20 mph Zones - Extend or Eliminate?

[pokemonsuper9]

Perhaps it's time to switch to kph. No need to change speed signs then, the former 30mph limit becomes 30kph.

I don't think motorway signs are able to display 3 digits, which might a problem for 60mph (96.56km/h probably rounded to 100km/h) limits.

 

[GusB]

I don't think motorway signs are able to display 3 digits, which might a problem for 60mph (96.56km/h probably rounded to 100km/h) limits.

It's not really relevant in a thread that's discussing whether or not to extend or eliminate 20mph zones, though...

 

[The Ham]

One of the positives of having new housing developments is that there is section 106 money to fund infrastructure like this.

It needs to be managed properly, so the money is spent where it really improves cycling safety, rather than having a quarter mile of a cycling motorway near a new housing estate that dumps you back on busy roads.

One area where this could see significant improvement going forwards is the existence of Active Travel England, who have the experience and the power to block planning, which would ensure that good quality well thought through designs are delivered.

For example, even very basic things like they will endure that any location where people may wish to go (including public open space and allotments) have cycle parking.

I don't think motorway signs are able to display 3 digits, which might a problem for 60mph (96.56km/h probably rounded to 100km/h) limits.

There's ways around it, for example displaying 99, which for most people would be easier to distinguish from 90 or 80 than between 50 and 60.

However on a matrix it's often not all that different to fit 100 rather than 60. However even if it was impossible (for example on older signs), having the posted limit of 90 (56mph) isn't all that much different from 60mph.

It's not really relevant in a thread that's discussing whether or not to extend or eliminate 20mph zones, though...

In part I agree, however it is a fairly good way to overcome one of the big complaints of changing to kph, that being the cost of replacing all the signs.

Having 30, 40, 60, 80 and 110 limits would allow a lot of sign reuse. However it's likely to be off topic, so much more discussion would likely need a new thread.

 

[AdamWW]

You can't have speed limits that end in a five. British speedos are only required to be graduated in 10s. Perhaps it's time to switch to kph. No need to change speed signs then, the former 30mph limit becomes 30kph.

Quite a few 5 mph signs around. Maybe on private land though?

 

[Krokodil]

I don't think motorway signs are able to display 3 digits, which might a problem for 60mph (96.56km/h probably rounded to 100km/h) limits.

Rounding down is probably better when you're putting a variable speed limit in place for congestion or whatever.

Quite a few 5 mph signs around. Maybe on private land though?

Yes, the only places I've ever seen 5mph or 15mph were on private land. You can't enforce it on a public road.

 

[Bletchleyite]

It's the changing gears that mostly causes high emissions in urban driving. Stopping at lights, then changing up to 4th, then back down to 2nd for the next junction, etc. If you just stay in a lower gear and that means changing up fewer times, it's going to reduce emissions.

This just isn't true. You should select appropriate gears based on your speed at the time.

 

[rapmastaj]

This just isn't true. You should select appropriate gears based on your speed at the time.

If the traffic lights are as frequent as they are in most of Leeds, then unnecesary speeding and revving in the short stretches between them will give no benefit to anything other than your boy racer reputation. 20mph zones are sometimes used by transport planners to improve traffic flow, as the more constant speeds, instead of frequent stopping and starting, helps to reduce congestion.

== Doublepost prevention - post automatically merged: 24 Jun 2024 ==

From the officer report on a 20mph scheme in London Road, Bath:

https://www.bathnes.gov.uk/sites/default/files/3_-_outcome_of_the_tro_process_-_20mph_bath.pdf

Air quality
We are not aware of evidence that supports some of the concerns that introducing a 20mph limit on roads where there are hills reduces air quality.

A number of studies have been conducted into the effects that 20mph limits have on air pollution as a whole. Overall, a smoothing of driving style and cutting out of the acceleration phase from 20mph to 30mph is considered to be beneficial for emissions of harmful pollutants.

