Wales set to reduce default speed limit to 20mph in residential areas

Have a look at the experience in Manchester.

20 mph zones seemed to perform worse than 30 mph zones in terms of KSIs.

If they work I'm all for them but let's not assume that they will.

Whilst looking for the Manchester report I Found this for Cheshire areas. 

Key stats are: 

Quote:

 

There was a 43 per cent reduction in road traffic collisions, a 24 per cent reduction in people killed or seriously injured (KSI), and a 46 per cent reduction in slight injuries.

The committee was told that across all schemes, 134 areas saw average speeds decrease, while 63 areas saw it increase.

The council said there was an annual cost saving to the community in road traffic collisions of £3.7m per year.

They also mention that the Police were enforcing these speed limits with 350 FPN's listed in the 12 months before the report was released. 

Agreed

Where did you get your data?

I found a report in the Manchester Evening News which said 20mph zones made little or no difference. The same article quoted RoSPA saying that 20mph zones were very effective. Cheshire West which is next door to Manchester gives a positive report - see AlsoSomniloquism below.

Common sense says 20mph zones are a No Brainer, just like Helmets and HiViz (Cynical comment)

I'm confused. Like a lot of topics, it's not black and white

Given that damage done in a collision is directly related to the energy of the impact, and that is in turn correlated to the square of the velocity, it stands to reason that a 30mph collision is going to KSI more often than a 20mph one.  There are two likely explanations for the amanchester results - either the lower limits tempted people into "chancing it" more often, or the drivers involved in the collisions had carried on driving at the same old speed.  I know which I'd bet on, as unless there was enforcement of the 20mph limit I doubt that compliance was high.

Apparently about 80% of drivers ignore the 20 mph limit vs 50-70% who ignore the 30mph limit (it depends on where you take your figures from).

20mph doesn't work in Manchester but does work in Cheshire West, and it also works for RoSPA. It may depend on the margin which drivers ignore the speed limit.

Strange old world

The Cheshire council report mentioned over 300 FPN's issued for speeding in the just the last 12 months of it. The BBC link for Manchester mentioned one driver was ticketed for speeding. So yes, I suspect Police non-enforcement probably had something to do with it. 

Richard D wrote:

Given that damage done in a collision is directly related to the energy of the impact...

I see that said a lot, but never with any source. I'd have thought the damage done to a human body was more to do with the forces acting on it and the acceleration it is subjected to, rather than the KE of the impacting object, which is not in any case transferred in whole to the person who is hit.

The human body may well have a non-linear response to force or acceleration, so possibly it comes back to the same thing, but I don't think it is helpful to think I terms of the energy of the vehicle, as if this is somehow the agent of damage.

The Kinetic Energy of a moving vehicle is given by:-

KE=1/2MV^2

When the vehicle comes to a halt it all has to go somewhere.
The ballistics which happen in a collision are extremely complex, but I think it is fair to say that in general the higher the KE of the vehicle the worse the outcome for the vulnerable road user.
As you say the big consideration must be the amount of KE that gets transferred to the cyclist.
Would you rather get hit by a Nissan Micra at 20mph or a Cement Mixer at 30mph?

Cycloid wrote:

The Kinetic Energy of a moving vehicle is given by:-

KE=1/2MV^2

thanks for the Yr9 tutorial

Quote:

When the vehicle comes to a halt it all has to go somewhere

that'd be the heat in the brakes - unless you are suggesting the impact with the cyclist has much effect on the car's velocity

Quote:

 I think it is fair to say that in general the higher the KE of the vehicle the worse the outcome for the vulnerable road user.

It's fair to say I'd rather be hit by a train doing 1mph than by a car doing 30.

"It All has to go somewhere"

Yes - The fist law of thermodynamics - Energy can neither be created or destroyed, only converted from one form into another. I think It all eventually finishes up as heat, whether it is in the brake linings of the vehicle or breaking bonds in the snapping bones of the cyclist. what Feynman called "jiggling molecules". The question is how much of the KE gets passed onto the cyclist.

