20mph Wales

LightFlare

Maybe more like this is the answer

WellKnownSid

MouldyOldDough said:

…especially in smalll (sub 1 litre) automatics - they are not designed to drive so slowly…

I hope never to meet you in a multi-storey car park in your out-of-control automatic!

Stubod

There are places where a 20 mph speed limit is appropriate, (eg our high street), then again there are places where it's just a complete nonsense, (eg our bypass, that is wide and has wide pavements both sides). 

Rather than take the logic approach of looking at each case Wales likes to be seen to be "the first", and it is simpler to just take a blanket approach, even though the evidence suggets otherwise. But when did facts actually matter to politicians?? ...that's Drakeford fo you...

Welcome to Wales!!

...slowly...

Herzlos

MouldyOldDough said:

born_again said:

MouldyOldDough said:

https://mol.im/a/12393711
I can't see this actually happening.....
It's going to cause more pollution due to low gear 
More noise due to high revs 

Does not compute
Unless you are a idiot & sit in 1st gear. Plenty or 20 zones round here on residential streets. Causes no problems & makes it safer for all road users.

Yes it does - if your car doesn't like top gear at 20mph (and I don't know what car does) - and remains in a lower gear - the engine will be turning at faster revs - than it would in top gear - so the exhaust  will be pumping out more noxious gases (per mile) !

Simple

I am not denying the safety side of it !!!!

You're not going to be in top gear at 30mph either though, so no need to be silly.

Some cars will need to be in a lower gear at 20 than 30, which may mean higher revs. But it should also mean less wind resistance, less accelerating and braking etc, so I'm not sure that it's safe to assume cars are notably less efficient at 20 than 30.

A huge amount of Edinburgh has become a 20 zone and I can't say that it's had any noticeable impact on my driving experience in an under powered 1.5 diesel manual, but I can't comment on sub 1 litre automatic petrols which I assume will struggle more in most circumstances. The biggest difference I think I've seen is that at lower speed limits cars tend to bunch up more making it harder to get out of junctions.

force_ten1

wee jimmy crankie and the snp cronies did in the Scottish Borders and now drippy drakeford is going to roll it out across wales 

and for the person that said "I can't see this actually happening" well it is happening and 30MPH signs are being bagged and road markings with 30mph are being erased from existence 

WellKnownSid

Herzlos said:

Some cars will need to be in a lower gear at 20 than 30, which may mean higher revs. But it should also mean less wind resistance, less accelerating and braking etc, so I'm not sure that it's safe to assume cars are notably less efficient at 20 than 30.

I think the assumption people are making is that the pollution produced by cars is proportional to engine speed, but that is not the case.  Engine load is the main factor (determining how much air and fuel is added) - but as you say, acceleration and braking are also extremely wasteful, especially in a pure ICE car where most of the braking results in shedding small amounts of particulate matter from the brake pads along with unrecovered energy which now requires more fuel.  That's why hybrids can get 80mpg in traffic.

CoastingHatbox

WellKnownSid said:

Herzlos said:

Some cars will need to be in a lower gear at 20 than 30, which may mean higher revs. But it should also mean less wind resistance, less accelerating and braking etc, so I'm not sure that it's safe to assume cars are notably less efficient at 20 than 30.

I think the assumption people are making is that the pollution produced by cars is proportional to engine speed, but that is not the case.  Engine load is the main factor (determining how much air and fuel is added) - but as you say, acceleration and braking are also extremely wasteful, especially in a pure ICE car where most of the braking results in shedding small amounts of particulate matter from the brake pads along with unrecovered energy which now requires more fuel.  That's why hybrids can get 80mpg in traffic.

Absolutely correct. There's plenty of research around this (easily searchable) which supports the hypothesis that a 20 mph speed limit will reduce emissions.

Given that average moving speeds of vehicles in urban areas are on the low side of 30 mph, I can't see it making much actual difference to journey times. In fact, by encouraging people to drive more slowly between junctions and other points at which there's often congestion, at peak times journey times may actually be reduced. In effect, lower speeds increase road capacity.

