20 mph Zones - Extend or Eliminate?

[stuu]

There is an argument to say it takes more effort to keep a car at exactly 20 mph, which in turn may well mean less attention focussed on surroundings. What certainly is the case is that you’re more likely to have people driving aggressively close behind, including some who know better such as professional drivers, which definitely *is* a distraction.

How is it any more difficult than any other speed limit? You get people aggressively tailgating at 30 too

 

[PGAT]

There is an argument to say it takes more effort to keep a car at exactly 20 mph, which in turn may well mean less attention focussed on surroundings. What certainly is the case is that you’re more likely to have people driving aggressively close behind, including some who know better such as professional drivers, which definitely *is* a distraction.

Great, now you’ve identified the most dangerous drivers, and can now easily remove them for everyone else’s safety

 

[Bletchleyite]

Based on the safety issues of the particular roads in question.


Across the board, this unlikely phenomenom will reduce to zero as IC vehicles are removed from the roads. As mentioned above, EVs aremore efficientat 20mph.

More revs doesn't automatically equal more fuel, in any case. Labouring the engine in too high a gear will use more fuel than using an appropriate gear for the speed.

How is it any more difficult than any other speed limit? You get people aggressively tailgating at 30 too

I went to Wales last weekend and found people respecting the 20s more than they did the 30s. The only tailgating I saw was a lorry driver on the motorway who should have given in his licence - he was at one point so close to someone he had to panic brake and stalled as a result.

 

[cactustwirly]

A car travelling at 20mph is travelling at 9m each second, whilst at 30mph it's 13.4m each second.

Given that the stopping distance (with a 0.7 second reaction time, which is very fast) at 30mph is 23m vs 14m at 20mph it means that at 20mph you could add 1 second to your reaction time and still stop within the same distance as if you were traveling at 30mph.

It's therefore unlikely that you would actually be so distracted that it would actually make your stopping distance worse than if you were traveling at 30mph.

How did you calculate those stopping distances?
Are they the ones taken from the highway code which are very out of date?

More revs doesn't automatically equal more fuel, in any case. Labouring the engine in too high a gear will use more fuel than using an appropriate gear for the speed.

Why would this be the case? More revs = engine spinning faster and burning more fuel surely?

 

[johncrossley]

I went to Wales last weekend and found people respecting the 20s more than they did the 30s. The only tailgating I saw was a lorry driver on the motorway who should have given in his licence - he was at one point so close to someone he had to panic brake and stalled as a result.

I went in November and I was surprised at the level of respect given to the 20 speed limit, despite the scaremongering on this forum and elsewhere.

 

[Ted633]

Why would this be the case? More revs = engine spinning faster and burning more fuel surely?

Usually, yes. But what Bletchleyite is getting at is if your revs are too low (ie close to stalling) you will burn more fuel than if you are in a suitable gear. Example, my car (diesel) can be in 5th at 30mph, (revs will be just under 1000rpm) however it is more efficient to be in 4th and rev higher as the engine in far happier

 

[Bletchleyite]

Usually, yes. But what Bletchleyite is getting at is if your revs are too low (ie close to stalling) you will burn more fuel than if you are in a suitable gear. Example, my car (diesel) can be in 5th at 30mph, (revs will be just under 1000rpm) however it is more efficient to be in 4th and rev higher as the engine in far happier

Yep this.

To be fair in old carburettor petrol engines fuel use being proportional to revs probably was to some extent true (the throttle is literally that in such cars - it controls the air intake), but it's not true of any fuel injection car where the amount injected is calculated separately, including pretty much every diesel ever.

 

[cactustwirly]

Usually, yes. But what Bletchleyite is getting at is if your revs are too low (ie close to stalling) you will burn more fuel than if you are in a suitable gear. Example, my car (diesel) can be in 5th at 30mph, (revs will be just under 1000rpm) however it is more efficient to be in 4th and rev higher as the engine in far happier

But you wouldn't ever drive like that!
Doing 30mph in 5th gear is just ridiculous

 

[bramling]

But you wouldn't ever drive like that!
Doing 30mph in 5th gear is just ridiculous

In some cars, yes. However there are plenty of cars out there where this is quite suitable.

 

[Chester1]

I went in November and I was surprised at the level of respect given to the 20 speed limit, despite the scaremongering on this forum and elsewhere.

