20mph limits: pros and cons?

[JohnL]

Cardelia Wrote:

-------------------------------------------------------

> rendelharris Wrote:

> --------------------------------------------------

> -----

> > Is there any evidence that at 30MPH you'd be

> > avoiding red lights? Surely you'd just be

> > arriving at the next red light more quickly

>

> Surely you'd arrive at the next *traffic* light

> more quickly. How do you know it's going to be

> red?

Experience - I think they're staggered.

The only time I've seen people gain road is when they go

60 or so - and that's dangerous

[Lowlander]

jimlad48 Wrote:

-------------------------------------------------------

> As you say, every car is different, but on the

> cars I've driven, trying to keep them to 20

> involves either signifantly overrevving the engine

> in 2nd, or borderline stalling it in 3rd.

>

> I, and plenty of other people I've spoken to about

> this find that driving at 20 is actually far more

> distracting to the driver than driving at 30

> because it paradoxically adds more work to monitor

> the car than would be the case at 30 where you can

> keep it in 3rd and not worry to the same level. I

> fully appreciate everyones experience is

> different, but as someone who has driven for 20

> years, I'd say that I find 20MPH limits more

> challenging than 30MPH limits to drive safely in.

> I would be very interested to see the stats on

> accidents - ultimately if a driver is more focused

> on the speed he is doing and processing the cars

> speed, he is not able to focus to the road to the

> same extent.

>

> As a driver I don't speed, don't break posted

> speed limits and drive defensively and anticipate

> likely changes to the road - but I still find

> driving at 20MPH more challenging than 30MPH

> because of the extra workload involved in managing

> to keep the car below the limit and not be done

> for speeding. Personally I think 25MPH would be a

> far more sensible half way house.

And I find it much easier to drive at 20, as do the only people I know who I've asked :-) You'll get used to it.

I've tried using cruise control myself, but am only able to keep to a max speed for 30 seconds or so in Dulwich before having to slow down again for lights, junctions or traffic.

Anyway, stats? How about some evidence from the British Medical Journal showing a 40% reduction in injuries:

http://www.bmj.com/content/339/bmj.b4469

Or Oxford University:

https://academic.oup.com/jpubhealth/article/37/3/515/2362676/Go-slow-an-umbrella-review-of-the-effects-of-20

[rendelharris]

Lowlander Wrote:

-------------------------------------------------------

> I've tried using cruise control myself, but am

> only able to keep to a max speed for 30 seconds or

> so in Dulwich before having to slow down again for

> lights, junctions or traffic.

Mrs.H's new Smart has both cruise control and a speed limiter, so she can actually tell the car not to let her go above a chosen speed. Is this not a common feature now?

Excellent links by the way, thanks.

[Cardelia]

rendelharris Wrote:

-------------------------------------------------------

> Cardelia Wrote:

> --------------------------------------------------

> -----

> > rendelharris Wrote:

> >

> --------------------------------------------------

>

> > -----

> > > Is there any evidence that at 30MPH you'd be

> > > avoiding red lights? Surely you'd just be

> > > arriving at the next red light more quickly

> >

> > Surely you'd arrive at the next *traffic* light

> > more quickly. How do you know it's going to be

> > red?

>

> And how do you know it's going to be green?

> Traffic lights in 20MPH zones are phased for 20MPH

> driving, so the smoothest flow through them will

> be at the posted speed limit.

I don't know that it's going to be green. But then I'm not the one making the assertion that at 30 mph you just get to the next red light more quickly when compared to 20 mph.

How do you know that traffic lights are phased for 20 mph driving in a 20 mph zone? Different roads will have different optimal traffic flows. Factors such as capacity, popularity, the number of side roads, bus stops, pedestrian crossings etc. will all affect the average traffic speed of a road, and this will determine how lights are phased. Traffic engineers have hideously complicated models which calculate the optimal traffic flow for whole areas, not just a single road, so there is no one size fits all approach.

[DuncanW]

I thought that the 20mph limit did not apply to red routes.

So there aren't may of those in ED, but OKR and NKR are obvious examples where traffic can flow a bit faster.