Research into 20mph limits carried out for the Department for Transport by Atkins stated “20mph limits (signed only) have the potential to affect vehicle emissions and air quality, reduce CO2 (greenhouse gas) emissions, and reduce noise levels, as a result of:
• a reduction in average speed and top percentile speeds;
• a smoother, more consistent driving speeds;
• a small-scale displacement of traffic; and
• mode shift away from car.
(Atkins, AECOM, Maher, M. (2018) 20mph Research Study, Process and Impact
Evaluation Headline Report)

20mph limits do not, therefore, appear to worsen air quality and they may reduce road danger. They also support a shift to walking and cycling, generate less traffic noise and reduce community severance. In 20mph limits vehicles move more smoothly with fewer accelerations and decelerations.

 

[The Ham]

An interesting analysis:

TikTok - Make Your Day

walesjournalist Will Hayward · 5h ago Follow The 20mph limit for Wales seems to be making a big difference to road safety despite being very controversial. Remember, this is early days but the signs are good. #walestiktok #cymru #wales #politics #welshtiktoker #welshtok #ukpolitics #20mph #20mphspeedlimit

For those without TikToc, basically in the first 3 months of the year there's been a reduction of an average of 25% of injury accidents on the roads subject to the 20mph limits in Wales.

When is broken down and compared to the cost of the average accident (by severity) it's likely that it's saved. £19.9 million in those three months, so if this trend continues it'll have saved the £32 million it cost to install the replacement speed limit signs in 6 months.

I'm not sure if the source for the numbers of accidents, however if accurate (and bearing in mind the caveats about you can't necessarily expect things to follow the pattern long term when you're looking at short term data) it does look like it's got the potential to be a good thing.

 

[greatkingrat]

I guess this is the source being referred to

Reported road casualties: 2023 [HTML] | GOV.WALES

Data on motorcyclist, pedal cyclist and pedestrian road accidents and casualties with analyses by demographic characteristics for 2023.

www.gov.wales

Figure 17 shows the long-term quarterly trend of the number of casualties from collisions that occurred on either 20mph or 30mph roads. The quarterly number of casualties in road collisions on roads with lower speed limits (20mph and 30mph), has generally been declining steadily over the last decade, but quarterly figures can be volatile. Care should be taken when interpreting this data over a short time period.

The figures have been consistently declining for over a decade, long before anyone though of introducing 20mph limits.

 

[The Ham]

I guess this is the source being referred to

Reported road casualties: 2023 [HTML] | GOV.WALES

Data on motorcyclist, pedal cyclist and pedestrian road accidents and casualties with analyses by demographic characteristics for 2023.

www.gov.wales

The figures have been consistently declining for over a decade, long before anyone though of introducing 20mph limits.

Indeed, for example accident rates (for the UK as a whole) have over the last 35 years have fallen by about half.

Which is why I did say about the normal caveats about short term data applied, however those falls haven't been just because, there has been a long term desire to make drivers more competent, build better road vehicles, improve the safety of the infrastructure and increased training of highway engineers.

As such change from 30mph to 20mph on many roads are likely to be a factor in that reduction (of course other factors will be important too), how much is going to be hard to define (unless we reverse the change in speed limits) and until we have longer term data it's going to be unclear if it's sustained.

 

[BrianW]

I guess this is the source being referred to

Reported road casualties: 2023 [HTML] | GOV.WALES

Data on motorcyclist, pedal cyclist and pedestrian road accidents and casualties with analyses by demographic characteristics for 2023.

www.gov.wales

The figures have been consistently declining for over a decade, long before anyone though of introducing 20mph limits.

Sadly that neither supports nor contradicts the contention that 20mph limits are effective in reducing incidents and/or their severity. Correlation is not causation. As a researcher or consultant may be expected to say: 'More research required'.

 

[Bletchleyite]

Sadly that neither supports nor contradicts the contention that 20mph limits are effective in reducing incidents and/or their severity. Correlation is not causation. As a researcher or consultant may be expected to say: 'More research required'.

Correlation doesn't automatically indicate causation, but unless something else significant has changed, then correlation probably is causation here.

