Cutting speed limits for cleaner air?

[WelshChris]

The Welsh government have announced plans to cut speed limits from 70 to 50 on certain stretches of busy roads to ‘improve air quality’...

http://www.bbc.co.uk/news/uk-wales-43881650

Is this snake oil? - my initial thought is that it won’t make a blind bit of difference. Another excuse to cut speed limits based upon flawed research. Surely air quality is determined by the volume of traffic rather than the speed it passes through a given zone? - or am I wrong?

[jmorgan]

Have they noticed what else is in Port Talbot?

[rustyuk]

My car does gets significantly better mpg when travelling at 50 rather than 70.

[davepoth]

http://www.dft.gov.uk/vca/fcb/smarter-driving-tips...

Driving at 50 rather than 70 uses 25% less fuel, as air resistance increases as a square of velocity. And as it turns out the principle that as traffic density increases speed reduces works the other way too - if you slow the traffic down you can fit more cars on the road.

https://www.sciencedirect.com/science/article/pii/...

So yes, slowing the traffic down will both reduce emissions and increase capacity.

It also reduces fun.

[ChemicalChaos]

Cars produce a fraction of the emissions of HGVs, which are already doing 50

[DurianIceCream]

Cars are more efficient at 50 than at 70. So as long as a limit reduction does not cause congestion, then it will reduce emissions. OTOH, how much clean air does anyone need? I'd like to see the limit increase to 80.

[Fermit The Krog and Sexy Sarah]

It's already happening in England. For 6 hours a day the M1 between Mansfield and Sheffield is at a permanent 60mph for this very reason

[dcb]

davepoth said:

Driving at 50 rather than 70 uses 25% less fuel,

Nonsense. At very least it depends on the car. Different cars have different efficiencies
at different speeds.

Saving 50p or a pound on fuel is a small saving, compared to the cost of the time saved.

An hour shorter car journey could easily save anywhere from £20 to £100 or more,
depending on the driver and the number of passengers.

[Welshbeef]

Are EVs permitted to continue at 70mph or PHeVs which have zero exhaust emission ?

[grumbledoak]

This is just yet another anti- private car measure.

Cars are way cleaner than buses or lorries or huge ships and ports. But pollution isn't the priority, it is just the excuse. For private motoring it is going to be death by a thousand excuses.

[turbobloke]

dcb said:

davepoth said:

Driving at 50 rather than 70 uses 25% less fuel,

Nonsense. At very least it depends on the car. Different cars have different efficiencies
at different speeds.

Saving 50p or a pound on fuel is a small saving, compared to the cost of the time saved.

An hour shorter car journey could easily save anywhere from £20 to £100 or more,
depending on the driver and the number of passengers.

Agreed on savings. This so-called analysis is merely cost-benefit without the benefit aspect included as it should be. Who could possibly fall for it?!

The driver and driving style are two key factors. That claimed fuel saving is a simplistic assertion. Nothing more. What's the mpg for speeds even slower than 50mph e.g. stop-start in motorway or dual carriageway roadworks?

A more pressing issue for politicians to prioritise, if they ever get to understand the meaning of the word priority, is indoor air. This is on average 10x more polluted than outdoor urban air according to the UK Buildings Research Establisment and USA EPA research. This will have far greater impacts on health but transport is a political milch cow.

Behind what passes for an idea with these people is the threat of EU fines for exceeding arbitrary standards that are already better than indoor air. The sources of pollution are another ironic laugh a minute. Trees, grass and shrubs emit as much NOx globally as combined transport and industrial emissions - see Hari et al "Ultraviolet Light and Leaf Emission of NOx" Nature 422, 134 (2003).

The really rich part, for urban air midlands to the south coast as opposed to roadside air, is that when arbitrary levels are exceeded and the prospect of EU fines makes people twitchy, the cause is almost always an anticyclone nearby or over the UK with a southerly airflow bringing trans-boundary pollution up to the UK from pollution sources in Europe to the south.

[Saleen836]

Apart from the fuel saving surely vehicles driving at 50mph will just be on the roads longer causing the same amount of pollution?

[turbobloke]

Saleen836 said:

Apart from the fuel saving surely vehicles driving at 50mph will just be on the roads longer causing the same amount of pollution?

AIUI the genius behind this sort of idea is that movement through particular 'hotspots' of pollution will be reduced (if you believe the simplistic mantra on offer) and arbitrary limits will no longer be exceeded. It matters not what happens before or after those sections of carriageway being targeted for speed limit reductions, including as part of a longer journey overall, as the EU hasn't got its sights on anywhere else just now.

https://news.sky.com/story/europes-final-warning-t...

[turbobloke]

Anyone who has fallen for the 40,000 deaths due to pollution should not only remember the 10x worse pollution indoors which is never taken into account, but also take a slow and careful read of the following clinical demolition of the fallacy involved.

http://wmbriggs.com/post/13029/

[garagewidow]

turbobloke said:

AIUI the genius behind this sort of idea is that movement through particular 'hotspots' of pollution will be reduced (if you believe the simplistic mantra on offer) and arbitrary limits will no longer be exceeded. It matters not what happens before or after those sections of carriageway being targeted for speed limit reductions, including as part of a longer journey overall, as the EU hasn't got its sights on anywhere else just now.

https://news.sky.com/story/europes-final-warning-t...

the next stage will be telling the wind not to blow the pollution around and leave it in those uncongested areas.

and it will probably be enforced by the flashing piggybanks

[dci]

Who comes up with this crap?

The A470 upper boat to Abercynon stretch is empty for large parts of the day. At the worst affected times which is when I assume that the pollution is at its worst you’d be lucky to reach 50 MPH.

Will there be a public consultation or has this already been signed, sealed and the 50 signs ordered?

[fatboy b]

dcb said:

davepoth said:

Driving at 50 rather than 70 uses 25% less fuel,

Nonsense. At very least it depends on the car. Different cars have different efficiencies
at different speeds.

Saving 50p or a pound on fuel is a small saving, compared to the cost of the time saved.

An hour shorter car journey could easily save anywhere from £20 to £100 or more,
depending on the driver and the number of passengers.

Yep. Mine is better at 80. At 50 it’s not in top gear.

[Welshbeef]

Saleen836 said:

Apart from the fuel saving surely vehicles driving at 50mph will just be on the roads longer causing the same amount of pollution?

Less fuel duty for Welsh Assembly due to higher MPG

[mike9009]

I presume the same will apply to all 20 mph, 30mph and 40mph zones and the speed limit will be increased to 50mph to cut emissions/ pollution in particularly densely populated areas. Makes more sense......

Or how about removing speed humps, traffic narrowing to reduce exhaust pollution in areas where pollution might directly affect the local population??

Mike

Edited by mike9009 on Tuesday 24th April 22:45

[Welshbeef]

No just EVs in Cardiff makes sense