East Dulwich (Mobile) Forum

20mph limits: pros and cons?

This started as an off-topic in other thread: [www.eastdulwichforum.co.uk] but I think it's more appropriate to have a separate topic.

I would like to learn more about the case for 20mph speed limits. I have a number of reservations, because I have not found convincing evidence presented by the councils that implemented this measure, nor by the campaigns that support it.

I am very much in favour of 20 mph speed limits in narrow residential roads. I am, however, not entirely convinced of the merits of 20mph speed limits on large roads, especially those, like most wide A roads in London, with multiple lanes, parking forbidden on both sides, and limited pedestrian crossings: any pedestrian who tried to cross outside of the designated crossings on this wide roads would be stupidly jeopardising his life and that of others – not to mention committing a sanctionable offence. I therefore do not understand how 20mph limits would make pedestrians any safer on these roads – if anything, they might achieve the opposite effect by encouraging the dangerous behaviour of crossing illegally outside of the designated pedestrian crossings - something I have already witnessed first hand.

During rush hour in busy cities like London, motor traffic very rarely reaches 30mph so I would expect the impact of 30mph speed limits to be negligible. However, 20mph limits would certainly increase journey times outside of rush hour, e.g. for night journeys. This can be a very big inconvenience for all road users, including users of public transport (bus users).

It is often mentioned that 20mph limits would ease congestion; I do not understand why this would be, since when roads are congested most vehicles already travel at speeds below 20mph. I understand variable speed limits may help ease congestion on motorways, but I don't understand how motorways may be comparable to urban roads - there are no traffic lights on motorways forcing motorists to stop every 3 minutes.

Of course being hit at 20mph is better than being hit at 30mph. Being hit at 10mph is even better, but this is not a valid reason to enforce 10mph limits! What is the evidence that 20mph limits actually have any effect on the number and severity of collisions and accidents? The London council of Lambeth mentions some very vague statistics on a supposed reduction in collisions in other parts of the country, but, without any details to substantiate these claims, these numbers are meaningless. The scientific way to analyse this problem would be a controlled experiment: taking two similar roads, one with 30mph and one with 20mph, are the collisions significantly different in the two? Do you know of any such study being carried out? I couldn’t find any data. These experiments are also hard to interpret because, luckily, the number of serious incidents is relatively small; according to TFL data, the number of road casualties in a given year in London is around 120. 120 too many, no doubt, but if 120 is the total, how many casualties can a single road account for? What is the natural variability? Making sense of rare data may well be impossible: if in a given road there was one accident in 2010, two in 2011 and one in 2012, does it really make sense to say that accidents first doubled then halved?

Does anyone have insights into what the main causes of collisions and casualties are? If it were all about speed, then there should be more collisions in the Northbound lanes of Park lane (the only urban rod with a 40mph speed limit in London, as far as I know) than in the southbound ones (30 mph); is this the case?

Finally, is it true, as some drivers' associations claim, that the Department for Trasnsport had commissioned reserach into the issue? If yes, when will it be published, and why did so many councils not wait for this to be made public?
During rush hour in busy cities like London, motor traffic very rarely reaches 30mph so I would expect the impact of 30mph speed limits to be negligible. However, 20mph limits would certainly increase journey times outside of rush hour, e.g. for night journeys. This can be a very big inconvenience for all road users, including users of public transport (bus users).

Two thirds of car journeys in London are of three miles or less. At 30MPH a three mile journey takes six minutes. At 20MPH it takes nine minutes. Is the difference between six and nine minutes for the majority of car journeys in London a "very big inconvenience"?
Yes
Nobody, Keano, and I include myself, who spends much time commenting on the EDF can claim their lives are so busy they haven't got the odd three minutes to spare.
You miss the point of your own analysis rh.

An extra 3 minutes x hundreds of journeys = more time in car burning fuel and spewing out emissions. Not good.
speed limit 20 driver go 25/30

speed limit 30 driver go 40
@rendelharris, And the source of the 2/3 statistics is? A statistic is utterly meaningless unless someone explains how it was calculated. For example, the statistical 'estimates' for the official immigration numbers were criticised because they were based on surveys taken in the main airports, at times which excluded arrivals from specific destinations. Oh, and you only need to look at the main political events in the UK and US over the last year or so to realise how wrong surveys and polls can be!

Also, your calculation is quite meaningless, because speed is far from constant, and there are lots of traffic lights everywhere. A lower speed limit means a greater likelihood of encountering more red lights and taking longer to complete your journey.

