20mph speed limits: what’s the data on pros and cons?

z1a

takman wrote: »

A car speedo will read atleast 2 mph an hour more than the speed you are going so you would need to be going an indicated 23 mph to have an actual speed of 21. But like the poster above said the chance of action being taken for 21 mph is very slim.

But why would it be more difficult to keep at 20 mpg mph than 30 mph?

Some, if not most, but not all. I recently got rid of a Saab, speedo pretty much agreed with satnav up to top speed.

unforeseen

Johno100 wrote: »

But it was central government that gave councils free reign to set speed limits. Unless in exceptional circumstances local councils used to have to set speed limits based on Department of Transport guidelines, in particular the 85th percentile rule.



http://www.safespeed.org.uk/speedlimits.html



20mpg is about what my car does at 20mph - really good for the environment.

Unless I'm misunderstanding that graph is saying that the crash risk increases when the speed is 30 mph above the 50% speed.

So at twenty the crash risk is still the lowest at 40

Apodemus

unforeseen wrote: »

Unless I'm misunderstanding that graph is saying that the crash risk increases when the speed is 30 mph above the 50% speed.

So at twenty the crash risk is still the lowest at 40

Sadly, no! You missed that the x-axis scale is "arbitrary units" not mph.

unholyangel

SouthLondonUser wrote: »

It is unlikely that any action would be taken at 21 mph. Unlikely but not impossible. The law has no leeway, no tolerance whatsoever.

The law does have leeway and tolerance. You can go any speed up to a maximum of 20mph - thats your leeway (providing you're not going so slow as to cause a danger to other road users).

As we keep hearing, a speed limit is the absolute maximum, not a target. Drivers should adjust their speed according to the surroundings - for example if you're driving down a busy street with heavy pedestrian traffic you should probably slow down to 15-25mph depending how bad it is but many drivers won't because "speed limits 30mph so I can go 30mph".

I believe 20mph is the magic number (rather than 5mph etc) because it the speed where the position significantly changes with regards to seriousness of harm and the probability of a pedestrian/cyclist dying if struck by a vehicle.

Not only that, it helps improve traffic flow (and fuel consumption). Constantly accelerating and braking at speeds of 30mph is worse for fuel consumption (and ergo worse for air pollution) than speeds of 20mph because less fuel is used to get up to speed and therefore less fuel is wasted when you have to stop again. If you remember the thread on here a while back complaining about people not driving forward a few metres in traffic and people explaining the concertina effect (which is caused by people driving too close to people and having to brake while if they stayed at a slower constant speed, it keeps traffic flowing), it shows that a slower constant speed is much better for fuel consumption & traffic flow than faster speeds where you have to brake/stop and then get back up to speed.

Plus it also apparently (according to figures from other EU countries who enacted similar speed limits) cuts down on traffic - perhaps someone who would have driven through there to get home will now take an alternative route where their speed wont be limited or someone who was going to take the car for a 5 min drive down the road will walk or take their bicycle instead.

nicechap

SouthLondonUser wrote: »

....... Note the question is “what’s the data?”, not “what is your totally unsubstantiated opinion?”....

SouthLondonUser wrote: »

..... I had read the document you posted. ... (and found unconvincing).

......

Not a lot of point engaging with someone who has already answered their own question but then overlays it with their own unsubstantiated opinion

SouthLondonUser

nicechap wrote: »

Not a lot of point engaging with someone who has already answered their own question but then overlays it with their own unsubstantiated opinion

(Text removed by MSE Forum Team)

Mine would be an unsubstantiated opinion if I had said I think that 20mph limits are or are not safer, without providing any evidence to back up this claim. This is not what I have said!

I have explained why I think assessing the effectiveness of 20mph limits is very challenging, and said that neither proponents, like the Lambeth council or the 20 is plenty campaign, nor opponents, like the association of British drivers who say they are ineffective, have addressed these points - this is why I find both unconvincing.

SouthLondonUser

unholyangel wrote: »

As we keep hearing, a speed limit is the absolute maximum, not a target. Drivers should adjust their speed according to the surroundings - for example if you're driving down a busy street with heavy pedestrian traffic you should probably slow down to 15-25mph depending how bad

True. There are plenty of roads in Westminster where the speed limit is 30mph but I always ride at 20 or below. However, there are also major roads in Lambeth and Southwark where the speed limit is 20 but I think the driving conditions would warrant 30; Kennington road is one that comes to mind, https://goo.gl/maps/vrThyiehXPq but it's far from the only one. It;s relatively wide and straight, generally good visibility, no schools, plenty of traffic lights for pedestrians to cross in absolute safety.

