Default 20mph for Wales

[mjr]

Unless I’m mistaken it takes 50% longer to cover a distance at 20mph than at 30mph ( you might like to alter your post accordingly ).

Ahem. Oops. So edited. Thanks.

There’s likely some truth in the assertion that it takes longer and uses more fuel to deliver goods in 20 mph zones than in 30 mph zones, how much more time and fuel is used isn’t clear to me and neither is its importance. The 20 mph zones aren’t about either fuel or time use they’re about safety and in that respect there’s been multiple real benefits to society.

I think the 1mph difference in average speed in such zone suggests it isn't significantly slower. While I think everyone accepts all the other benefits, I'd really like to refute the claim that moving faster uses less fuel too. I'm fairly sure another gear change up or two then dumping more energy into friction brakes will negate any efficiency improvements from a short spell in higher gears, but how to show this?

Of course there might well be frustrations and pressures on delivery drivers who are prevented from driving faster than they might otherwise go; similarly there are times when I find local 20 mph limits frustrating but there are also times when I’d think it unwise to do as much as 30 mph (in a 30 mph zone). Speed limits aren’t a target speed, though a delivery drivers boss might think so, they’re a cap imposed by the State.

Yes, I believe the fuel argument is more an attempt at retrospective justification of higher speeds in built-up areas due to the frustration of 'waaah, I can't brrrrm-brrrrm as fast any more' more than it's a rational argument based on concern for the planet. But the oily driver is never going to admit that.

[Cugel]

The 20 mph zones aren’t about either fuel or time use they’re about safety and in that respect there’s been multiple real benefits to society.

Of course there might well be frustrations and pressures on delivery drivers who are prevented from driving faster than they might otherwise go; similarly there are times when I find local 20 mph limits frustrating but there are also times when I’d think it unwise to do as much as 30 mph (in a 30 mph zone). Speed limits aren’t a target speed, though a delivery drivers boss might think so, they’re a cap imposed by the State.

As an aside I welcome speed limits and find myself driving more slowly than I used to do, my journey times don’t seem noticeably longer but my stress levels are less and at the end of the journey I’m not needing recovery time.

The stats show that 20mph regions have significantly reduced deaths and serious injuries in those regions, even when a portion of drivers ignore those limits ; not to mention lower driving insurance rates; and lower pollution. Are there stats that show increased fuel use (and pollution) or some sort of economic damage to firms delivering goods via van in these regions? Or any other practical drawback? Not any that I can find.

An interesting question would be that attempting to discover the reason why some drivers find 20 mph limits "frustrating". Have you introspected yourself if and when you find such a frustration manifesting in your own psyche as you drive through a 20 mph region? What gives rise to the feeling a need to go faster?

Personally I have no such frustration but observing others and their remarks about their own "frustrations" when directed to slow down, it seems they have no pressing reason for feeling that way other than their habit of driving too fast everywhere because the powerful car and the psychology surrounding its selling and possession have induced a speeding habit in them. It seems to be some sort of joy-in-power thing that, frankly, seems a bit infantile.

They don't seem to actually be significantly delayed in their important driving mission - which when looked at objectively is usually very unimportant indeed.

Are there stats that show the degree of "delay" in getting from A to B when traversing a route containing various stretches of 20mph limit (that one actually obeys)? I do several habitual journeys along such routes in West Wales, going through half a dozen or so villages that have 20mph rather than the previous 30 or even 40 mph limits. I've not noticed any significant delays. I suspect that the maths might show that a typical journey takes 24 minutes rather than 23.

In many more densely populated areas, I expect that those disobeying the 20 mph limit to do their customary 36mph just get to the next traffic jam faster (by 30 seconds or so).

********
People can be very stupid, eh - especially when driving a car.

[853]

Are there stats that show the degree of "delay" in getting from A to B when traversing a route containing various stretches of 20mph limit (that one actually obeys)? I do several habitual journeys along such routes in West Wales, going through half a dozen or so villages that have 20mph rather than the previous 30 or even 40 mph limits. I've not noticed any significant delays. I suspect that the maths might show that a typical journey takes 24 minutes rather than 23.

