Cutting speed limits for cleaner air?

[Evanivitch]

Kawasicki said:

Evanivitch said:

Kawasicki said:

Full underbody aero

Kind of what I said previously, flat underbody with the exception of Exhaust and suspension. For those reasons I wouldn't call it full underbody aero.

So do you think it was a Friday afternoon and they just said “that’ll do”, or maybe they considered and engineered the entire underbody function, including aerodynamics.

No, I think they reached a compromise, as all engineers must, between their desire for improved underbody aerodynamics and the need to use cheap plastics which aren't suitable up-close to exhaust pipes and installing the necessary emissions control equipment under the car, and not just a straight pipe out the side.

[Kawasicki]

Evanivitch said:

No, I think they reached a compromise, as all engineers must, between their desire for improved underbody aerodynamics and the need to use cheap plastics which aren't suitable up-close to exhaust pipes and installing the necessary emissions control equipment under the car, and not just a straight pipe out the side.

My point is that aero is considered for the complete car, underbody and over. The complete car is aerodynamically designed.

[anonymous-user]

The improvement in air quality expected is a secondary effect, the primary intent of reducing the speed limit is to avoid getting fined by the EU.

Might be a cynical view but it is a scheme from politicians and seems rather knee jerk, there has been plenty of time to work on this but very little has been achieved until a big fine is looming.

Edited by anonymous-user on Thursday 5th July 18:11

[Evanivitch]

Kawasicki said:

My point is that aero is considered for the complete car, underbody and over.

I agree.

Kawasicki said:

The complete car is aerodynamically designed.

Not true. If all cars were aerodynamically designed we'd be driving variations of tear drops. Aero is a consideration (as you rightly said) amongst money other design factors (such as styling, legislation, practicality, cost, drivetrain etc etc).

[Kawasicki]

Evanivitch said:

Kawasicki said:

My point is that aero is considered for the complete car, underbody and over.

I agree.

Kawasicki said:

The complete car is aerodynamically designed.

Not true. If all cars were aerodynamically designed we'd be driving variations of tear drops. Aero is a consideration (as you rightly said) amongst money other design factors (such as styling, legislation, practicality, cost, drivetrain etc etc).

Designed, to the exclusion of other requirements isn’t really good design, is it? If an aero engineer decides that it is better design not to have flat underbody panels in certain areas, does that mean those areas haven’t been aerodynamically designed? No.

[anonymous-user]

My rudimentary understanding is that, all other things being equal, it takes more energy to push an object through the atmosphere at 70mph than 50mph.

Is that not settled physics?

[oyster]

janesmith1950 said:

My rudimentary understanding is that, all other things being equal, it takes more energy to push an object through the atmosphere at 70mph than 50mph.

Is that not settled physics?

Correct.

Some posters on PH will argue that some cars have engine thermodynamic properties and gearbox ratios that mean the energy required to travel at 70mph can be created with the same amount of fuel use (or even less) than to produce the energy required to travel at 50mph.

And you know what - some of them may well be correct. Some people may well have cars like that.


But here's where their argument falls down. Such cars (if they do exist) are used by about 0.01% of the population. The other 99.99% use cars that use less fuel to travel at 50 than 70.


Never mind, we should base policy on the 0.01% and not the 99.99%.

[JagLover]

oyster said:

janesmith1950 said:

My rudimentary understanding is that, all other things being equal, it takes more energy to push an object through the atmosphere at 70mph than 50mph.

Is that not settled physics?

Correct.

Some posters on PH will argue that some cars have engine thermodynamic properties and gearbox ratios that mean the energy required to travel at 70mph can be created with the same amount of fuel use (or even less) than to produce the energy required to travel at 50mph.

And you know what - some of them may well be correct. Some people may well have cars like that.


But here's where their argument falls down. Such cars (if they do exist) are used by about 0.01% of the population. The other 99.99% use cars that use less fuel to travel at 50 than 70.


Never mind, we should base policy on the 0.01% and not the 99.99%.

I questioned why 50 and not 60?, given that the most fuel efficient speed for the average car is most often cited as 56mph?

Has anyone supporting a 50mph speed limit for "air quality reasons" actually have any data that air quality is better at 50 mph rather than 60?