Getting hit by a train at 1mph gently accelerates you in the direction of the train - Newton's first law. (You will probably fall off and go under the wheels). I did a quick calculation and the car doing 30mph has more KE than the train, so your preference is correct. (I assumed the weight of the car was 1000 kilos and the train 100, 000 kilos).

As I have already said the ballistics which take place in a collision are extremely complex and fooling around with Newtonian mechanics can give us a feeling for the maximum (worst) outcome in a collision, but are not a simple predictor.

I think you are deliberately missing the essential point that the cyclist does not absorb the KE of the vehicle, hence it is of little significance. Back to the example of being hit by a slow moving train - are you really suggesting that the outcome would vary depending on the mass of the train? What does matter is the force to which the cyclist is subjected.

Moreover, if acquiring KE were so harmful to the human body, how do the vehicle passengers survive travelling at such great speed to begin with? Surely they would be killed by all the KE? Unless it all turns to heat and cooks them, they will be quite OK.

The Kinetic energy which is transferred to a cyclist in a collision is important.

A golf ball and a ping pong ball are about the same size but have very different masses.
Would you rather be hit by a ping pong ball or a golf ball at (say) 30mph?

The golf ball has the greater mass and therefore greater kinetic energy for a given velocity. The relationship is in the equation KE = 1/2MV^2.

As I said the ballistics are complicated, but KE is important.

5 mph is demonstrably safer than 20 mph.. where does this statistical nonsense stop? 

It Stops exactly where we want it to.

Nearly everything we do has a risk associated with it, and we balance the risk against the outcome.
In a typical year there are about 4000 cyclists killed or seriously injured on our roads, if we think this is a reasonable price to pay for driving the way we do then we do nothing.

If we are not happy with the situation we make changes to reduce the number  Safer cycling infrastructure, Safer vehicles, Helmets+Hiviz? SPEED LIMITS!

Statistical information is virtually the only tool we have to help us make decisions.

It's probably something to do with dead children.

Cycloid wrote:

"Driving at 20mph is innefiecient for the engine and will cause more pollution" - We'll all be driving Electric in a few years time.

I've also heard that line trotted out, and even aside from a switch to electric cars I'm not convinced it's true.

Even if we take it as a given that a car is more efficient cruising at a steady 30mph than a steady 20mph, that's not how urban driving normally works - there's a lot of stopping and starting. I very much expect accelerating to 30mph, driving a short while, and then stopping, requires more fuel than the same distance but only accelerating to 20mph.

If 20mph enables traffic to flow better, then that would also lower pollution. Traffic jams and repeated accelerating and braking are both bad. I don't know about urban areas but I'm pretty sure the variable speed limits/advisory speed signs on motorways are meant to improve traffic flow and increase capacity by reducing speeds.

And of course the point already made that encouraging people out of cars and onto either active travel or public transport will reduce pollution. 

Claims that reducing speed will increase pollution and/or harm the engine don't seem credible to me.

Travelling at 30mph clearly takes more energy than travelling at 20mph, so more fuel will be needed. Braking from 30mph is going to produce more brake dust than from 20mph and I don't see how tyres rolling at faster speeds are going to create less friction on the road.

If someone has a car that doesn't have a low enough gear to travel at 20mph with revs in the right zone, then either the car's not designed for city travel or they don't know how to use the gears correctly.

hawkinspeter wrote:

Claims that reducing speed will increase pollution and/or harm the engine don't seem credible to me.

Travelling at 30mph clearly takes more energy than travelling at 20mph, so more fuel will be needed. Braking from 30mph is going to produce more brake dust than from 20mph and I don't see how tyres rolling at faster speeds are going to create less friction on the road.

If someone has a car that doesn't have a low enough gear to travel at 20mph with revs in the right zone, then either the car's not designed for city travel or they don't know how to use the gears correctly.

"The Energy Saving Trust says that the most efficient speed you can travel in a car in terms of achieving the best fuel economy is 55-65mph"

(based on travelling at a steady speed, over extended period)

This is due to the combination of maximum engine efficiency speed and highest gear.

As electric cars become the norm, the most efficient speed will be much lower, as wind resistance will dominate over engine efficiency variance. 