Sydenham Hill sticks out as somewhere where 30mmph could be reintroduced - it feels unnecessary there and as soon as you get to the end of it, you're out of Southwark and it's all 30 mph again anyway.

[rendelharris]

DuncanW Wrote:

-------------------------------------------------------

> I thought that the 20mph limit did not apply to

> red routes.

> So there aren't may of those in ED, but OKR and

> NKR are obvious examples where traffic can flow a

> bit faster.

>

> Sydenham Hill sticks out as somewhere where 30mmph

> could be reintroduced - it feels unnecessary there

> and as soon as you get to the end of it, you're

> out of Southwark and it's all 30 mph again anyway.

Lewisham's had a borough-wide 20MPH limit since last September.

[JohnL]

DuncanW Wrote:

-------------------------------------------------------

> I thought that the 20mph limit did not apply to

> red routes.

> So there aren't may of those in ED, but OKR and

> NKR are obvious examples where traffic can flow a

> bit faster.

>

> Sydenham Hill sticks out as somewhere where 30mmph

> could be reintroduced - it feels unnecessary there

> and as soon as you get to the end of it, you're

> out of Southwark and it's all 30 mph again anyway.

But most of my time on urban roads is spent standing still.

The speed you travel in between being stationary doesn't seem to matter in comparison to the long stationary periods.

I use my car for shopping mainly - so to Tescos or Sainsbury.

Peckham Rye -> Sainsburys for example. Most of the time is spent waiting to turn right at Peckham Rye or at East Dulwich roundabout - the rest is insignificant to the total time IMHO.

[DulwichLondoner]

rendelharris Wrote:

> And how do you know it's going to be green?

> Traffic lights in 20MPH zones are phased for 20MPH

> driving, so the smoothest flow through them will

> be at the posted speed limit.

The difference in busy urban areas at rush hour will be minimal. But outside of rush hour it won't. And let's please ignore the 'average' journey within London - that's about as relevant as calculating the 'average' salary between myself and one of the residents of a Belgravia villa. Consider going from south to North London at night, when the roads will be mostly empty. That's when the speed limit can make a difference. A shorter journey time can mean less pollution. Traffic lights programmed for 30mph limits can mean vehicles will spend less time at traffic lights at night, and, again, pollute less.

[rendelharris]

DulwichLondoner Wrote:

>

> The difference in busy urban areas at rush hour

> will be minimal. But outside of rush hour it

> won't. And let's please ignore the 'average'

> journey within London - that's about as relevant

> as calculating the 'average' salary between myself

> and one of the residents of a Belgravia villa.

> Consider going from south to North London at

> night, when the roads will be mostly empty. That's

> when the speed limit can make a difference. A

> shorter journey time can mean less pollution.

> Traffic lights programmed for 30mph limits can

> mean vehicles will spend less time at traffic

> lights at night, and, again, pollute less.

Not quite sure of the logic there, most traffic lights (as opposed to pelican crossings) are at intersections so they have to be programmed for a certain amount of time allowing north-south traffic through, a certain amount east-west, no matter what the speed of traffic. Let's say there are two intersections a mile apart and they alternately allow N/S and E/W traffic through for two minutes at a time each. If I leave intersection A as it turns green and drive at 30MPH, I'll get to intersection B just in time for the light to turn red as I approach and have to wait for two minutes with engine idling. If I leave A at 20MPH I'll get to B with the light on red but only have to wait one minute - and I'll be behind the car which did 30MPH but won't have burned the extra fuel required to accelerate to 30.

A simplistic example but the point is that whatever the traffic speed lights have to change to allow cross traffic, they're not there for no reason.

You can't ignore the "average" journey in London, if the majority of journeys are of a certain distance then traffic systems have to be designed for that distance - you surely can't argue that 30MPH limits should be introduced simply because they'll make it quicker to get across London in the middle of the night? Also nighttime is when people are more tired, visibility is obviously worse, there will be more inebriated pedestrians about - absolutely not the time to have a higher limit, I'd say.