 

[Krokodil]

As such change from 30mph to 20mph on many roads are likely to be a factor in that reduction (of course other factors will be important too)

Those other factors will include other pet whinges of petrolheads: speed cameras, traffic calming, proper cycle infrastructure, pedestrianisation...

 

[BrianW]

Those other factors will include other pet whinges of petrolheads: speed cameras, traffic calming, proper cycle infrastructure, pedestrianisation...

It's reassuring to know that most drivers are (ie consider themselves to be!) above average drivers.
I think research evidence may suggest that drivers tend to drive to or exceed the designated speed limit (rather than that which rational observation may suggest). Improved mechanical acceleration and braking capabilities together with car safety improvements are said, I believe, to increased 'speeding'. Pedestrians, including children, remain as soft as ever.

 

[The Ham]

It's reassuring to know that most drivers are (ie consider themselves to be!) above average drivers.
I think research evidence may suggest that drivers tend to drive to or exceed the designated speed limit (rather than that which rational observation may suggest). Improved mechanical acceleration and braking capabilities together with car safety improvements are said, I believe, to increased 'speeding'. Pedestrians, including children, remain as soft as ever.

It's often joked that the safest car would be one with a massive spike in the middle of the steering wheel.

 

[Merle Haggard]

It's reassuring to know that most drivers are (ie consider themselves to be!) above average drivers.
I think research evidence may suggest that drivers tend to drive to or exceed the designated speed limit (rather than that which rational observation may suggest). Improved mechanical acceleration and braking capabilities together with car safety improvements are said, I believe, to increased 'speeding'. Pedestrians, including children, remain as soft as ever.

Calculating an average does not result in a level at which there's an equal number above and below that level.

There was interesting correspondence in a (then) broadsheet about a railway worker who said 'most people tip below the average'. This remark was initially ridiculed but it was subsequently shown it could well be true; if most tip around, say, £1 and one person tips extravagantly then the latter will move the average above all the others.

The are many examples of where the use of averages (not least by politicians!) are misleading, and why the use of mean is much more meaningful.

 

[Krokodil]

Calculating an average does not result in a level at which there's an equal number above and below that level.

If the measure of "average" is the median then it does

 

[Merle Haggard]

If the measure of "average" is the median then it does

A (mathematical) average is calculated by adding a group of numbers and dividing by the count of numbers - as in my example of tips. The median is the middle of a list of numbers, and overcomes the bias of a single outlier on an 'average' calculation. (I'm enclosing the word average in ' marks only to show it's not an adjective of 'calculation'...).

I think that average and median are two different things and language should not be loose, but I'm a mathematician at heart, cold logic .

 

[Oxfordblues]

In terms of enforcement, at least here in England, I understand that where the limit is 30mph the police only take action if drivers are doing 35mph-plus. So in effect the enforced limit is 35mph. Therefore where it's reduced to 20mph, as in Welsh urban areas, the effective limit is 25mph. If the police in Wales were to prosecute every motorist caught doing 21mph or above the criminal justice system would collapse!

 

[AdamWW]

If the police in Wales were to prosecute every motorist caught doing 21mph or above the criminal justice system would collapse!

I think if they did actually start doing that, driver behaviour would change quite quickly.

 

[Bletchleyite]

I think if they did actually start doing that, driver behaviour would change quite quickly.

In my experience of going there, people are largely complying. I was surprised!

== Doublepost prevention - post automatically merged: 1 Aug 2024 ==

In terms of enforcement, at least here in England, I understand that where the limit is 30mph the police only take action if drivers are doing 35mph-plus. So in effect the enforced limit is 35mph. Therefore where it's reduced to 20mph, as in Welsh urban areas, the effective limit is 25mph. If the police in Wales were to prosecute every motorist caught doing 21mph or above the criminal justice system would collapse!

10% + 2mph is what is usually used, i.e. 24mph for a 20.

It's still slower than 30. Reason they have it is that it reduces the calibration level required of the cameras.

 

[Merle Haggard]

10% + 2mph is what is usually used, i.e. 24mph for a 20.