The point is that riding at 21mph does not make me a dangerous, hardened criminal, and riding at 15 (to be sure I am well within the limit) is too slow, would cause too much congestion, and would incentivise other motorists to overtake me, even dangerously.

unholyangel wrote: »

it is but many drivers won't because "speed limits 30mph so I can go 30mph".

Idiots exist in all categories of road users, unfortunately. Btw, one of the difficulties of respecting the 20mph limits on roads like the one I mentioned is that many motorists get angry if I do and attempt to overtake dangerously.

unholyangel wrote: »

I believe 20mph is the magic number (rather than 5mph etc) because it the speed where the position significantly changes with regards to seriousness of harm and the probability of a pedestrian/cyclist dying if struck by a vehicle.

Like I said, I think it is more complex than that. Speed is only one of the factors. Park lane (northbound) is one of the few, if not the only one, road in central London with a 40mph limit. Yet I'm ready to bet there are way fewer pedestrian collisions there than on a busy high street with cars driving at average speeds of 10mph.
Two of the greatest annoyances of my motorcycle commute in London are pedestrians crossing where they shouldn't and bicycles undertaking me when I am turning left (which should prove that I ride slowly!). 20mph are unfortunately an incentive for this dangerous behaviour.

Also, 20mph limits make it harder, and in some cases impossible, to overtake slow bicycles, even when it would be safe to do so. This means the potential for more congestion.

unholyangel wrote: »

Not only that, it helps improve traffic flow (and fuel consumption). Constantly accelerating and braking at speeds of 30mph is worse for fuel consumption (and ergo worse for air pollution) than speeds of 20mph because less fuel is used to get up to speed and therefore less fuel is wasted when you have to stop again.

But the fact remains that, if you are travelling longish distances through London outside of rush hour, a 20mph limit increases your journey substantially. Not by 50%, but certainly by some material amount. Is this totally irrelevant for congestion and pollution?

unholyangel wrote: »

Plus it also apparently (according to figures from other EU countries who enacted similar speed limits) cuts down on traffic - perhaps someone who would have driven through there to get home will now take an alternative route where their speed wont be limited

???????? What alternative routes? There aren't any in the councils that have implemented council-wide 20mph limits.

unholyangel wrote: »

or someone who was going to take the car for a 5 min drive down the road will walk or take their bicycle instead.

Yes, a favourite argument of the 20mph lobby. Totally false in London, however, at least in central London. Driving in central London is so expensive, between congestion charge parking and what not, that almost no one drives there. The notion that London is full of lazy motorists who take their car instead of public transport or a pushbike for a 5-minute journey is ridiculous. Anyone commuting on the road in London will notice a huge amount of minicabs (we probably have too many, but that's a separate topic), HGVs, delivery vans, etc. The often-quotes statistic about the average car journey in London being something like 3 miles is also equally misleading, because the TFL report where that figure comes from - unsurprisingly - specifies that car ownership and usage is way way lower in the inner London boroughs, so that average may well be driven (pun intended...) by people in zone 6 driving to the supermarket or the train station because they have no alternative.

Note that I do not even own a car (only a motorcycle); I am very much in favour of any initiative to reduce car usage in city centres. But I also hate it when false statistics which don't mean anything are quoted to try to prove a shaky point.

Me, I started commuting by motorcycle when the SouthernFail shambles made my commute by train a nightmare (one train per hour instead of one every 10 minutes both last summer and this one).

Car_54

SouthLondonUser wrote: »

@Car54, with a digital speedometer a quick glance tells me whether I'm at 19, 21 or whatever. With an analogue one, a quick glance at the needle tells me that I am very close to 20, but I cannot with absolute certainty tell whether I am doing 19 or 21 or else. And, before you ask, I regularly have my eyesight checked and I do wear the correct type of glasses when driving.
Also, even if the speedometer is digital, it doesn't take a Ferrari or an MV F4 Serie Oro to go from 20 to 21mph without actually realising...

It is unlikely that any action would be taken at 21 mph. Unlikely but not impossible. The law has no leeway, no tolerance whatsoever.

Similarly, the offer of a course is very probable, but is neither a requirement not certain. The fact remains it is illegal to drive at 21mph in a 20mph zone. I think it's wrong and stupid, not because speed limits must be breached, but because spending more time looking at the speedometer than at the road makes driving more dangerous, not less.