[DulwichLondoner]

Lowlander Wrote:

> Anyway, stats? How about some evidence from the

> British Medical Journal showing a 40% reduction in

> injuries:

First of all, this is precisely the kind of research that the 20mph campaigners should be publishing on their website and shouting from the rooftops. The fact that they don't makes me extremely suspicious and makes me think that either evidence does not exist, or that these campaigns don't feel they need them because they have a dogmatic ideological approach.

Isn't it odd that the conclusions of these studies are radically different from those I quoted, which prompted the DfT to define the first trials 'inconclusive', and to commission more research into the matter? The DfT did the right thing: inconclusive doesn't mean it works, doesn't mean it doesn't, it means: "we can't really tell". Why did so many councils not wait for the DfT to finish its job?

For example, in another article on the BMJ, the same researches are called to defend their conclusion because, well, practical experience seems to show the exact opposite, ie that casualties have * cough cough* increased after the introduction of 20mph limits! http://www.bmj.com/content/345/bmj.e5580

I don't have access to the full article - if someone does and could comment here it would be great.

Running a quick pubmed I have also found this other article:

https://www.ncbi.nlm.nih.gov/pubmed/27173361

which supports the 20mph limits. I don't have access to the full article but I'll see if I can get it through some friends.

Like I said, I genuinely want to understand more. I am skeptical because the 20mph campaigns have not presented much evidence, because the initial conclusion of the DfT was that the results were inconclusive, and because it seems fishy that councils wouldn't want to wait for the DfT to finish its study, but if there really is overwhelmening evidence that lower limits work so be it - I will gladly support them despite the inconvenience as a motorcyclist.

The point is, I want to reach an informed opinion based on facts as evidence rather than ideology!

I would also add that it will soon be possible to compare accidents and casualties between 20mph and 30mph councils in London - that will be very interesting.

[Lowlander]

DulwichLondoner Wrote:

-------------------------------------------------------

>

> For example, in another article on the BMJ, the

> same researches are called to defend their

> conclusion because, well, practical experience

> seems to show the exact opposite, ie that

> casualties have * cough cough* increased after the

> introduction of 20mph limits!

> http://www.bmj.com/content/345/bmj.e5580

>

I can help you - attached.

Essentially - "Commenting on the Department for Transport figures, Grundy told the BMJ. ?We can?t tell anything from the raw numbers alone.?"

You are right - why are councils - including Wandsworth, arguably the most conservative and Conservative council in the UK, with the lowest council tax - implementing a costly reduction in speed when there is no monetary benefit?

[keano77]

To be seen to be politically correct? On message?

[Lowlander]

keano77 Wrote:

-------------------------------------------------------

> To be seen to be politically correct? On message?

Carry on, elaborate; how is a 20mph limit "politically correct"!? Does it offend you? Do you feel marginalised? are you from the snowflake generation?

[keano77]

You've obviously missed my AA link above.

20mph roads and CO2 emissions

Lower limits can increase fuel consumption and CO2 emissions

Here's the link again http://www.theaa.com/public_affairs/news/20mph-roads-emissions.html

Lower speeds do not necessarily protect the planet. 20mph limits would seem like a formula 1 race track to many a flatulent grazing cow but that doesn't stop scientists blaming them for global warming.

Cow 'emissions' more damaging to planet than CO2 from cars

http://www.independent.co.uk/environment/climate-change/cow-emissions-more-damaging-to-planet-than-co2-from-cars-427843.html

[Lowlander]

keano77 Wrote:

-------------------------------------------------------

> You've obviously missed my AA link above.

>

> 20mph roads and CO2 emissions

> Lower limits can increase fuel consumption and CO2

> emissions

>

> Here's the link again

> http://www.theaa.com/public_affairs/news/20mph-roa

> ds-emissions.html

>

> Lower speeds do not necessarily protect the

> planet. 20mph limits would seem like a formula 1

> race track to many a flatulent grazing cow but

> that doesn't stop scientists blaming them for

> global warming.