However on urban/residential roads where the driver will natully decelerate every feww hunder metres for traffic lights/junctions/hazards etc, then 20mph may well be more efficient. E.g entering a road and accelerating to 30  instead of 20 will more than consume the savings from travelling 1/4 mile at 30 instead of 20

wycombewheeler wrote:

"The Energy Saving Trust says that the most efficient speed you can travel in a car in terms of achieving the best fuel economy is 55-65mph"

I'd like to see how that's worked out. Surely the increased air resistance at 55mph is going to dwarf any engine efficiency savings.

I've just had a quick look and it seems that the 55mph optimum is for AVERAGE speed not STEADY speed. Here's a link that says mpg at a STEADY 20mph is huge.

https://www.20splenty.org/do_emission_increase

Only a quick look and I'd be happy to be corrected if I'm wrong. It does seem more logical to me though.

Good point.

I was giving one liners as obvious counter arguments. I have found that giving reasoned, but more complex complex arguments to people who have already made up their minds (bigots) is a waste of time.

I am amazed at the number of drivers who speed throught the 30mph zone, doing 40+ in my village just to get on the end of a queue at the crossroads. If they drove at 20mph the queue would be a bit shorter when the got there.

HoarseMann wrote:

This is a step in the right direction. Hopefully when the rage has subsided and people get used to this, a blanket ban on overtaking in 20mph limits can be quietly slipped in. 

Hopefully not, for several reasons:

1.a) Few people can cycle at a sustained speed over, say, 15mph. Reduce that to as little as 5mph up hill. 

b) Few people enjoy cycling at the head of a queue of, probably frustrated and impatient, traffic.

2. In certain situations, 20mph limits give faster cyclists the opportunity of overtaking moving cars, which feels good.

3.a) Strictly speaking, it would then be illegal to ride to the front of a queue of stationary traffic.

b) Even if this ban only applied to motor vehicles, it would similarly prevent motorcyclists filtering. Motorbikes are far more space efficient (or at least far less space inefficient) than cars, so should be encouraged in urban areas.

Bmblbzzz wrote:

Hopefully not, for several reasons:

Nah, it'll work perfectly. We're talking about residential streets here, they already do this on some roads in the Netherlands (Fietsstraats).

1.a.) If a car has to wait behind a cyclist doing 5mph uphill, then they wait. One of the main impacts of this is making a short urban journey in a car take a similar time (or longer) than doing that journey by bicycle. Thus making car travel less desirable for these journeys that can easily be done by bike.

b) Cycling at the head of a queue is only a problem because vehicles behind are impatient to pass. If the rules were changed so that a pass was not possible and having to follow at the speed of a bicycle was just one of those things, then it would be fine.

2.) Hell no! Whilst filtering past slow moving or stationary cars would still be ok, cyclists should not be passing vehicles doing 20mph in these areas.

3.a) There is a differentiation drawn between overtaking and filtering. So this would still be ok.

b) ditto for motorbikes, they could still filter.

'making car travel less desirable'.. this is the real reason for these very slow limits; car hating greenies.

It is certainly one reason. Do you have a problem with that? I know it is lovely to step into n air conditioned armoured box with two luxury sofas, but maybe it would be better for you to walk to the shops?

This is a good point. The car that's left at home is less damaging (in all ways, it's not just safety) than the car that's driven safely and considerately. The car that's not built is better still.

Just residential streets? Default should mean applicable to all roads in built-up areas, including main roads. That's the way it works here (Bristol). Obviously there are always a few exceptions, but they are, well, exceptions to the default and therefore exceptional... 

Good point on differentiating overtaking and filtering, though I'm not sure of the actual law on this.

yes, I think it would apply to all urban roads, but some of the larger city centre through routes could be 'upgraded' to 30mph.

There's no law on what constitutes filtering vs. overtaking, but you can't filter in anything with more than 2 wheels, and that is all that matters!

Banning overtaking in 20mph limits would make prosecutions far easier. Rather than having to prove a 'close pass' was careless, which is quite difficult to do from bicycle camera footage, you just have to demonstrate that they overtook. Bang to rights, FPN issued, happy days.