It's still slower than 30. Reason they have it is that it reduces the calibration level required of the cameras.

Just to point out that some drivers seem to misunderstand this leeway - not just for 20, but other speed limits too.

In the example you make, people think that, if they drive at 24 m.p.h. they won't be prosecuted but, as you say, it's connected with the tolerance of the speed sensors and the 10 % could be in an unfavourable direction to them.

To spell it out; cameras record speed with a + or - 10% accuracy and if one is actually overstating the speed it might record an actual 24 m.p.h. as 26 m.p.h. - that would be within the camera's stated tolerance - and would result in an offence being recorded. There's the accuracy of the speed indicator you're relying on, too.

Edited to correct my arithmetic oops

 

[Bletchleyite]

Speedometers always read over, as it's a Construction and Use offence for one to read even 1mph under.

 

[Merle Haggard]

Speedometers always read over, as it's a Construction and Use offence for one to read even 1mph under.

I think many drivers now rely more on sat-nav devices or similar phone apps and rely on the 'correct' speed shown on them though - they don't seem to be included in the legislation, and show a slightly lower speed than indicated by the car speedo. Dash cams can record 'actual' speeds, too.

 

[AM9]

I think many drivers now rely more on sat-nav devices or similar phone apps and rely on the 'correct' speed shown on them though - they don't seem to be included in the legislation, and show a slightly lower speed than indicated by the car speedo. Dash cams can record 'actual' speeds, too.

But subject to the granularity and reliability of GPS reception. I doubt that a defence citing a GPS derived speed value would stand up that well without a record of speed and certified calibration. At least vehicle speedometers are nominally likely to be reading actual or over by design.

 

[Merle Haggard]

But subject to the granularity and reliability of GPS reception.

Exactly - speed seems to be calculated from the change in angles detected by satellites in the upper atmosphere.

An interesting debunking of the (in-) accuracy of satnavs is to take one on a train (with windows that allow the passage of radio waves, of course) - it will show you moving on the nearest road. I did it as an easy way of getting (approximate) speeds but had to put in a destination to get mine to work, with that result. As an amusing but perhaps irrelevant aside; I read somewhere that the Netherlands police were downloading data from sat-nav suppliers (which somehow recorded data from their products) to identify where speed limits were being regularly exceeded. If a similar operation was carried out in Britain they might discover an apparently reckless driver who was regularly achieving speeds of up to 125 m.p.h. on the back streets of large towns

 

[bspahh]

But subject to the granularity and reliability of GPS reception. I doubt that a defence citing a GPS derived speed value would stand up that well without a record of speed and certified calibration. At least vehicle speedometers are nominally likely to be reading actual or over by design.

I set my cruise control to 23/33/44/54/64/74 mph, which are usually reported as 20/30/40/50/60/70 mph by Waze. I don't need to rely on the live speed from Waze. My car speedometer is consistent, but inaccurate. I haven't had any tickets from average speed cameras, so I haven't had to use a defence.

 

[Cloud Strife]

I set my cruise control to 23/33/44/54/64/74 mph, which are usually reported as 20/30/40/50/60/70 mph by Waze. I don't need to rely on the live speed from Waze. My car speedometer is consistent, but inaccurate. I haven't had any tickets from average speed cameras, so I haven't had to use a defence.

With my car, 135km/h on the speed is 129-130km/h in reality.

 

[Krokodil]

If a similar operation was carried out in Britain they might discover an apparently reckless driver who was regularly achieving speeds of up to 125 m.p.h. on the back streets of large towns

The modeller who built the 009 Dduallt layout recently posted a screenshot from his car insurer's "spying app". It was asking him if he was the driver, the map showed a journey from Tan-y-Bwlch to Tan-y-Grisiau, including the spiral.

 

[dcbwhaley]

CCalculating an average does not result in a level at which there's an equal number above and below that level.

The are many examples of where the use of averages (not least by politicians!) are misleading, and why the use of mean is much more meaningful.

My statistics teacher liked to remind us that 99.999% of people have more than the average number of legs