[dcb]

oyster said:

Some posters on PH will argue that some cars have engine thermodynamic properties and gearbox ratios that mean the energy required to travel at 70mph can be created with the same amount of fuel use (or even less) than to produce the energy required to travel at 50mph.

And you know what - some of them may well be correct. Some people may well have cars like that.

Surely fuel consumption is the mouse in the room. A couple of mpg
here and there isn't much to worry about.

I think time wasted is the elephant in the room. It takes a lot longer
to get somewhere at 50 mph versus 70 mph.

Time is money, and most waged folks are on more than £10 an hour.
Some much more.

So for some notional 10 mile journey, do you want to get there in 12 minutes
under the new rules or under 9 minutes under the old rules ?

Multiply that up by thousands of road users and it's clear that
the cleaner air is costing us a lot of money.

We will leave aside the effects diesel HGVs and buses have on cleaner air.
Let's just make sure we punish petrol and diesel cars, eh ?

[Willy Nilly]

dcb said:

oyster said:

Some posters on PH will argue that some cars have engine thermodynamic properties and gearbox ratios that mean the energy required to travel at 70mph can be created with the same amount of fuel use (or even less) than to produce the energy required to travel at 50mph.

And you know what - some of them may well be correct. Some people may well have cars like that.

Surely fuel consumption is the mouse in the room. A couple of mpg
here and there isn't much to worry about.

I think time wasted is the elephant in the room. It takes a lot longer
to get somewhere at 50 mph versus 70 mph.

Time is money, and most waged folks are on more than £10 an hour.
Some much more.

So for some notional 10 mile journey, do you want to get there in 12 minutes
under the new rules or under 9 minutes under the old rules ?

Multiply that up by thousands of road users and it's clear that
the cleaner air is costing us a lot of money.

We will leave aside the effects diesel HGVs and buses have on cleaner air.
Let's just make sure we punish petrol and diesel cars, eh ?

'thing is, every day the national fleet gets cleaner. Even with VW's exploits, engines are massively cleaner than they ever have been when an old pre-emissioned car drives by the smell is noticeable. We can wag our fingers at good vehicles either because they are all complying with the emissions regs and likely better maintain than most cars.

Not only that, the motoring public were incentivised to by diesel cars, now it turns out they are being regulated against. Besides, my car has passed of the EU Type Approval regulations for emissions, safety and bystander noise and by any measure is pretty damn quiet and doesn't use much fuel, but that's still not good enough. If it is about reducing emissions and emissions are equal to fuel used, why are small economical cars made to crawl along at the same speed as 2-tonne SUV's?

[cherryowen]

We've just spent a coupe of days in Pembrokeshire, and on the way back to Shropshire we visited close relatives in Porthcawl so we had to use the M4 past Port Talbot.

The last time we were down in that neck of the woods was back in December, and the 50 limit was in force past Port Talbot only. Now it extends (travelling West from PT) towards the viaduct at Earlswood. Now, I noted that the 50 limit started just before Earlswood and that it wasn't a "gateway" (i.e. no roundels either side of the carriageway) which - to me - invalidates the speed limit. However, for the additional length of 50 limit from Port Talbot to Earlswood, I did wonder how much difference it would make to air quality in Port Talbot.

[Pan Pan Pan]

janesmith1950 said:

My rudimentary understanding is that, all other things being equal, it takes more energy to push an object through the atmosphere at 70mph than 50mph.

Is that not settled physics?

