where the economically minded driver is not using fuel.
It's more complex than your simple sums.
kinetic energy = 1/2 m * V^2 (half mass multiplied by velocity squared)
Now there well may be modifiers due to engine efficiently at different revs, (although you've argued that one out for yourself) but because you are squaring the velocity bit of the equation, that's the dominant factor in the calculation.
1 tonne car at 20mph = 39,605 kg m/s^2 of kinetic engergy
1 tonne car at 30mph = 89,930 kg m/s^2
Which is suppled by the fuel used, so definitely less mpg at higher speeds. Butchers' phyical demonstration, I like that too.
same rpm/emisions but for 30% longer and higher fuel consumption (less distance travelled for the same consumption).
Emissions aren't only related to engine speed. You're doing less work against air resistance at lower speeds, so you need less energy (try riding at 20mph vs 30mph on your bike). In a petrol car that's less of a difference because (unless it's direct injection) you need a certain amount of fuel in the cylinder for ignition. But if you have a TFSI engine or a diesel then it holds true.
And the acceleration point as above. But it's not simple, you probably won't be on the roads for 50% longer or even 30%. A local road that has had a 20 on it recently is about 1km long, but most of your time on it you are queueing at the roundabout at the end. Slowing that roundabout up would actually have a far greater effect on emissions on all the joining streets than the actual speed limit.
If you are genuinely concerned about emissions then drive less. It's a just a load of whataboutery.
Slower streets make them nicer places to be for all users.
If you are genuinely concerned about emissions then drive less.
If I cycle it doesn't affect everyone else's emissions 🙂
It’s doing what it needs to do which is to say that it’s a bit more complicated than slower = more efficient when it comes to ICE cars.
Feel free to wrap up the debate with some real numbers, I’m quite keen to know. I would be testing this out if I could.
But at the basic level, as previously defined, a lower speed requires less energy to reach and maintain, therefore a given road vehicle will use less fuel going slower rather than faster, no?
therefore a given road vehicle will use less fuel going slower rather than faster, no?
Not necessarily because whilst you are doing less work the efficiency of the whole car is lower at that kind of low speed. For various reasons. And whilst dropping from 60mph to 40mph saves a lot of required energy, dropping from 30 to 20 doesn't save as much because as we know air resistance is proportional to the square of speed. That means that inefficiencies in the powertrain can become more significant. But on the other hand, as above, the energy required for acceleration may also dominate depending on the road itself.
From memory, in the Passat that had the instant readout, driving at 30-40mph didn't seem to be much more efficient than 50mph, if at all.
Find the RAC "research " if you want a laugh. They tried to prove 20mph caused greater fuel consumption and emissions but its laughable and was quickly picked apart and they had to admit that in the real world 20mph cut fuel consumption and emmision.
Im sure you guys would love it
Has no one mentioned the BBC article yet?
https://www.bbc.co.uk/news/uk-wales-62020427
angry gammon says....
"Cyclists are having a whale of a time on Liverpool Road because they can do more than 20mph on their bikes and we can't do more than 20mph in a car - so I've had quite a lot of reports of people being overtaken by cycles."
Tried yesterday on a long flat straight in town 20 limit going to a 40.
In 20was a smidge over 1200rpm in 3rd only need to change down if in traffic.
30 needle just touching 1200 in 4th but weirdly need to drop to 3rd if slowing to 25 or on hills.
We've had it for. A couple of years and it is a nice speed for town. Just stick it in cruise control and all's good. We had/have the usual "its too slow" folk but it honestly make no real difference to ETAs and the time you have to leave by the time you take into account lights and traffic and junctions etc.
its got a 6th of the population density of wales and is still absolutely fine
Which bit of Wales?
We have it here in the Scottish Borders and is ace. Yes people go faster but they also broke the 30mph limit so average speeds are lower.
The arguments against it are weak and don't stand up to any sensible challenge. The people who object the most are your Daily Mail comments types. It's safer, quieter, greener and doesn't really take any more time. It encourages active travel and when the roads are busier I'm sure it speeds things up as much easier to pull out of junctions.
People just don't want to be told what to do in their cars. The thing is, roads are jot just for cars.
I have no problem with 20 on residential roads, and I hope it will mean we can get rid of road humps. They are a particular disincentive to use of small cars, as they are disproportionately affected. A 30mph speed limit with obstructions that can't be safely taken at over 15mph is not useful. They're also bad for bikes, and even worse for trikes.
I just did a trip involving lengthy sections of suburban 20mph. I set the speed limiter on the car, so I didn't have to watch my speedo or use fine throttle control and it was great. Much more relaxing. I think part of the issue people have is that it's more difficult to simply drive at that speed. I suppose because you need a lower gear so the tiniest change in throttle position results in more acceleration or deceleration.
Which bit of Wales?
The bit west of the border?
My point as well you know is that the scottish borders is a very very rural area, the largest twon is 14000 people (abotu the same as abergavenny) The least pupulated region of wales is more densely populated than the borders. The same very very rural area has had a 20mph in all towns and villages for three years (including the trials). So someone tryign to claim it only works in cities is talking out their backside.
Report for the Scottish Border below. 6mph reduction in some areas and an average reduction of 3mph across the region.
All the villages in Perthshire are now 20.
Most, but not all. My village is oddly still 30. I'm only pointing this out because I'm able to see the difference it makes just by going one village over. I've decided to be the change I want to see and stick to 20 in my village now.
A small point currently but will become more relevant over time: EVs are way more pleasant to drive at low speeds than ICE cars in my experience.