And, as keano rightly points out, more time x all the vehicles in London means more emissions and more pollution!
I'm all for the 20 mph speed limit in fact I think that it should be applied to all the roads in Southwarks jurisdiction however I do feel that on the main road that goes across the top of Sydenhnam towards Crystal Palace that signage should be clearer or more frequent signage and the reason is that there are constantly vehicles riding right up my back bumper when I'm trying to adhere to the stipulated 20mph.
alice Wrote:

speed limit 20 driver go 25/30

speed limit 30 driver go 40

Not quite, because it's full of cameras everywhere and, especially in North London, lots of fines have been handed out to motorists going over 20
keano77 Wrote:

You miss the point of your own analysis rh.

An extra 3 minutes x hundreds of journeys = more
time in car burning fuel and spewing out
emissions. Not good.

You do know that it takes more fuel (hence more emissions) for a car to drive at 30MPH than 20MPH? Rather basic physics.

ETA When including the acceleration requirements, I should add.



Edited 1 time(s). Last edit was april 20, 10:07pm by rendelharris.

I'm looking forward to the time flying cars become the norm.

No doubt Southwark Council currently has a working group looking it the matter trying to devise absurd rules and drone traffic lights in another money-making wheeze.
DulwichLondoner Wrote:

@rendelharris, And the source of the 2/3
statistics is?

TfL: [content.tfl.gov.uk] page three. Really, you are capable of looking this stuff up yourself instead of every time someone says something with which you disagree demanding sources - something notably lacking from your own posts. Good night.



Edited 1 time(s). Last edit was april 20, 10:05pm by rendelharris.

KidKruger Wrote:

I'm all for the 20 mph speed limit in fact I think
that it should be applied to all the roads in
Southwarks jurisdiction however I do feel that on
the main road that goes across the top of
Sydenhnam towards Crystal Palace that signage
should be clearer or more frequent signage and the
reason is that there are constantly vehicles
riding right up my back bumper when I'm trying to
adhere to the stipulated 20mph.

Agreed - I'd quite like drivers on East Dulwich Grove to stop getting up to the back wheel of my bicycle and hooting as well when I'm doing 20MPH going down towards Lordship Lane - pointing at the huge 20 circles clearly visible on the road just seems to annoy them further...
@rendelharris, you mentioned this: [www.rospa.com] Have you read it? It agrees with my key points. Do you agree with me, too, or have you simply not read the document you mentioned? smiling smiley

The document mentions that:

"the relatively low numbers of recorded KSI casualties
in Portsmouth mean that small fluctuations up and down by chance can have an undue influence on this.
Much of this increase came from the number of pedestrians injured and it was not possible to measure
whether the amount of pedestrian activity had increased following the introduction of the 20mph limits."

Its conclusions are:

"RoSPA does not believe that 20mph
speed limits are suitable for every road in a local authority area. They should be targeted at roads that are
primarily residential in nature and on town or city streets where pedestrian and cyclist movements are high (or
potentially high), such as around schools, shops, markets, playgrounds and other areas. Roads which are not
suitable for 20mph limits are major through routes. "

Both the criticism above and the conclusions are exactly my points! I am all for 20mph in narrow residential roads, or in roads which may be wider but are busy high streets, with cars parked on both sides, people everywhere, and lots of children who might unexpectedly jump onto the street from a parked car which hides them from view (eg the Lordship Lane high street). Like I said, I am not convinced of the benefits of 20mph limits on "major through routes".
"RoSPA does not believe that 20mph
speed limits are suitable for every road in a local authority area. They should be targeted at roads that are
primarily residential in nature and on town or city streets where pedestrian and cyclist movements are high (or
potentially high), such as around schools, shops, markets, playgrounds and other areas. Roads which are not
suitable for 20mph limits are major through routes. "

Both the criticism above and the conclusions are exactly my points! I am all for 20mph in narrow residential roads, or in roads which may be wider but are busy high streets, with cars parked on both sides, people everywhere, and lots of children who might unexpectedly jump onto the street from a parked car which hides them from view (eg the Lordship Lane high street). Like I said, I am not convinced of the benefits of 20mph limits on "major through routes".


How many "major through routes" of that nature (with low pedestrian and cyclist movement and away from schools, shops, markets and playgrounds) can you think of in London? If you want to keep Park Lane 40MPH that's fine, where else in our local area or in central London can you apply that description to?
rendelharris Wrote:

TfL:
[content.tfl.gov.uk]
avels-by-car-in-london.pdf page three.