This statement would indeed be correct if as you say all other things are equal, but they are not.
I have just returned from another long road trip, and can categorically state from experience that my car uses more fuel when travelling at 50mph in top gear, than it does when traveling at 70 mph in top gear.
Car manufacturers design their vehicles to operate in a range of conditions which actually exist, and in which cars are expected to travel., Consequently they are designed to be operated in urban and motorway conditions. In terms of fuel consumption, all vehicles perform less well in slower stop/start urban conditions where aerodynamics have far less effect owing to the kind of stop start, low speed motoring that usually occurs in urban/built up areas.
Manufacturers own figures show that they achieve better mpg on motorways, than they do in urban areas. Since most developed countries have motorway speed limits of just below or above 70 mph, that is the speed they design their vehicles to achieve optimum performance. I know of no car manufacturer who design their vehicles to achieve its optimum performance at 50 mph.
Whenever the overhead gantries show speeds below the NSL, all that happens is that vehicles bunch up, and then travel at a fuel consuming, emissions increasing crawl, and only rarely travel at even the reduced speed limit shown on the overhead gantries.
Having been caught in a major traffic jam on the M25 recently where speeds ranged from 5 mph to 50 mph, my fuel consumption went from the 52 mpg it achieves at 70 mph down to 28mpg, So no, in the real world reduced speed limits do not help to reduce fuel use, and emissions.
In the real world reduced limits actually increase fuel use, and emissions, and because at 50 mph all drivers will take longer to complete a given journey, it means that all drivers will be on the roads for longer, thereby increasing the number of cars on the roads at any given time, and increasing the likelihood of traffic congestion. Lower speed limits will also negatively effect the efficiency of businesses, since they cannot reach as many meetings/customers/ deliveries etc in a day. and will impact people lives even more than slow traffic/ hold ups etc already do.
Perhaps a Pistonhead poll of how many Pistonheaders want to travel at 50mph instead of 70mph would be one way of testing the water on this subject.
Those that want the 50 mph limit would seem to be those who don't have to do high mileages themselves, and who fore heaven only knows what reason want to make long distance car use as slow inefficient, and inconvenient as possible (especially for high mileage drivers) so that people abandon their cars in favour of other faster options, such as high speed rail or air travel.

[eccles]

Evanivitch said:

The original part of the 50mph section in Port Talbot was brought in for noise reasons.

No It wasn't it was brought in for safety reasons due to the very short slip roads on the elevated section.

[eccles]

cherryowen said:

We've just spent a coupe of days in Pembrokeshire, and on the way back to Shropshire we visited close relatives in Porthcawl so we had to use the M4 past Port Talbot.

The last time we were down in that neck of the woods was back in December, and the 50 limit was in force past Port Talbot only. Now it extends (travelling West from PT) towards the viaduct at Earlswood. Now, I noted that the 50 limit started just before Earlswood and that it wasn't a "gateway" (i.e. no roundels either side of the carriageway) which - to me - invalidates the speed limit. However, for the additional length of 50 limit from Port Talbot to Earlswood, I did wonder how much difference it would make to air quality in Port Talbot.

I was home seeing my Mum (west of Swansea, in Gower) in the middle of June, we had very wet weather with low cloud, what happens then is all the crap that's pumped out of the steel works fills up the gap between the clouds and the hills above the M4, so you are driving along in a foul smelling fog. It's been like that for the last 40 years, and whilst it's still considerably better than it was, it's still awful.

[Evanivitch]

eccles said:

Evanivitch said:

The original part of the 50mph section in Port Talbot was brought in for noise reasons.

No It wasn't it was brought in for safety reasons due to the very short slip roads on the elevated section.

So I guess all NSL dual carriageways have the same thing? No. It always was because the carriageway passed within metres of people's roofs.

Despite this idea that the slip roads in Port Talbot are dangerous the only hazard that ever truly existed along that stretch was the West-bound fixed speed camera.

[cherryowen]

eccles said:

cherryowen said:

We've just spent a coupe of days in Pembrokeshire, and on the way back to Shropshire we visited close relatives in Porthcawl so we had to use the M4 past Port Talbot.

The last time we were down in that neck of the woods was back in December, and the 50 limit was in force past Port Talbot only. Now it extends (travelling West from PT) towards the viaduct at Earlswood. Now, I noted that the 50 limit started just before Earlswood and that it wasn't a "gateway" (i.e. no roundels either side of the carriageway) which - to me - invalidates the speed limit. However, for the additional length of 50 limit from Port Talbot to Earlswood, I did wonder how much difference it would make to air quality in Port Talbot.

I was home seeing my Mum (west of Swansea, in Gower) in the middle of June, we had very wet weather with low cloud, what happens then is all the crap that's pumped out of the steel works fills up the gap between the clouds and the hills above the M4, so you are driving along in a foul smelling fog. It's been like that for the last 40 years, and whilst it's still considerably better than it was, it's still awful.

You should have lived in Skewen when the Llandarcy refinery was operating! Damp days could take the skin off the back of your throat

[eccles]

Evanivitch said:

eccles said:

Evanivitch said:

The original part of the 50mph section in Port Talbot was brought in for noise reasons.