The maths is one minute slower per mile travelled with a reduction in speed from 30mph to 20mph. (At 30mph it takes 2 minutes to cover one mile, and at 20mph it takes 3 minutes.). This means if your six villages all have one sixth of a mile of 20mph limit, which was previously a 30mph limit, then your journey will be one minute slower.

What this doesn't show is the effect that traffic lights will have. At 30mph the lights on green will let, say, 9 cars through but at 20mph this will be only 6 cars. In cases where there are lots of traffic lights in the villages the extra delay at 20mph from the traffic lights could be considerably greater than the 1 minute extra in the example you mentioned.

[mjr]

Are there stats that show the degree of "delay" in getting from A to B when traversing a route containing various stretches of 20mph limit (that one actually obeys)? I do several habitual journeys along such routes in West Wales, going through half a dozen or so villages that have 20mph rather than the previous 30 or even 40 mph limits. I've not noticed any significant delays. I suspect that the maths might show that a typical journey takes 24 minutes rather than 23.

The maths is one minute slower per mile travelled with a reduction in speed from 30mph to 20mph. (At 30mph it takes 2 minutes to cover one mile, and at 20mph it takes 3 minutes.). This means if your six villages all have one sixth of a mile of 20mph limit, which was previously a 30mph limit, then your journey will be one minute slower.

That's repeating the mistake I previously mentioned. Your journey would only be a minute slower if you previously did 30mph for the entire stretch, whereas in reality corners, others road users, parked cars, people crossing the road and so much more means competent drivers go slower most of the time in the sorts of areas that are now 20mph. In practice, the 2018 research found a 1mph reduction in average speed, not 10mph.

What this doesn't show is the effect that traffic lights will have. At 30mph the lights on green will let, say, 9 cars through but at 20mph this will be only 6 cars. In cases where there are lots of traffic lights in the villages the extra delay at 20mph from the traffic lights could be considerably greater than the 1 minute extra in the example you mentioned.

Traffic lights? In residential areas which became 20mph? And with a minor road busy enough for 9 cars to queue in the time between greens? That's pretty rare, at most.

[Bmblbzzz]

A well implemented 20mph zone should enable the removal of some, certainly not all, traffic lights and (mini) roundabouts.

[853]

Traffic lights? In residential areas which became 20mph? And with a minor road busy enough for 9 cars to queue in the time between greens? That's pretty rare, at most.

Don't think you've understood what I've written.

I was answering Cugel about their journey through six or so villages. Never mentioned residential areas.

Never mentioned 9 cars queing, either.

I drove on the A87 last week through Portree. Major road and 20mph at the traffic light controlled junction with the A855

https://www.google.co.uk/maps/@57.41327 ... FQAw%3D%3D

If you travel across Skye by road you have to negotiate this 20mph junction of two A roads (unless you want a very long detour). What I found last week is that when the lights are on green (for however long that may be) there is enough time for 6 cars to head south on the A87 at 20mph. The maths says more cars would pass through the junction in a change of lights if the limit was put back to its previous 30mph

[drossall]

What I found last week is that when the lights are on green (for however long that may be) there is enough time for 6 cars to head south on the A87 at 20mph. The maths says more cars would pass through the junction in a change of lights if the limit was put back to its previous 30mph

But not as many more as you might think?

For a moment, let's ignore anything about cars starting up when lights go green, and slowing down when they turn amber (if any driver should happen to obey the law) or red (if no driver should) or after red (in the real world), and assume that we are just talking about cars going through at whatever speed limit might apply.

We have to remember that the envelope of a faster vehicle is bigger than that of a slower one. By envelope I mean the amount of road space that it needs, when you take into account that it should have its stopping distance in front of it. So, according to the AA tables, a car at 20mph needs 53 ft of road space (40ft stopping distance plus 13 ft assumed car length). At 30mph, it needs 88ft. That large increase in envelope is only somewhat outweighed by travelling faster, because a large component of the stopping distance increases as the square of speed. That's why, when traffic gets heavy, motorway speed limits are sometimes decreased in an effort to increase vehicle throughput.

Back to the 20/30 debate; my back-of-an-envelope calculation suggests that you might even get fewer cars through at 30mph than 20mph, because of the larger envelope. In the real world, of course, cars are moving off from, and slowing to, a standing start, so it's even more complicated, and envelopes are smaller for most of the time.

