20mph limits: pros and cons?

[DulwichLondoner]

malumbu Wrote:

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> 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.

???????

[edcam]

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]

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

[rendelharris]

keano77 Wrote:

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> 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.

>

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

> 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.

[rendelharris]

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:

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> 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.

[rendelharris]

DulwichLondoner Wrote:

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> > 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. and has schools, shops and playgrounds on it and high cyclist usage

> East Dulwich Grove is neither a high street nor

> narrow. and has two (soon to be three) schools and a hospital on it, plus high cyclist and pedestrian usage

> Many stretches of the South Circular are not high

> street. high cyclist and pedestrian usage - anyway, the South Circular's 30MPH anyway isn't it, coming under TfL rather than local council control (and maybe uncoincidentally has a pretty high accident rate)

> Parts of Camberwell road. Lots of shops, high pedestrian and cyclist use (and good luck getting to 30MPH there anyway)

> Most of the A2 from New Kent road, to Old Kent

> road and New Cross gate at least. OK you can have 30MPH there

> Brixton Hill. OK

> Streatham High road (the limit may already be

> 30mph there, I'm not sure). Many shops, very heavy pedestrian and cyclist use

So most of the areas you mention do not fit the away from schools, shopping areas and areas of high pedestrian or cycle use criteria. One or two areas you could maybe have 30MPH - though good luck trying to reach it on most of them - and the first thing we'd get (as already happens with the 20MPH Lordship Lane turning into 30MPH South Circular) people whining it was a money-raising trick to catch them out.

[KidKruger]

I feel more confident at 20mph (having now been doing it since it was imposed) that I've more time to react / stop against unexpected events, the outcome of an event will be less serious / damaging, other people on or near the road have more time to react to me and my vehicle, everyone is safer.

[DulwichLondoner]

rendelharris Wrote:

> So most of the areas you mention do not fit the

> away from schools, shopping areas and areas of

> high pedestrian or cycle use criteria.

I disagree, for the reasons I mentioned above.

Let's remember that pedestrians do not have a constitutional right to jump out of nowhere into the street, crossing where they shouldn't. This must be one of the reasons why Park Lane (40mph limit northbound, 30mph southbound) is not the road with the most accidents in London, despite being very busy and full of pedestrians on both sides.

The key point is not so much whether there are many pedestrians, but the road layout. In a narrow and curved residential roads, with cars parked on both sides, and no pedestrian crossings, visibility will be limited and pedestrians will cross the street pretty much everywhere. Near a primary school or playground, young kids may run away from their parents and jump onto the road, despite the parents' best attempts at stopping them. All reasons for a 20mph limit. On the Lordship lane high street pedestrians shouldn't cross wherever they feel like doing it, but many still do... On the Streatham high street there are more pedestrians than on Lordship Lane, but most of the high street is a dual carriageway, with very clear separation between lanes, and between pedestrians and cars. I don't see a huge risk in 30mph limits there. Unless you tell me the whole policy should be based on safeguarding the occasional idiot who decides to jump in the middle of the road crossing where he shouldn't... Let's also halve train speeds for the same reason, then!

Also, the fact that the South Circular has more accidents is to be expected, since it is one of the main arteries of the city! Roads with more traffic flow are more likely to have accidents, it is a very self-evident concept.

Don't believe me? Fine. Let's look at some data for 20mph schemes elsewhere. The National Archive website has the PDF of a report commissioned by the Department for Transport on 20mph zones in Portsmouth: http://webarchive.nationalarchives.gov.uk/+/http:/www.dft.gov.uk/pgr/roadsafety/research/rsrr/theme4/interimeval20mphspeedlimits.pdf

The key is on page 3, section "safety":

"None of these results were statistically significant when compared against national trends".

I also remember other newspaper articles and reports quoting the Department for Transport saying the results were "inconclusive" http://www.roadsafetygb.org.uk/news/3451.html

I see no one has commented on the fact that the Department for Transport commissioned a multi-year study on the topic, yet councils rushed to implement 20mph zones before the results were public. Why?

By the way, for once the DfT did the right thing: the initial assessment was inconclusive, so it commissioned a more detailed, multi-year study, instead of rushing to implement expensive and disruptive changes which are not backed by clear evidence. This is the right thing to do. Is it such a weird concept, for xxxx's sake? Am I the only one who doesn't want his tax money frittered away in initiatives of dubious effectiveness?

Lambeth introduced 20mph zones about a year ago. I don't remember exactly when Southwark did, but soon we should be able to compare accident statistics in 20 vs 30mph councils. That will be interesting.

[DulwichLondoner]

edcam Wrote:

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> 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.