>

> Cow 'emissions' more damaging to planet than CO2

> from cars

> http://www.independent.co.uk/environment/climate-c

> hange/cow-emissions-more-damaging-to-planet-than-c

> o2-from-cars-427843.html

The main - proven - argument for 20mph limits is based on fewer physical injuries and deaths caused by vehicles hitting humans at lower speeds.

There is a secondary environmental concern regarding the environment, which as you state in your two links is unproven.

[keano77]

The main unproven reason for Wandsworth to introduce 20 mph I suspect (no facts) is a money making racket to fine motorists

[rendelharris]

keano77 Wrote:

-------------------------------------------------------

> The main unproven reason for Wandsworth to

> introduce 20 mph I suspect (no facts) is a money

> making racket to fine motorists

Just to repeat as one has to many, many times, local councils do not benefit from speeding fines, all revenue from speeding fines goes to H.M.Treasury.

[keano77]

Thanks, I didn't realise that. Do councils not get a percentage of money raised in their local council grants?

[rendelharris]

They can request money for new anti-speeding measures - speed cameras etc - but that's all, it can't be spent on anything else, it all goes back into general taxation.

[DulwichLondoner]

rendelharris Wrote:

> Just to repeat as one has to many, many times,

> local councils do not benefit from speeding fines,

> all revenue from speeding fines goes to

> H.M.Treasury.

That's not what I had understood from Kingston's official website:

https://www.kingston.gov.uk/info/200195/parking/627/penalty_charge_notices_-_parking_moving_traffic_and_bus_lane_penalties/6

I'd assume speeding tickets are part of the penalty charge notices "from moving tarffic".

"Where does the money go?

Any profit made from parking, moving traffic or bus lane penalties once the cost of providing, managing and enforcing on-street parking have been deducted must be used by law to fund off street parking or any of the following:

public passenger transport services

highway improvement works

highway maintenance

the cost of anything that has the approval of the Mayor of London and which facilitates the implementation of the Mayor's transport strategy."

[DulwichLondoner]

Lowlander Wrote:

> I can help you - attached.

Thank you. Do you work in the biomedical field? Can you get access to the other article I found on PubMed, too?

As for the BMJ article, I honestly struggle to follow it. Maybe you can clarify some of my doubts.

I would have expected an analysis of 20mph limits to be similar to the analyses of a new drug vs placebo: you compare two similar sets of roads, one with 20mph and the other with 30mph limits, and se if there is any statistically significant difference. Or, similarly, you analyse the accident frequencies on a set of roads the year before and the year after the introduction of 20mph limits.

Of course the set of roads must be large enough to have a meaningful number of events to compare; there is an inherent variability in this phenomena which makes them ahrd to study unless they are frequent enough. For example, in 2015 there were ca. 2000 fatalities and serious injuries in London ( https://tfl.gov.uk/info-for/media/press-releases/2016/june/road-casualties-in-london-continue-to-fall-but-concerns-remain-about-motorbike-collisions ); if you drill down into too much detail, you'll be left with not enough data points for a meaningful analysis. These are some of the reasons why the DfT (see my previous links) said the evidence on 20mph is inconclusive.

Anyway, I would have expected these two kinds of analyses, but it is not clear to me at all that this is what was done in the BMJ article. The authors have information for 385 of the 399 20mph zones introduced from 1991 to 2007, but that's too long a period for this kind of analysis, because all kinds of trends and factors over such a long period of time may contribute to a change in the number of accidents: safer cars, stricter driving tests, more speed cameras, dramatic changes in the population of motorists and road users, etc.

The authors say they reach similar conclusions when they analyse the most recent years only, but it is not clear to me what was compared over the last few years: how many 20mph zones were introduced over that period? How comparable are they to the non-20 mph zones? Table 1 gives some indications on these points over the entire period (which, as I said, I consider too long). One thing that jumps out is that, over the entire period, they analyse 2006 kms of 20mph roads, vs almost 14,000 kms of non-20mph zones. My main objections/doubts are 2:

1) how comparable are the 20 vs non-20 zones? E.g. to what extent are there fewer accidents in the 20mph zones because there is less traffic flow anyway?

