No Road Safety Benefits For 20 Limits

[bad company]

What should we make of this:-

http://www.abd.org.uk/press-release-are-20-mph-spe...

I’m not against all 20 limits but it does seem to be getting out of hand.

[Graveworm]

bad company said:

What should we make of this:-

http://www.abd.org.uk/press-release-are-20-mph-spe...

I’m not against all 20 limits but it does seem to be getting out of hand.

I am in favour of anything that improves overall outcomes. Locally (Surbiton) they have recently introduced a load of 20mph limits like much of London.
There was a consultation and I did respond. I was partially impressed in that I did get a call from them about my response but less so with the content. The initial proposal was far more widespread than the original plan. I asked how many KSIs there had been on the proposed roads and it was less than one a year. I asked what the negative impact of slowing traffic was and what it was going to cost. This is where it got weird. They said that the cost was the best part as it was "Free". Free meant that it was paid for by a grant and they couldn't see the irony of that.
Pushing on the negative aspect they didn't seem to know what I was talking about.
This has has not caused any real changes so the impact is minimal. Unlike the huge upheaval for the cycling lanes (also grant funded) that are definitely not in any way cost effective.

[pavarotti1980]

Graveworm said:

I am in favour of anything that improves overall outcomes. Locally (Surbiton) they have recently introduced a load of 20mph limits like much of London.
There was a consultation and I did respond. I was partially impressed in that I did get a call from them about my response but less so with the content. The initial proposal was far more widespread than the original plan. I asked how many KSIs there had been on the proposed roads and it was less than one a year. I asked what the negative impact of slowing traffic was and what it was going to cost. This is where it got weird. They said that the cost was the best part as it was "Free". Free meant that it was paid for by a grant and they couldn't see the irony of that.
Pushing on the negative aspect they didn't seem to know what I was talking about.
This has has not caused any real changes so the impact is minimal. Unlike the huge upheaval for the cycling lanes (also grant funded) that are definitely not in any way cost effective.

The irony is that whilst implementing blanket 20mph limits on towns the same LAs are complaining about vehicle emissions without seeing the correlation with lower speed increasing emissions. Post implemtation data was collected and showed that 90% of traffic ignored the limits and continued at between 25-30mph anyway and the police have been outspoken and said they will not enforce the lower limit either

My LA has imposed blanket limits across whole towns due to the EU grant whilst completely ignoring the there was no need for them anyway due to the lack of any serious injury (let alone fatal) RTCs in any of the towns.

[Wooda80]

pavarotti1980 said:

The irony is that whilst implementing blanket 20mph limits on towns the same LAs are complaining about vehicle emissions without seeing the correlation with lower speed increasing emissions.

I'm interested in this bit.

Do vehicles emit increased emissions at lower speeds? Not to be confused with increased emissions from stationary traffic which also reduces average speeds.

Perhaps we could use this evidence to campaign for a change in the motorway speed limit to 100mph on environmental grounds?

[pavarotti1980]

Wooda80 said:

I'm interested in this bit.

Do vehicles emit increased emissions at lower speeds? Not to be confused with increased emissions from stationary traffic which also reduces average speeds.

Perhaps we could use this evidence to campaign for a change in the motorway speed limit to 100mph on environmental grounds?

Well travelling at lower speeds means they are on the roads for longer, therefore more emissions are released

[anonymous-user]

ABD are at the other end of the spectrum to BRAKE. Just as extreme, except pro-speed rather than anti. Both are as bad as each other.

And for every piece of evidence brake has, 20s Plenty will have something that flies against it.

[coldel]

pavarotti1980 said:

Well travelling at lower speeds means they are on the roads for longer, therefore more emissions are released

But the car will emit more emissions burning more fuel to go faster? It should be easily doable with a bit of math to look at time spent vs emission levels over the same stretch of road.

[TriumphStag3.0V8]

Wooda80 said:

I'm interested in this bit.

Do vehicles emit increased emissions at lower speeds? Not to be confused with increased emissions from stationary traffic which also reduces average speeds.

Perhaps we could use this evidence to campaign for a change in the motorway speed limit to 100mph on environmental grounds?