I did say very explicitly that I don't expect huge differences during rush hour, but I do expect differences outside the peak times, eg travelling from South to North London at night will take longer. It's also about the extra time you spend waiting at traffic lights. If the speed limit is lower, outside peak times you are more likely to encounter more red lights, and stay still at more traffic lights for more time, with the engine running, polluting more. Some cars and scooters automatically switch off when stopped at traffic lights, but they're a minuscule minority.

It's not only about the inconvenience: it's about the inconvenience plus the additional pollution this causes.

Oh, by the way, speed humps and bumps are also terrible for pollution because they cause motorists to brake and accelerate all the time, rather than keeping a more constant pace.

http://www.telegraph.co.uk/news/2016/12/01/speed-bumps-could-removed-cut-traffic-pollution-save-lives/

http://www.telegraph.co.uk/news/2017/04/09/speed-bumps-could-disappear-uk-roads-part-new-plan-cut-pollution/

[Lowlander]

I've never in my 30 years of driving managed to reach 40mph on Park Lane, and am lucky to hit 30!

In East Dulwich, there are so few places you could get to 30mph for more than a minute - or two at the most - before having to slow down for lights/junctions.

As a driver, 20mph is a nice speed to drive at - less stressful and I'm able to take in a lot more of what is going on. Less stopping and braking needed too - just seem to cruise along as you can anticipate what's going to happen at junctions and traffic lights.

[keano77]

rendelharris Wrote:

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> 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.

On the contrary rh. That 16mph average was on the basis of 30mph limits. Cars stuck in traffic, accelerating to 30mph when lights change or conditions allow. I don't do that route now but I'd take a guess (no facts) that the average would drop to 13 or 14mph - hardly any quicker than Victorian horse-drawn carriages.

So the moral of the story appears to be stick with Southwark and the Green Party and they'll show you what progress ain't.

[DulwichLondoner]

Lowlander Wrote:

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> I've never in my 30 years of driving managed to

> reach 40mph on Park Lane, and am lucky to hit 30!

??? Are we talking about the same road? The one from Hyde park corner to Marble Arch? I ride my motorcycle there very, very often. I almost always manage to reach 40mph (on the northbound lane, the southbound lane has a 30mph limit), even during the morning rush hour. In fact, speeding there is so easy that police motorcyclists very often hide there to catch speeding motorists, especially at rush hour!

> In East Dulwich, there are so few places you could

> get to 30mph for more than a minute - or two at

> the most - before having to slow down for

> lights/junctions.

Yes, but if driving at 30mph lets you spend less time at red traffic lights, then you'll be polluting less. Also, small differences add up, once multiplied by the number of drivers in London.

[Earl Aelfheah]

We shouldn't be looking to increase vehicle speeds on London?s roads but rather to smooth flow and reduce ?stop-start driving?. Because drivers cut their spacing as braking distances contract, it might even be the case that more vehicles can use the available road space, leading to a reduction in standing traffic. It's possible that lower speeds lead to better 'flow', more predictable journey times and even lower emissions. Or maybe not - I don't know what the evidence is either way, but certainty wouldn't assume that faster speeds in London necessarily means faster journeys.

[rendelharris]

DulwichLondoner Wrote:

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> Yes, but if driving at 30mph lets you spend less

> time at red traffic lights, then you'll be

> polluting less. Also, small differences add up,

> once multiplied by the number of drivers in

> London.

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 and spending longer with the engine idling, having used more fuel to get up to 30MPH before braking again. If the lights are synced for 20MPH flow, 20MPH will be the best speed at which to take them.

[Lowlander]

DulwichLondoner Wrote:

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> Lowlander Wrote:

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

> -----

> > I've never in my 30 years of driving managed to

> > reach 40mph on Park Lane, and am lucky to hit

> 30!

>

> ??? Are we talking about the same road? The one

> from Hyde park corner to Marble Arch? I ride my

> motorcycle there very, very often. I almost always

> manage to reach 40mph (on the northbound lane, the

> southbound lane has a 30mph limit), even during

> the morning rush hour. In fact, speeding there is

> so easy that police motorcyclists very often hide

> there to catch speeding motorists, especially at

> rush hour!

>

> > In East Dulwich, there are so few places you

> could

> > get to 30mph for more than a minute - or two at

> > the most - before having to slow down for

> > lights/junctions.

>

> Yes, but if driving at 30mph lets you spend less

> time at red traffic lights, then you'll be

> polluting less. Also, small differences add up,

> once multiplied by the number of drivers in

> London.

Try in a car. I've come close to 35 or so before the lights change or I hit more traffic. Never 40mph.

There is zero evidence to suggest either way that driving at 20 instead of 30 increases pollution. I could argue that I pollute less at 20mph because I accelerate less - and can better anticipate.

Driving at 20mph instead of 30mph takes one minute longer per mile - so a ten mile journey across London will take 10 minutes longer - assuming 30mph non-stop from start to finish. Most of my journeys in London are through 20mph zones now anyway - Wandsworth is going 20mph in May - and I can only see a benefit.