No It wasn't it was brought in for safety reasons due to the very short slip roads on the elevated section.

So I guess all NSL dual carriageways have the same thing? No. It always was because the carriageway passed within metres of people's roofs.

Despite this idea that the slip roads in Port Talbot are dangerous the only hazard that ever truly existed along that stretch was the West-bound fixed speed camera.

Look at the on slips, they are just yards long, cars can't get up to speed, so you have slow moving cars coming out into fast moving traffic. It's nothing like normal NSL dual carriageways.
It used to be 70 there, passing by peoples roofs, it was only after a spate of accidents there (caused by the short on slips) that they dropped it to 50 on the raised section.

[saaby93]

eccles said:

Evanivitch said:

eccles said:

Evanivitch said:

The original part of the 50mph section in Port Talbot was brought in for noise reasons.

No It wasn't it was brought in for safety reasons due to the very short slip roads on the elevated section.

So I guess all NSL dual carriageways have the same thing? No. It always was because the carriageway passed within metres of people's roofs.

Despite this idea that the slip roads in Port Talbot are dangerous the only hazard that ever truly existed along that stretch was the West-bound fixed speed camera.

Look at the on slips, they are just yards long, cars can't get up to speed, so you have slow moving cars coming out into fast moving traffic. It's nothing like normal NSL dual carriageways.
It used to be 70 there, passing by peoples roofs, it was only after a spate of accidents there (caused by the short on slips) that they dropped it to 50 on the raised section.

When was it built?

If it was being built today, where would it go?

[TheDrBrian]

Pan Pan Pan said:

janesmith1950 said:

My rudimentary understanding is that, all other things being equal, it takes more energy to push an object through the atmosphere at 70mph than 50mph.

Is that not settled physics?

This statement would indeed be correct if as you say all other things are equal, but they are not.
I have just returned from another long road trip, and can categorically state from experience that my car uses more fuel when travelling at 50mph in top gear, than it does when traveling at 70 mph in top gear.
Car manufacturers design their vehicles to operate in a range of conditions which actually exist, and in which cars are expected to travel., Consequently they are designed to be operated in urban and motorway conditions. In terms of fuel consumption, all vehicles perform less well in slower stop/start urban conditions where aerodynamics have far less effect owing to the kind of stop start, low speed motoring that usually occurs in urban/built up areas.
Manufacturers own figures show that they achieve better mpg on motorways, than they do in urban areas. Since most developed countries have motorway speed limits of just below or above 70 mph, that is the speed they design their vehicles to achieve optimum performance. I know of no car manufacturer who design their vehicles to achieve its optimum performance at 50 mph.
Whenever the overhead gantries show speeds below the NSL, all that happens is that vehicles bunch up, and then travel at a fuel consuming, emissions increasing crawl, and only rarely travel at even the reduced speed limit shown on the overhead gantries.
Having been caught in a major traffic jam on the M25 recently where speeds ranged from 5 mph to 50 mph, my fuel consumption went from the 52 mpg it achieves at 70 mph down to 28mpg, So no, in the real world reduced speed limits do not help to reduce fuel use, and emissions.
In the real world reduced limits actually increase fuel use, and emissions, and because at 50 mph all drivers will take longer to complete a given journey, it means that all drivers will be on the roads for longer, thereby increasing the number of cars on the roads at any given time, and increasing the likelihood of traffic congestion. Lower speed limits will also negatively effect the efficiency of businesses, since they cannot reach as many meetings/customers/ deliveries etc in a day. and will impact people lives even more than slow traffic/ hold ups etc already do.
Perhaps a Pistonhead poll of how many Pistonheaders want to travel at 50mph instead of 70mph would be one way of testing the water on this subject.
Those that want the 50 mph limit would seem to be those who don't have to do high mileages themselves, and who fore heaven only knows what reason want to make long distance car use as slow inefficient, and inconvenient as possible (especially for high mileage drivers) so that people abandon their cars in favour of other faster options, such as high speed rail or air travel.

It’s like you’re so close but can’t see the forest for the empty road answer.

[saaby93]

Would it help to think about planes
Is there an optimum speed that a plane travels?
If I remember Concorde was more economical travelling above the speed of sound than below it
Below a few hundred mph it wouldnt fly