2) even if they are comparable (and it's a big if), the mere fact that the vast majority of the dataset relates to the non-20 zones makes the results for the 20mph zones much harder to interpret and less statistically significant, because we are basically comparing a large vs a small dataset.

>

> Essentially - "Commenting on the Department for

> Transport figures, Grundy told the BMJ. ?We can?t

> tell anything from the raw numbers alone.?"

Makes sense. Just like I struggle to make sense of their analysis without clarifications to my two points above.

Also, I note that the data of the Metropolitan police, which classifies collisions in London by main cause, was not used. I can't find the link now (I'll try to refine my google skills later...) but that would be useful to look at, because clearly some types of collisions would be reduced by lower limits, but not all, e.g. drunk driving, low-speed collisions at dangerous junctions with limited visibility, etc.

Finally, I can't help but continue thinking that there must be something wrong in either the BMJ study or the DfT's; we're not talking about small differences, we are talking about a 40% reduction vs inconclusive results. One of the two must be hideously wrong!

[rendelharris]

DulwichLondoner Wrote:

-------------------------------------------------------

> rendelharris Wrote:

>

> > Just to repeat as one has to many, many times,

> > local councils do not benefit from speeding

> fines,

> > all revenue from speeding fines goes to

> > H.M.Treasury.

>

> That's not what I had understood from Kingston's

> official website:

>

> https://www.kingston.gov.uk/info/200195/parking/62

> 7/penalty_charge_notices_-_parking_moving_traffic_

> and_bus_lane_penalties/6

>

> I'd assume speeding tickets are part of the

> penalty charge notices "from moving tarffic".

No, moving traffic offences are the likes of blocking stopboxes, making illegal turns, driving the wrong way up one way streets etc, speeding offences are separate and the revenue goes to the treasury.

[Lowlander]

DulwichLondoner Wrote:

-------------------------------------------------------

> Lowlander Wrote:

>

> > I can help you - attached.

>

> Thank you. Do you work in the biomedical field?

> Can you get access to the other article I found on

> PubMed, too?

>

> As for the BMJ article, I honestly struggle to

> follow it. Maybe you can clarify some of my

> doubts.

> I would have expected an analysis of 20mph limits

> to be similar to the analyses of a new drug vs

> placebo: you compare two similar sets of roads,

> one with 20mph and the other with 30mph limits,

> and se if there is any statistically significant

> difference. Or, similarly, you analyse the

> accident frequencies on a set of roads the year

> before and the year after the introduction of

> 20mph limits.

>

> Of course the set of roads must be large enough to

> have a meaningful number of events to compare;

> there is an inherent variability in this phenomena

> which makes them ahrd to study unless they are

> frequent enough. For example, in 2015 there were

> ca. 2000 fatalities and serious injuries in London

> (

> https://tfl.gov.uk/info-for/media/press-releases/2

> 016/june/road-casualties-in-london-continue-to-fal

> l-but-concerns-remain-about-motorbike-collisions

> ); if you drill down into too much detail, you'll

> be left with not enough data points for a

> meaningful analysis. These are some of the reasons

> why the DfT (see my previous links) said the

> evidence on 20mph is inconclusive.

>

> Anyway, I would have expected these two kinds of

> analyses, but it is not clear to me at all that

> this is what was done in the BMJ article. The

> authors have information for 385 of the 399 20mph

> zones introduced from 1991 to 2007, but that's too

> long a period for this kind of analysis, because

> all kinds of trends and factors over such a long

> period of time may contribute to a change in the

> number of accidents: safer cars, stricter driving

> tests, more speed cameras, dramatic changes in the

> population of motorists and road users, etc.

> The authors say they reach similar conclusions

> when they analyse the most recent years only, but

> it is not clear to me what was compared over the

> last few years: how many 20mph zones were

> introduced over that period? How comparable are

> they to the non-20 mph zones? Table 1 gives some

> indications on these points over the entire period

> (which, as I said, I consider too long). One thing

> that jumps out is that, over the entire period,

> they analyse 2006 kms of 20mph roads, vs almost

> 14,000 kms of non-20mph zones. My main

> objections/doubts are 2:

> 1) how comparable are the 20 vs non-20 zones? E.g.