[Earl Aelfheah]

rendelharris Wrote:

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> DulwichLondoner Wrote:

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

> -----

>

> > Yes, but if driving at 30mph lets you spend

> less

> > time at red traffic lights, then you'll be

> > polluting less. Also, small differences add up,

> > once multiplied by the number of drivers in

> > London.

>

> 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 and

> spending longer with the engine idling, having

> used more fuel to get up to 30MPH before braking

> again. If the lights are synced for 20MPH flow,

> 20MPH will be the best speed at which to take

> them.

There is no reason to think that driving faster means avoiding red lights.

[Earl Aelfheah]

Lowlander Wrote:

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> Driving at 20mph instead of 30mph takes one minute

> longer per mile - so a ten mile journey across

> London will take 10 minutes longer - assuming

> 30mph non-stop from start to finish.

And that's the point - one doesn't travel without stopping. When you cycle, quite often you'll observe the cars zooming off at one set of lights only to catch them up at the next. The fact is that sauntering along on a bicycle watching cars disregarding the speed limit, it is quite apparent that it does not get them to their destination more quickly. But they would of course swear it does.

[DulwichLondoner]

Lowlander Wrote:

> Try in a car. I've come close to 35 or so before

> the lights change or I hit more traffic. Never

> 40mph.

I don't need to try in a car: seeing all the cars and lorries riding at my same speed, if not higher, is sufficient proof that 40mph are very feasible there even on 4 wheels.

[jimlad48]

Biggest problem that I have with the 20MPH limits is that I find myself spending more time monitoring the dashboard than the road to make sure I don't break the limit.

Its a very difficult speed to keep the car at - it naturally wants to be going a bit faster for the engine to justify going into 3rd gear - 20 is for my car the point where its struggling to keep in 2nd, but struggles to stay in 3rd - its also a speed that, particularly with cameras, has low tolerance for even a slight excess (e.g. a slight over revving by accident means you can easily breach the 10% + 1 rule) and find yourself fined / points on licence.

Consequently, I find that when in 20MPH zones, I have to spend a lot more of my driving time focusing on the speedo for fear of breaking the limit than I do in a 30MPH zone where the car is comfortably driving along and where you don't need to worry to the same level about going over the limit by accident.

I personally think 20MPH zones are more dangerous because drivers who want to drive within the law have to spend more time focusing on their speed, and not on the environment around them.

[Lowlander]

DulwichLondoner Wrote:

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> Lowlander Wrote:

>

> > Try in a car. I've come close to 35 or so

> before

> > the lights change or I hit more traffic. Never

> > 40mph.

>

> I don't need to try in a car: seeing all the cars

> and lorries riding at my same speed, if not

> higher, is sufficient proof that 40mph are very

> feasible there even on 4 wheels.

I've no doubt as I'm often being overtaken there - personally, I've just never felt comfortable enough to reach 40mph with the amount of traffic.

[Lowlander]

jimlad48 Wrote:

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> Biggest problem that I have with the 20MPH limits

> is that I find myself spending more time

> monitoring the dashboard than the road to make

> sure I don't break the limit.

>

> Its a very difficult speed to keep the car at - it

> naturally wants to be going a bit faster for the

> engine to justify going into 3rd gear - 20 is for

> my car the point where its struggling to keep in

> 2nd, but struggles to stay in 3rd - its also a

> speed that, particularly with cameras, has low

> tolerance for even a slight excess (e.g. a slight

> over revving by accident means you can easily

> breach the 10% + 1 rule) and find yourself fined /

> points on licence.

>

> Consequently, I find that when in 20MPH zones, I

> have to spend a lot more of my driving time

> focusing on the speedo for fear of breaking the

> limit than I do in a 30MPH zone where the car is

> comfortably driving along and where you don't need

> to worry to the same level about going over the

> limit by accident.

>

> I personally think 20MPH zones are more dangerous

> because drivers who want to drive within the law

> have to spend more time focusing on their speed,

> and not on the environment around them.

Oh come on - firstly, every car is different. Secondly, East Dulwich is not flat, so 20mph going up Dog Kennel Hill or Lordship Lane will be quite different to going down. Thirdly, you get used to it. Fourth - going at 30mph is usually at the change between third and fourth gear.

[rendelharris]

Also many (most?) new cars have the facility to set a limit, and I don't get the argument that one has to concentrate less hard on not exceeding a 30 limit than a 20 one: firstly as RRR points out, the gear change issue identified only applies to certain cars on certain terrain, and secondly I'm not sure someone who finds it difficult and distracting to keep to the marked speed limits is really terribly competent (no personal slight intended), it's just part and parcel of being on the road, isn't it?

[jimlad48]

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.

[Cardelia]

rendelharris Wrote:

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> 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?

[rendelharris]

Cardelia Wrote:

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> 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.