But, in summary, higher speeds may have more to do with getting one individual driver to a destination faster (which they will provided that others keep out of the way), than with efficiently getting everybody where we are going as soon as possible. Only if the roads are sufficiently empty that drivers do not have to interact too much will higher speeds necessarily benefit everyone. In the real world, if the challenge were to get everyone on a busy road home in the lowest possible total time, the answer might be quite a surprising speed limit. And that's before you start thinking about the effects of someone braking and it passing down the chain on a motorway for a mile or more.

This is all quite tentative; I'm not a traffic engineer of any description, so by all means correct me.

[Pete Owens]

Can you think of other ways to debunk this bizarre argument? I've spared you all the pseudophysics about differentials, technical requirements and piston motion as a wave model through space-time because I don't think they matter. They seem like sciencey waffle to try to distract people and discourage arguments in favour of 20mph.

You have pretty much already debunked the argument already. As you point out, in urban conditions the energy use is dominated by accelerating and breaking so if you cut out the repeated accelerations from 20mph to 30mph you are saving energy (not that KE increases with the square of speed).

Theoretical calculations based on constant cruising speed are irrelevant in towns - you need to go for empirical observations under real world conditions and this shows that 20mph speed limits improve fuel efficiency.

One point you can make to someone who insists that speed limits should be set to optimise fuel efficiency rather than improve road safety is to apply their own argument to the motorway speed limit and ask them for their opinion on reducing the motorway speed limit to 40 mph - where it would undoubtedly make a very big difference.

[Pete Owens]

What this doesn't show is the effect that traffic lights will have. At 30mph the lights on green will let, say, 9 cars through but at 20mph this will be only 6 cars. In cases where there are lots of traffic lights in the villages the extra delay at 20mph from the traffic lights could be considerably greater than the 1 minute extra in the example you mentioned.

The opposite is the case.

20 mph limits INCREASE the capacity of urban roads in terms of vehicles per minute because the gap you need to maintain between vehicles to keep traffic moving smoothly goes up with the square of the speed.

[853]

What this doesn't show is the effect that traffic lights will have. At 30mph the lights on green will let, say, 9 cars through but at 20mph this will be only 6 cars. In cases where there are lots of traffic lights in the villages the extra delay at 20mph from the traffic lights could be considerably greater than the 1 minute extra in the example you mentioned.

The opposite is the case.

20 mph limits INCREASE the capacity of urban roads in terms of vehicles per minute because the gap you need to maintain between vehicles to keep traffic moving smoothly goes up with the square of the speed.

It's a nice theory, but it doesn't work in practice.

I've driven in Wales and Scotland in 20mph zones on A roads, and what I have seen is that drivers maintain the same gap as they do in 30mph zones (when they may in fact be driving with less than the safe distance required for the speed). Years of driving a certain distance from the car in front, in built up areas, persists.

A further factor is that the modest 2024 car I drive will brake for me if I get too close. How this works on the road is that if the car in front drops below a speed of around 15mph the car will not allow me to drive any closer than then we were driving at 16mph; if the car drops to 10mph and then accelerates back up to 20mph the gap is bigger than it was before and unless I exceed 20mph (and break the law) the gap will be bigger than the optimum gap (envelope) for 20mph. If I drive with a bigger gap (envelope) this rarely happens. A lot (probably most) modern cars have features such as this.

[Ron]

In cases where there are lots of traffic lights in the villages

I've been trying to compile a list of "villages with lots of traffic lights" but cannot think of even a single example.
These cases are so rare they can be discounted.

[the snail]

Can you think of other ways to debunk this bizarre argument? I've spared you all the pseudophysics about differentials, technical requirements and piston motion as a wave model through space-time because I don't think they matter. They seem like sciencey waffle to try to distract people and discourage arguments in favour of 20mph.

You have pretty much already debunked the argument already. As you point out, in urban conditions the energy use is dominated by accelerating and breaking so if you cut out the repeated accelerations from 20mph to 30mph you are saving energy (not that KE increases with the square of speed).

Theoretical calculations based on constant cruising speed are irrelevant in towns - you need to go for empirical observations under real world conditions and this shows that 20mph speed limits improve fuel efficiency.