But a digital speedo is simply more precise, not more accurate.

THe law may have no leeway, but the police and prosecutors certainly do and they work within published guidelines. Action outside those guidelines is virtually unheard of.

The offer of a course may not be certain, but the offer of a fixed penalty certainly is, within the published limits and provided the driver is identified unequivocally and in a timely manner.

brat

Johno100 wrote: »

But it was central government that gave councils free reign to set speed limits. Unless in exceptional circumstances local councils used to have to set speed limits based on Department of Transport guidelines, in particular the 85th percentile rule.



http://www.safespeed.org.uk/speedlimits.html



20mpg is about what my car does at 20mph - really good for the environment.

I had a look at the data from Portsmouth a few months ago, and found it entirely unconvincing. As usual, those who were keen to extol the virtues of 20mph limits made much of the slight reduction in KSIs in the 20mph zones, without taking full account of the reduced traffic volumes, while they ignored the fact that accidents had increased outside the zones showing that the KSIs had simply been displaced rather than reduced.

I'm not against 20mph on residential roads, although imho they'd benefit from being advisory rather than mandatory - to imbue a sense of responsibility on the motorist.

Nice to see Smithy's charts linked. Miss that guy!

Mercdriver

SouthLondonUser wrote: »

True. There are plenty of roads in Westminster where the speed limit is 30mph but I always ride at 20 or below. However, there are also major roads in Lambeth and Southwark where the speed limit is 20 but I think the driving conditions would warrant 30; Kennington road is one that comes to mind, https://goo.gl/maps/vrThyiehXPq but it's far from the only one. It;s relatively wide and straight, generally good visibility, no schools, plenty of traffic lights for pedestrians to cross in absolute safety.

The point is that riding at 21mph does not make me a dangerous, hardened criminal, and riding at 15 (to be sure I am well within the limit) is too slow, would cause too much congestion, and would incentivise other motorists to overtake me, even dangerously.


Idiots exist in all categories of road users, unfortunately. Btw, one of the difficulties of respecting the 20mph limits on roads like the one I mentioned is that many motorists get angry if I do and attempt to overtake dangerously.

Like I said, I think it is more complex than that. Speed is only one of the factors. Park lane (northbound) is one of the few, if not the only one, road in central London with a 40mph limit. Yet I'm ready to bet there are way fewer pedestrian collisions there than on a busy high street with cars driving at average speeds of 10mph.
Two of the greatest annoyances of my motorcycle commute in London are pedestrians crossing where they shouldn't and bicycles undertaking me when I am turning left (which should prove that I ride slowly!). 20mph are unfortunately an incentive for this dangerous behaviour.

Also, 20mph limits make it harder, and in some cases impossible, to overtake slow bicycles, even when it would be safe to do so. This means the potential for more congestion.
But the fact remains that, if you are travelling longish distances through London outside of rush hour, a 20mph limit increases your journey substantially. Not by 50%, but certainly by some material amount. Is this totally irrelevant for congestion and pollution?

???????? What alternative routes? There aren't any in the councils that have implemented council-wide 20mph limits.

Yes, a favourite argument of the 20mph lobby. Totally false in London, however, at least in central London. Driving in central London is so expensive, between congestion charge parking and what not, that almost no one drives there. The notion that London is full of lazy motorists who take their car instead of public transport or a pushbike for a 5-minute journey is ridiculous. Anyone commuting on the road in London will notice a huge amount of minicabs (we probably have too many, but that's a separate topic), HGVs, delivery vans, etc. The often-quotes statistic about the average car journey in London being something like 3 miles is also equally misleading, because the TFL report where that figure comes from - unsurprisingly - specifies that car ownership and usage is way way lower in the inner London boroughs, so that average may well be driven (pun intended...) by people in zone 6 driving to the supermarket or the train station because they have no alternative.

Note that I do not even own a car (only a motorcycle); I am very much in favour of any initiative to reduce car usage in city centres. But I also hate it when false statistics which don't mean anything are quoted to try to prove a shaky point.

Me, I started commuting by motorcycle when the SouthernFail shambles made my commute by train a nightmare (one train per hour instead of one every 10 minutes both last summer and this one).

Seems to me that you need to ride more positively in the way some cyclists do so that others can't try and pass you in an unsafe way. If you can't safely pass a slow cyclist, how wide is your bike? Is it a Gold Wing?

As far as traffic in central London is concerned, it still exists in the congestion charge zone. Bus travel is a nightmare because of it. I walk rather than be a road user in central London.