I don't know where you were visiting but few in North Wales keep strictly to 20 mph. Where I visit regularly there are a huge proportion of people driving at about 25 mph. There is an extra 4 mph leniency in addition to the normal 10% so you need to be driving above 26mph to be stopped. If you are driving at 26-30 mph on a 20 mph road then you will only recieve a warning. The additional 4 mph was a partial u turn, although the Welsh Government denies it.

 

[Bletchleyite]

But you wouldn't ever drive like that!
Doing 30mph in 5th gear is just ridiculous

In my experience just about every taxi driver does it!

Second or third is similarly best for 20, depending on the car.

 

[johncrossley]

I don't know where you were visiting but few in North Wales keep strictly to 20 mph. Where I visit regularly there are a huge proportion of people driving at about 25 mph. There is an extra 4 mph leniency in addition to the normal 10% so you need to be driving above 26mph to be stopped. If you are driving at 26-30 mph on a 20 mph road then you will only recieve a warning. The additional 4 mph was a partial u turn, although the Welsh Government denies it.

It was south Wales around Swansea and Bridgend. Much of London has a 20 mph limit but if you to stick to the limit then you are likely to be overtaken, even on a narrow road, but that wasn't the case where I was. If many people are staying at 25 or lower then that is a great result, meaning there must be some actual reduction in speed.

 

[Bletchleyite]

It was south Wales around Swansea and Bridgend. Much of London has a 20 mph limit but if you to stick to the limit then you are likely to be overtaken, even on a narrow road, but that wasn't the case where I was. If many people are staying at 25 or lower then that is a great result, meaning there must be some actual reduction in speed.

Almost certainly a 10mph reduction as intended. Many people after all do an indicated 35 in 30s (more like a real 33).

 

[AdamWW]

It was south Wales around Swansea and Bridgend. Much of London has a 20 mph limit but if you to stick to the limit then you are likely to be overtaken, even on a narrow road, but that wasn't the case where I was. If many people are staying at 25 or lower then that is a great result, meaning there must be some actual reduction in speed.

I read a survey done shortly after the new limits came in looking at a few places in Wales. There was a reduction in speed but the average (I presume mean) reduction was a lot less than 10 mph!

 

[Chester1]

It was south Wales around Swansea and Bridgend. Much of London has a 20 mph limit but if you to stick to the limit then you are likely to be overtaken, even on a narrow road, but that wasn't the case where I was. If many people are staying at 25 or lower then that is a great result, meaning there must be some actual reduction in speed.

Almost certainly a 10mph reduction as intended. Many people after all do an indicated 35 in 30s (more like a real 33).

I read a survey done shortly after the new limits came in looking at a few places in Wales. There was a reduction in speed but the average (I presume mean) reduction was a lot less than 10 mph!

Certainly around where I visit the reduction is much less than 10 mph. People didn't drive at 35 mph around housing estates! Perhaps on main roads its high single digits but elsewhere its probably about 5mph or less.

There is a lot to be learned from the Welsh government in how not to introduce the change. Wales is basically a one party state but the SNP need to avoid major electoral backlash, especially for polcies being implemented in 2026. Wildly inconsistent interest in exemptions really hurt the policy. Its not great for the policy if Labour run councils don't exempt all the roads that meet the criteria! The website showing exempted roads took far too long to update which made it look they were not exempt.

 

[johncrossley]

Certainly around where I visit the reduction is much less than 10 mph. People didn't drive at 35 mph around housing estates! Perhaps on main roads its high single digits but elsewhere its probably about 5mph or less.

The policy is primarily about cutting speeds where people used to drive fast, and where most of the casualties occur, so if there is a reduction of over 5 mph on those roads that makes the policy worthwhile. Obviously there are many narrow residential roads with lots of parked cars and/or traffic calming where driving over 20 mph is difficult if not impossible, and where there were not many accidents. Those roads are irrelevant.

 

[Bletchleyite]

Wales is basically a one party state

Is it? Both Labour and Plaid seem strong.

Scotland WAS but the glory days of the SNP seem to be waning.

 

[pokemonsuper9]

I'm a very new driver (under a month since passing), I find no trouble with 20 limits.
My only problem is plenty of cars (and busses too!) ignore the limit.
I'd be happy for more 20mph limits to be around.

The few remaining 30mph zones are on roads that are main through routes, which are annoying to cross (both walking and driving) because of the traffic and speed, not really one or the other, they both make each other worse.

 

[jon0844]

I have no issue doing 20mph either. Not stalled yet (and I drive a manual car right now). I can drive even slower still - it's called crawling in traffic!