> to what extent are there fewer accidents in the

> 20mph zones because there is less traffic flow

> anyway?

> 2) even if they are comparable (and it's a big

> if), the mere fact that the vast majority of the

> dataset relates to the non-20 zones makes the

> results for the 20mph zones much harder to

> interpret and less statistically significant,

> because we are basically comparing a large vs a

> small dataset.

>

> >

> > Essentially - "Commenting on the Department for

> > Transport figures, Grundy told the BMJ. ?We

> can?t

> > tell anything from the raw numbers alone.?"

>

> Makes sense. Just like I struggle to make sense of

> their analysis without clarifications to my two

> points above.

>

> Also, I note that the data of the Metropolitan

> police, which classifies collisions in London by

> main cause, was not used. I can't find the link

> now (I'll try to refine my google skills later...)

> but that would be useful to look at, because

> clearly some types of collisions would be reduced

> by lower limits, but not all, e.g. drunk driving,

> low-speed collisions at dangerous junctions with

> limited visibility, etc.

>

> Finally, I can't help but continue thinking that

> there must be something wrong in either the BMJ

> study or the DfT's; we're not talking about small

> differences, we are talking about a 40% reduction

> vs inconclusive results. One of the two must be

> hideously wrong!

Look, I just signed up for the free 14 day trial to access the publications. The reason it's not for general public consumption is that it is just an article with some free-form opinion.

It is not a study.

The evidence is there - I posted BMJ and Oxford as they are impartial.

Your DfT study has been debunked by independent statisticians - see http://www.telegraph.co.uk/news/uknews/road-and-rail-transport/8038821/20mph-limit-has-not-made-roads-safer.html

"Professor Stephen Senn, an expert in statistics at the University of Glasgow, said: "The design of the report is very bad. Various statistical terms are used incorrectly and they've probably used the wrong statistical test."

Here are some more studies suggesting that 20mph limits reduce casualties from road safety charities:

http://www.rospa.com/road-safety/advice/drivers/speed/20mph-zones-and-limits/

http://www.brake.org.uk/assets/docs/Whatshappening/Influencegovernment/GO20_campaign_briefing-Oct12.pdf

Those against the 20mph limit are using questionable data and processes.

Those supporting 20mph limits are using the proper processes consistent with those used in medical studies.

[DulwichLondoner]

rendelharris Wrote:

> No, moving traffic offences are the likes of

> blocking stopboxes, making illegal turns, driving

> the wrong way up one way streets etc, speeding

> offences are separate and the revenue goes to the

> treasury.

I see - my bad, then.

Do councils still somehow get the money back, i.e. is that money reallocated back to road safety, etc, or can the government use it for whatever it wants? In other words, do councils really have no financial incentives whatsoever in speeding tickets? I hope you'll forgive my scepticism :)

[rendelharris]

DulwichLondoner Wrote:

-------------------------------------------------------

> rendelharris Wrote:

>

> > No, moving traffic offences are the likes of

> > blocking stopboxes, making illegal turns,

> driving

> > the wrong way up one way streets etc, speeding

> > offences are separate and the revenue goes to

> the

> > treasury.

>

> I see - my bad, then.

>

> Do councils still somehow get the money back, i.e.

> is that money reallocated back to road safety,

> etc, or can the government use it for whatever it

> wants? In other words, do councils really have no

> financial incentives whatsoever in speeding

> tickets? I hope you'll forgive my scepticism :)

No, it goes into the general treasury expenditure and can be spent in any way the government allocates. If you want to be cynical to the nth degree, obviously it's in the interests of the relevant council officers to push for more speed cameras etc as the consultations etc required will keep them in work (though that reasoning can be applied to virtually any workers, private or public), but unless I'm completely missing something there is not a financial benefit to any council in issuing speeding tickets. Maybe, just maybe, they actually genuinely care about the safety of their residents on their roads? ;-)

(By the way in some areas (though not the Met, as far as I know) police are permitted to keep a percentage of speeding fines, but councils definitely don't)