Well my car as an example will not sit in third at 20mph but will do so happily at 28-30, so at 20mph i am at higher engine revs and lower gear, and as i am travelling more slowly i will be on the road for longer. I would therefore say that yes, it would increase pollution.

People need to get away from the sheep mindset that says lower speed = less pollution.
More efficient speed does = less pollution. 20mph is bot an efficient speed for a car

[pavarotti1980]

coldel said:

But the car will emit more emissions burning more fuel to go faster? It should be easily doable with a bit of math to look at time spent vs emission levels over the same stretch of road.

https://www.cityoflondon.gov.uk/business/environmental-health/environmental-protection/air-quality/Documents/speed-restriction-air-quality-report-2013-for-web.pdf

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

Edited by pavarotti1980 on Wednesday 5th December 12:41

[Neonblau]

OpulentBob said:

ABD are at the other end of the spectrum to BRAKE. Just as extreme, except pro-speed rather than anti. Both are as bad as each other.

And for every piece of evidence brake has, 20s Plenty will have something that flies against it.

That's my impression too. Do they have a sizeable membership - always sounded as though the whole set up was about 20 nutters bigging themselves up.

[Graveworm]

Wooda80 said:

I'm interested in this bit.

Do vehicles emit increased emissions at lower speeds? Not to be confused with increased emissions from stationary traffic which also reduces average speeds.

Perhaps we could use this evidence to campaign for a change in the motorway speed limit to 100mph on environmental grounds?

It depends on the vehicle and gearing but the best constant speed in environmental terms of emissions vs distance travelled is usually 50-70 with an inverse asymmetric bell curve either side. However its not so straightforward as acelleration, deceleration and length of time stationary with the engine running have a huge impact. This of course again is vehicle dependent. We don't really know the exact impact of more cars with start stop, hybrid cylinder shut down etc.
But in general all other things being equal, purely environmentally, 30 is better than 20, 70 is better than 100 and traffic calming is worst of all.
Higher peak speeds don't always result in higher average speeds but they are a factor
An often missed/avoided factor is lower average speeds (Which would result from 20 limits if they were strictly enforce), has other very serious downsides which are seldom factored in the by single issue groups.

Edited by Graveworm on Wednesday 5th December 12:48

[Wooda80]

But emissions are proportional to the amount of fuel being burned

The amount of fuel being burned is proportional to the amount of energy being used.

You will use more energy to propel a given mass over a given distance in a shorter time than in a longer time ( irrespective of any quaint notions about gearing! )

[Graveworm]

Wooda80 said:

But emissions are proportional to the amount of fuel being burned

The amount of fuel being burned is proportional to the amount of energy being used.

You will use more energy to propel a given mass over a given distance in a shorter time than in a longer time ( irrespective of any quaint notions about gearing! )

So why do cars do more Mpg at higher speeds?

[stargazer30]

Graveworm said:

Wooda80 said:

But emissions are proportional to the amount of fuel being burned

The amount of fuel being burned is proportional to the amount of energy being used.

You will use more energy to propel a given mass over a given distance in a shorter time than in a longer time ( irrespective of any quaint notions about gearing! )

So why do cars do more Mpg at higher speeds?

I'm guessing its the last line thats flawed. As a car in a lower gear might use as much energy as in a higher gear but be going slower. The wasted energy is output as torque which is likely to not be needed in this example. I'm sure someone more physics savvi will explain it better though.

[coldel]

pavarotti1980 said:

https://www.cityoflondon.gov.uk/business/environme...

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

Edited by pavarotti1980 on Wednesday 5th December 12:41

Clearly the gearing is the issue in the AA test - 30mph in 4th gear vs 20mph in 3rd gear is whats not creating an apples with apples test. If you left the car in 3rd gear and accelerated to 30mph you would be I suspect be using more fuel. Our renault megane can easily cruise at 20mph in 4th gear without labouring the engine at lower revs than the car in 4th gear at 30mph. I can only assume all things being equal that 30mph would require more energy thus more emissions.

The reality is of course that speed humps, crossings, traffic etc. all make it hard to pull apart in the real world.