This issue being politicised and the actual well established science is being ignored. The top gears on cars are actually overdrive gears, which do increase fuel economy at motorway speeds by keeping the engine turning over in it's optimum range - but because RPM at the drive shaft is lower than the engine RPM - they will actually be less efficient in other situations.

Lower gears significantly reduce engine load (the amount of work the engine has to do in each revolution) which reduces fuel burn and in consequence tail pipe emissions. It is a lot more nuanced then simply looking at engine RPM.

To select a low gear to travel down a hill - the engine will achieve high RPM with minimal fuel burned. To get the effect of engine braking - the engine has to be purposefully pushed into the range where it would be less efficient due to it's internal losses. Those are significantly higher than what would be considered normal driving, at any speed.

Anecdotally, my van will do comfortably north of 80 mpg (typically 100 mpg) with a terminal velocity of 20 mph against a more typical average fuel consumption of around 65 mpg. The best I've consistently obtained from it on long motorway journeys where I've managed to sustain a constant 53 mph (the same speed as most lorries, by GPS indication) for much of the journey is 70 mpg.

daivid

There's is a lot of misinformation in here. Wales is not getting a new national speed limit: that remains 60/70, further to this it's not quite true all 30s are becoming 20s - the default is switched so under the old criteria as was for 30 it now recommends 20. There will however be some roads remaining at 30 but these will be on a case by case basis.

I have to say I'm pretty relaxed about it all, many of the non national limit roads I drive are such that driving to the limit would be irresponsible or impossible in the day due to traffic and vulnerable road users. In an ideal world I'd have variable limits for time of day - going past a school 20 in the day, 40 at night… - but that will never happen.

Martyn1981

Some roads in Cardiff have been 20mph for many years, especially near schools. Now that many more are changing, I'm not convinced it's a major issue. Journeys don't seem much longer, you just spend less time accelerating and braking.

And folk joining from side roads, or leaving the main road, have far less impact on speeds. You don't need to slow down either way, or just a light lift off is enough. So much more relaxed.

Not sure I buy into the more fuel consumption/emissions argument. I appreciate that a lower gear may be needed, but put against higher air resistance, and wasted energy (and brake pads) from having to remove greater kinetic energy, I suspect the 20mph are better.

Plus of course, for BEV's (and partially for HEV's and PHEV's operating in electric only), there are no gears, so less energy is consumed (reduced air resistance). And whilst some of that 20-30mph kinetic energy can be re-gened back to the batts, it's not as efficient as not committing it in the first place.

I also feel that the roads flow more easily / smoothly, and the safety aspect is important.

Purely my experience, so others may disagree, but about 30yrs ago coming home, I struck a child on a scooter. Luckily, he came out of our neighbours drive so I was slowing ready to turn, and was down to about 20mph. He came out from behind a works van that was in their drive, but sticking out level with the outside edge of the pavement, so appeared, literally in the road in front of me. 

I hit the brakes, and stopped with the car bonnet pushing against his handlebar. So he appeared at a distance roughly equal to the thinking and stopping distance at ~20mph. As I understand it, the thinking distance at 30mph is almost as much as the thinking + stopping distance at 20mph, so had I not lived next door, I may have been travelling at 30mph, and also struck him at roughly 20-25mph.


Edit - Just a thought, but in case anyone is interested I thought I'd check the kinetic energy, since velocity is more 'important', as the calculation is 1/2mv2, or half mass x velocity squared.

So, for a 1,000kg car (back then), it's kinetic energy at ~1mph would be about 100J, but at 25mph it would be 62,451J, or roughly 624x greater.

For comparison, at 20mph it would be ~40,000J, and at 30mph it would be ~90,000J.

northwalesd

daivid said:

There's is a lot of misinformation in here. Wales is not getting a new national speed limit: that remains 60/70, further to this it's not quite true all 30s are becoming 20s - the default is switched so under the old criteria as was for 30 it now recommends 20. There will however be some roads remaining at 30 but these will be on a case by case basis.

Crikey, don't let facts get in the way of a good rant 😁