One point you can make to someone who insists that speed limits should be set to optimise fuel efficiency rather than improve road safety is to apply their own argument to the motorway speed limit and ask them for their opinion on reducing the motorway speed limit to 40 mph - where it would undoubtedly make a very big difference.

Correct. IIRC 30 mph is only more fuel efficient than 20mph if you travel over 1 mile at a steady speed,which almost never happens.

[Pete Owens]

What this doesn't show is the effect that traffic lights will have. At 30mph the lights on green will let, say, 9 cars through but at 20mph this will be only 6 cars. In cases where there are lots of traffic lights in the villages the extra delay at 20mph from the traffic lights could be considerably greater than the 1 minute extra in the example you mentioned.

The opposite is the case.

20 mph limits INCREASE the capacity of urban roads in terms of vehicles per minute because the gap you need to maintain between vehicles to keep traffic moving smoothly goes up with the square of the speed.

It's a nice theory, but it doesn't work in practice.

Yes it does.

It is the same mechanism that variable speed limits increase the capacity of congested motorways.
While you may see an INDIVIDUAL driver tailgating at any speed, the whole traffic flow cannot maintain if every driver does so. If the gaps get too close for the traffic speed then any slight slowing of the vehicle in front means that the one behind has to break sharply and the one behind that even more sharply until the traffic grinds to a complete standstill - then slowly starts to move again - until it reaches the critical speed and the flow breaks down again.

The critical speed at which capacity is maximised varies from road to road. On motorways it tends to be in the mid 30s (hence variable speed limits don't tend to go below 40mph) In urban conditions (where there is a lot more going on) the critical speed is very much lower.

This is why empirical measurements of real world traffic show a higher volume of free flowing traffic on 20 mph roads.

[Pete Owens]

Never mentioned 9 cars queing, either.

I drove on the A87 last week through Portree. Major road and 20mph at the traffic light controlled junction with the A855

https://www.google.co.uk/maps/@57.41327 ... FQAw%3D%3D

If you travel across Skye by road you have to negotiate this 20mph junction of two A roads (unless you want a very long detour). What I found last week is that when the lights are on green (for however long that may be) there is enough time for 6 cars to head south on the A87 at 20mph. The maths says more cars would pass through the junction in a change of lights if the limit was put back to its previous 30mph

A curious choice of location to highlight.

Note that it is just outside a school and the only other traffic actually queueing to use the junction are a cyclist and a pedestrian. This rather justifies the need for the 20mph speed limit at that location.

Also note that it is a T junction so no-one would actually be travelling at 30mph at that point even if the lights were permanently green and the speed limit increased (whatever bogus theory you are imagining)

If you are following the A87 past Portree you are within the 20mph zone for just a quarter of a mile and that includes 2 sets of traffic lights (one of those at this T junction) so the actual length of road you would be able to exceed 20mph is very much less than that - even assuming hard braking and acceleration between junctions. So you are moaning about an absolute maximum theoretical delay of the order of 10 seconds on a journey across Skye. Does the safety of the folk of Portree really mean so little to you?

[853]

The opposite is the case.

20 mph limits INCREASE the capacity of urban roads in terms of vehicles per minute because the gap you need to maintain between vehicles to keep traffic moving smoothly goes up with the square of the speed.

It's a nice theory, but it doesn't work in practice.

Yes it does.

It is the same mechanism that variable speed limits increase the capacity of congested motorways.
While you may see an INDIVIDUAL driver tailgating at any speed, the whole traffic flow cannot maintain if every driver does so. If the gaps get too close for the traffic speed then any slight slowing of the vehicle in front means that the one behind has to break sharply and the one behind that even more sharply until the traffic grinds to a complete standstill - then slowly starts to move again - until it reaches the critical speed and the flow breaks down again.

The critical speed at which capacity is maximised varies from road to road. On motorways it tends to be in the mid 30s (hence variable speed limits don't tend to go below 40mph) In urban conditions (where there is a lot more going on) the critical speed is very much lower.

This is why empirical measurements of real world traffic show a higher volume of free flowing traffic on 20 mph roads.

No it doesn't increase the capacity of urban roads in 20mph limits; and if you'd have read all nine lines of my reply you'd have learnt why