I do think in this day and age we have the technology to do variable speed limits in more areas than just motorways. There are many roads that I am sure everyone can think of that either have a surprisingly high speed limit for the various hazards around, and others that are 30mph despite having no side roads and would likely be 40 or 50mph if it wasn't for the fact that the council has probably never got around to seeking a change.

Good road design also plays a big part in reducing accidents. We seem to be totally incapable of prioritising the painting of white lines at dangerous junctions on unlit country roads, as just one example of many situations where we could be far more pro-active than just reducing speed limits.

You do need people to respect the lower limits, so when 20mph zones become political, you naturally end up with people who will speed as some sort of protest.

 

[Chester1]

Is it? Both Labour and Plaid seem strong.

Scotland WAS but the glory days of the SNP seem to be waning.

Plaid was pushed into third place by Tories in 2016. They are second largest now but they have no chance of having a First Minister, short of a massive Labour collapse. Off the top of my head I don't think Labour has dropped below 40% of AMs since devolution in 1999. The elections basically just determine whether Labour need a coalition partner, and whether they have a choice between Lib Dems and Plaid. Welsh Labour can mess up on any policy, either on the principle or the implementation and it will make little difference.

SNP dominated Scottish politics from winning a majority in 2011 until Sturgeon's resignation last year. Its more comparable with the Tories since 2010 than Welsh Labour. The latter have been virtually guaranteed the job of First Minister at every election, including next one, taking their time in power to 31 years. Scottish Labour are well placed to form the next Scottish government because the Tories and Lib Dems will put them into office because the unionist cause is the priority. Plaid are put in same category as SNP by London media but the former has never seemed plausible to be largest party. They struggle to expand past their core vote of Welsh speakers.

 

[philthetube]

I've had front and rear cams for a while now and they produce interesting memories sometimes. As to 20MPH zones, our estate has been that for a while and generally it's been ok, except of course it's become obvious that the 20MPH limit does not apply to Royal Mail or numerous couriers (except UPS for some reason) or builders. On my trip to work I tend to use cruise control in 30's and 20's which makes it easier to spot annoyed drivers like you find while not being tempted to adjust speed to suite their desires. The A65 through the centre of Ilkley (and all side roads) has now become 20 and you can't tell because no-one does that (well, except me any anyone behind me!!)

I spent many summers and winters south of Montreal and the limits there, often 30KPH which is near enough 20MPH just seems fine to everyone. Mind you, the roads are a heck of a lot nicer than ours... (the only thing that caught me out first time is cycles go in all directions on both sides of the road!)

If you check your speed using a gps tracker then you may find that you are doing considerably less than your intended speed, possibly 10% less, I know my car reads 7% below corredt.

 

[jmh59]

If you check your speed using a gps tracker then you may find that you are doing considerably less than your intended speed, possibly 10% less, I know my car reads 7% below corredt.

Indeed. I checked mine against various SID boards in the area, all of which agreed that 22MPH on the speedo is 20MPH according to Mr SID. All may be wrong of course! Same for several 30MPH ones, 32=30 in my case (digital speedo). And the cruise control is no guarantee i.e. it sometimes goes over on downhill sections even with regen braking. Driver's responsibility etc etc

 

[AM9]

Indeed. I checked mine against various SID boards in the area, all of which agreed that 22MPH on the speedo is 20MPH according to Mr SID. All may be wrong of course! Same for several 30MPH ones, 32=30 in my case (digital speedo). And the cruise control is no guarantee i.e. it sometimes goes over on downhill sections even with regen braking. Driver's responsibility etc etc

That's fair enough, I used to have a Mercedes A180 with a CVT which would hold my set speed down some pretty steep hills by changing down, and also keep at the set speed up fairly steep hills, (it was a 2L diesel).
It's strange that so many here would balk at any mandated speed control where a system takes control of their car's speed according to road conditions, yet say that it is unreasonable to expect them to keep to the advertised speed limit* despit them having full control of their vehicle's speed.

* The imposition of a speed limit does not expect drivers to control the vehicle to an exact figure +/- 0mph, - it is a maximum speed that should be easy to avoid exceeding.

 

[Jim the Jim]

I'm all for more 20mph roads, but the biggest difference for pedestrians surely comes from less traffic and/or more pedestrian crossings. 20mph schemes I think can be one of those things that are trumped as "pro-pedestrian" in place of actually bothering to make more effective interventions.

 

[jon0844]

I'm all for more 20mph roads, but the biggest difference for pedestrians surely comes from less traffic and/or more pedestrian crossings. 20mph schemes I think can be one of those things that are trumped as "pro-pedestrian" in place of actually bothering to make more effective interventions.