Thank you. I find the statistic you quoted utterly meaningless, for the simple reason that it is the average across extremely different areas. Unsurprisingly, the report mentions that car use is much lower in inner London boroughs. For all we know, that statistic could very easily be driven by people living in outer London boroughs and needing to reach destinations not well served by public transport: the local railway station, the local supermarket, etc.; after all, these are the very people who'd need a car in the first place, whereas, unsurprisingly, car ownership and use are much lower in the inner boroughs.

I have seen the same statistic mentioned numerous times by cycle and green activists, who say it with great outrage, as if this proved that the average central London resident was so lazy to drive everywhere instead of using public transport, while, in fact, the reality could well be different!

> Really,
> you are capable of looking this stuff up yourself
> instead of every time someone says something with
> which you disagree demanding sources - something
> notably lacking from your own posts. Good night.

I only quote information I am reasonably sure of. Sure, this causes me to quote fewer sources and data points than other people smiling smiley
Jolly good - you ask for the source of my statistic, I show you it comes from a TfL report, you dismiss it as totally meaningless. Care to answer my question above about the roads in this area or central London "with low pedestrian and cyclist movement and away from schools, shops, markets and playgrounds" suitable for higher speed limits?
Can't think of many to be fair. However, it doesn't seem to worry Lambeth. Driving on the South Circular towards Dulwich College and realising I've left Southwark's 20mph gives me a Brexit-like feeling of freedom.
DulwichLondoner Wrote:

@rendelharris, you mentioned this:
[www.rospa.com]
/road-safety/drivers/20-mph-zone-factsheet.pdf
Have you read it? It agrees with my key points. Do
you agree with me, too, or have you simply not
read the document you mentioned? smiling smiley

The document mentions that:

"the relatively low numbers of recorded KSI
casualties
in Portsmouth mean that small fluctuations up and
down by chance can have an undue influence on
this.
Much of this increase came from the number of
pedestrians injured and it was not possible to
measure
whether the amount of pedestrian activity had
increased following the introduction of the 20mph
limits."

Its conclusions are:

"RoSPA does not believe that 20mph
speed limits are suitable for every road in a
local authority area. They should be targeted at
roads that are
primarily residential in nature and on town or
city streets where pedestrian and cyclist
movements are high (or
potentially high), such as around schools, shops,
markets, playgrounds and other areas. Roads which
are not
suitable for 20mph limits are major through
routes. "

Both the criticism above and the conclusions are
exactly my points! I am all for 20mph in narrow
residential roads, or in roads which may be wider
but are busy high streets, with cars parked on
both sides, people everywhere, and lots of
children who might unexpectedly jump onto the
street from a parked car which hides them from
view (eg the Lordship Lane high street). Like I
said, I am not convinced of the benefits of 20mph
limits on "major through routes".


Totally agree.
@rendelharris, yes, your figure comes from TFL, but I have explained in great detail why I dare consider it meaningless. Would you like to challenge my opinion with some concrete argument, or do you think we should just switch our brain off and shut up when an official body writes something?
Yes, your number comes from TFL, but so does the (rather obvious and unsurprising) statement that car usage and ownership vary a great deal between the inner and outer boroughs, which was the basis of my argument. Do you disagree with that? Do you disagree that that statistic could well be driven by people living in the outer boroughs and needing the car for relatively short trips to destinations not served by public transport? Again, after all those are the very people who'd need a car, whereas lots of people in the inner boroughs don't even have one, and those who do use it little.
It's like saying that official data from HMRC show the average London income to be £X. It would be quite meaningless as an average because the variability would be huge!
rendelharris Wrote:

How many "major through routes" of that nature
(with low pedestrian and cyclist movement and away
from schools, shops, markets and playgrounds) can
you think of in London? If you want to keep Park
Lane 40MPH that's fine, where else in our local
area or in central London can you apply that
description to?

Let's see...

Lordship lane runs all the way to Forest Hill and the South Circular. Most of it is not a high street.
East Dulwich Grove is neither a high street nor narrow.
Many stretches of the South Circular are not high street.
Parts of Camberwell road.
Most of the A2 from New Kent road, to Old Kent road and New Cross gate at least.
Brixton Hill.
Streatham High road (the limit may already be 30mph there, I'm not sure).