[Graveworm]

coldel said:

Clearly the gearing is the issue in the AA test - 30mph in 4th gear vs 20mph in 3rd gear is whats not creating an apples with apples test. If you left the car in 3rd gear and accelerated to 30mph you would be I suspect be using more fuel. Our renault megane can easily cruise at 20mph in 4th gear without labouring the engine at lower revs than the car in 4th gear at 30mph. I can only assume all things being equal that 30mph would require more energy thus more emissions.

Not necessarily, in fact not at all.

[untakenname]

Unsurprisingly the Guardian article doesn't allow comments

https://www.theguardian.com/world/2018/nov/23/litt...

The last paragraph by Brake really grates, I don't see why anyone supports these fools.

[coldel]

Graveworm said:

coldel said:

Clearly the gearing is the issue in the AA test - 30mph in 4th gear vs 20mph in 3rd gear is whats not creating an apples with apples test. If you left the car in 3rd gear and accelerated to 30mph you would be I suspect be using more fuel. Our renault megane can easily cruise at 20mph in 4th gear without labouring the engine at lower revs than the car in 4th gear at 30mph. I can only assume all things being equal that 30mph would require more energy thus more emissions.

Not necessarily, in fact not at all.

But your chart is not applicable to the point I made? It has on your chart the gear ratios which change at different speeds. If you read again what I wrote I said the AA test wasn't fair because it compared MPG at 20mph vs 30mph 'in different gearing' which in effect means the MPG being created is not like for like. Additionally the chart shows an accelerating car not something travelling at a constant speed. Again an additional variable.

The test would have been fairer had both the 20mph and 30mph test done over say 1 mile been done in the same gear to remove that variable from the calculations. It would then show if the extra fuel used to generate 30mph vs the time saved by doing that 1 mile means less MPG. I hope thats a bit clearer?

[Graveworm]

coldel said:

But your chart is not applicable to the point I made? It has on your chart the gear ratios which change at different speeds. If you read again what I wrote I said the AA test wasn't fair because it compared MPG at 20mph vs 30mph 'in different gearing' which in effect means the MPG being created is not like for like. Additionally the chart shows an accelerating car not something travelling at a constant speed. Again an additional variable.

The test would have been fairer had both the 20mph and 30mph test done over say 1 mile been done in the same gear to remove that variable from the calculations. It would then show if the extra fuel used to generate 30mph vs the time saved by doing that 1 mile means less MPG. I hope thats a bit clearer?

But in that (old) graph, higher speeds in the same gear below 50 also give better mpg.

[coldel]

Graveworm said:

coldel said:

But your chart is not applicable to the point I made? It has on your chart the gear ratios which change at different speeds. If you read again what I wrote I said the AA test wasn't fair because it compared MPG at 20mph vs 30mph 'in different gearing' which in effect means the MPG being created is not like for like. Additionally the chart shows an accelerating car not something travelling at a constant speed. Again an additional variable.

The test would have been fairer had both the 20mph and 30mph test done over say 1 mile been done in the same gear to remove that variable from the calculations. It would then show if the extra fuel used to generate 30mph vs the time saved by doing that 1 mile means less MPG. I hope thats a bit clearer?

But in that (old) graph, higher speeds in the same gear below 50 also give better mpg.

You would need to check the source of the data but presume the car is in a constant state of acceleration rather than a constant state of speed. This will impact the numbers. You could argue then according to your chart why the mpg goes down the more revs you put through in 5th gear. The fact that 6th or 7th gear is also missing from the chart that the data could well be old data.

I guess (and we need an experts view on this) would a car revving at 3000rpm in say 4th gear use each second more fuel than one revving at 2000rpm in 4th gear? If yes, then we can assume the mpg debate for the AA article comes down to does the difference = more or less than the time saved.

The way I see it in my head in simple terms is if a car takes at 20mph 3 minutes to do 1 mile in 4th gear, and one doing 30mph 2 minutes to do 1 mile in 4th gear does the slower car burn less fuel at a single point in time and if so does the extra minute it takes to do the mile mean that the fuel saving beats or loses to the other car

Edited by coldel on Wednesday 5th December 14:40