As I said elsewhere, councils cannot afford to fix potholes, resurface roads, re-paint white lines even at dangerous junctions or remodel roads to design out conflict - so while I fully support better road design, and the investment in more crossings (but 10+ years ago I was told a traffic light crossing cost upwards of £140k, so goodness knows what one would cost today), I acknowledge that this isn't likely to happen without major changes of priorities in funding.

Zebra crossings seem out of fashion, and a simple traffic island is something that - going by my local area - ends up with vehicles mounting them on a regular basic and destroying the bollards/signs.

Even roadside dot-matrix information signs have been wiped out in recent years, and on any given road there are plastic barriers where a metal railing once was after being hit.

This demonstrates just how bad driving is these days, as the police are far too stretched to enforce anything and the best we can hope for are fixed cameras that aren't going to spot street racers slowing down, or drink/drug drivers, or those with no license.

20mph zones aren't perfect but they do seem to bring the average speed down. They won't stop the idiots, so pedestrians will still need to be on their guard but they're better than nothing which is pretty much what the alternative would be.

 

[stuu]

Zebra crossings seem out of fashion, and a simple traffic island is something that - going by my local area - ends up with vehicles mounting them on a regular basic and destroying the bollards/signs.

There was an attempt to get the DfT to allow just stripes like you find in most of Europe, but it was turned down. New zebra crossings do get put in, but even they cost a fortune as you need power for the beacons

 

[The Ham]

How did you calculate those stopping distances?
Are they the ones taken from the highway code which are very out of date?

Yes it's the doubt distances from the Highway code, however I would disagree that's is out of date.

In highway engineering there's two stopping distances which are used this is how they compare with the highway code:

0.67 second reaction time, and braking force of 6.57m/s/s - highway code (23m at 30mph)
2 seconds reaction time and braking force of 2.45m/s/s - Design Manual for Roads and Bridges (70m at 30mph)
1.5 seconds reaction time and a braking force of 4.41m/s/s - Manual for Streets (40m.at 30mph)

Whilst most production cars can achieve 7.82m/s/s (0.8g) and so can stop (assuming a 0 second reaction time) in just 11.5m, you'd still need a reaction time off 0.86 seconds to still stop within the 23m cited in the highway code for 30mph.

Likewise at 20 you'd stop in 5.1m giving you a thinking time of 0.77 seconds to stop within the highway code 12m

That means that the amount of thinking time from being at 20mph is 2 seconds to achieve the same 23m stopping distance. That's still about a second more. At those speeds, the distance reduction from better braking isn't really all that much, given that the 30mph reaction time is still noticeable below 1 second.

As an example of we set the braking force at 5 times the force of gravity (which F1 cars can achieve, whist most production cars compare at 0.8 times the force of gravity) from 30mph you'd do this in 1.8m, add in 1 second for a reaction time and the total stopping distance would be 15m, now from 20mph with the same breaking force you would stop in 0.81m, leaving you with 1.6 second reaction time to still stop in the same 15m distance. Whilst the gap has closed from about 1 second to 0.6 of a second, the braking force is significantly better than what most road cars could achieve (for example airbags deploy at 2g). Even if the car was capable, in not sure that the road surface would allow you to reliability achieve it and you'd need new or nearly new tyres all the time

Therefore, even with better braking it's likely that in most circumstances someone travelling at 20mph has about an extra 1 second of reaction time compared to someone travelling at 30mph.

 

[mpthomson]

How did you calculate those stopping distances?
Are they the ones taken from the highway code which are very out of date?

They're much less out of date at slow speeds up to about 35mph (from the last ahem... speed awareness course that I did where this came up). At speeds above this then the reaction time is a far lower %age of the total process and braking distances on a modern well maintained car with anti-lock disc brakes is significantly lower than the Highway Code states. But not all cars are like that.

 

[DC1989]

It's not often said much but the main benefits of 20mph zones seems to be that it makes people more likely to go 30 or thereabouts. When a road is a 30 many may do 35ish. When a road is a 20 they then do 26-30ish

Perhaps one day car tech will be at a point where we can drive the 'real' speed limit and many roads can go back to being a 30 safe in the knowledge that that limit won't be broken

 

[CaramellDansen]

Not Just Bikes is perfect on this issue: you can't just put a "20" sign there and hope it works.

(His recommendations on how to actually achieve this, however, fall flat.)