Also, let's not forget that a 30mph limit does not mean that the speed will always be 30mph, everywhere. I think the speed limit is 30 in most, if not all, of Westminster, yet I do not see all motorcyclists and drivers crazily rushing at 30mph in narrow residential roads there - sure, there is the occasional idiot, but the occasional idiot will always be present, even with 20mph limits.
rendelharris Wrote:

You do know that it takes more fuel (hence more
emissions) for a car to drive at 30MPH than 20MPH?
Rather basic physics.

Yes, but at the same time journeys tend to take longer. I genuinely do not know if the two effects offset each other. Do you?
From the AA

Cutting the speed limit from 30 mph to 20 mph on the wrong roads can increase CO2 emissions by more than 10% with the result that well-intentioned safety schemes may backfire in environmental terms.

On average, petrol car fuel consumption on longer and relatively free-flowing 20mph urban streets can worsen by 5.8 miles per gallon (1.3 miles/litre). Over a year this will significantly increase CO2 emissions – burning 1 litre of unleaded petrol produces 2.36kg of CO2.

[www.theaa.com]

The AA's fuel consumption tests were carried out at Millbrook proving ground by an independent engineer and car tester, using a fuel flow meter.

In 2000, the then Department of Environment, Transport and the Regions argued against reducing the 30 mph limit for fear of increasing emissions. (See above link)
Also, some deaths and injuries are caused by behaviour which is unlikely to be affected by a 20mph limit. E.g. drunk driving, crazy speeding (ie if you're stupid enough to go at 50mph in an urban area, a 20mph limit is unlikely to deter you), low-speed collisions at intersections, etc. Increases or reductions in these types of collisions have little to do with speed limits.
There's no point in saving a few deaths at 20mph if you're helping to destroy the planet. Ergo, 30mph please Southwark and other misguided councils.
If some of you insist that you have the right to abuse speed limits to save time I have the same right to run unecessary red lights when cycling.
malumbu Wrote:

If some of you insist that you have the right to
abuse speed limits to save time I have the same
right to run unecessary red lights when cycling.

???????
The point being missed here is that on any given journey I'd be amazed if anyone could travel at more than an average of 20 mph in London. Therefore the limit seems reasonable.
Good point edcam.

Before the 20mph limit was introduced I regularly drove to Tower Bridge each day (ED>Peckham>Old Kent Rd>Bermondsey rat run) 8 miles according to my mileometer. Took 30 mins on average (hold ups, heavy traffic, lights etc).

If my sums are correct 8 miles in 30 mins is 16mph
keano77 Wrote:

From the AA

Cutting the speed limit from 30 mph to 20 mph on
the wrong roads can increase CO2 emissions by more
than 10% with the result that well-intentioned
safety schemes may backfire in environmental
terms.

On average, petrol car fuel consumption on longer
and relatively free-flowing 20mph urban streets
can worsen by 5.8 miles per gallon (1.3
miles/litre). Over a year this will significantly
increase CO2 emissions – burning 1 litre of
unleaded petrol produces 2.36kg of CO2.

[www.theaa.com]
ds-emissions.html

The AA's fuel consumption tests were carried out
at Millbrook proving ground by an independent
engineer and car tester, using a fuel flow meter.

In 2000, the then Department of Environment,
Transport and the Regions argued against reducing
the 30 mph limit for fear of increasing emissions.
(See above link)

That report only assesses MPG in completely ideal conditions, and I agree that if you have two cars driving for ten miles on a completely clear road the one travelling 30MPH will (for most models) have lower fuel consumption than the one going 20MPH. However, no account is taken in their figures of acceleration and deceleration, in particular the extra fuel required to accelerate to 30MPH as opposed to 20MPH. In practice, in London, it's virtually impossible to travel more than about 300 yards before having to slow and/or stop for an intersection, lights, jam etc, in which case a car accelerating to 20MPH and down again will use less fuel (and, incidentally, lose no time over its more leaden-footed counterpart). The AA test is a clear example of the problem of carrying out a test in "laboratory" conditions with no regard for real world circumstances.
edacm wrote:

>The point being missed here is that on any given journey
>I'd be amazed if anyone could travel at more than an average
>of 20 mph in London. Therefore the limit seems reasonable.

keano77 Wrote:

Good point edcam.

Before the 20mph limit was introduced I regularly
drove to Tower Bridge each day (ED>Peckham>Old
Kent Rd>Bermondsey rat run) 8 miles according to
my mileometer. Took 30 mins on average (hold ups,
heavy traffic, lights etc).

If my sums are correct 8 miles in 30 mins is 16mph

So you agree that 20MPH limits are reasonable, it seems. Excellent.

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