Variable speed limit reasons - noise & pollution?

[MarkwG]

Good evening - I've done some searching but nothing definitive appears, so figured I might get something here. I was chatting to a friend who recently attended a speed awareness course, due to misreading a variable speed limit sign on a smart motorway. She was advised that reduced limits are sometimes applied when going through populated areas at night, to reduce noise & air pollution? Does that sound right? I vaguely recall some debate, but had no idea it had already been applied, or is this a case of the course deliverer making stuff up...? Thanks in advance!

[covboy]

It has been posted on here that a section of the M1 through Derby/Notts is proposed/alreadyhas a 60mph limit to reduce polution

[kiethton]

And on the M4 in Port Toilet, blame the motorway for the pollution, not the gunning great steel works that’s overshadowing the town...

[meatballs]

Speed limit exclusions for electric?

[Pica-Pica]

A494 at Welsh/England border. 50mph length has been extended and made permanent.

http://www.deeside.com/temporary-50mph-speed-limit...

[Alucidnation]

Makes sense if engines are at their most efficient at 50mph ish.

[Driversmatter]

It is true - and not just us Brits that are at it. The French lowered their national limits by 10mph for the same reason. The 20mph limit in towns isn't solely for road safety it also reduces noise and emission levels.

[kiethton]

Driversmatter said:

The 20mph limit in towns isn't solely for road safety it also reduces noise and emission levels.

Sorry but it doesn't....especially when speed bumps are added - I have to stop near completely to get over them given my ride height. This means 1st gear which I might as well hold onto until the next one a hundred yards or so away - not particularly quiet with a Larini sports exhaust system....

Ignoring speed bumps, at 20mph I'm in second gear where as 32 I can just about run with 4th, a significant RPM difference.....

[GreatGranny]

kiethton said:

Sorry but it doesn't....especially when speed bumps are added - I have to stop near completely to get over them given my ride height. This means 1st gear which I might as well hold onto until the next one a hundred yards or so away - not particularly quiet with a Larini sports exhaust system....

Ignoring speed bumps, at 20mph I'm in second gear where as 32 I can just about run with 4th, a significant RPM difference.....

But most of us don't have to slow down to crawling speed to negotiate them or have Larini sports exhausts!

[rdjohn]

This long sweeping curve on the new A556 leading leading to the M56 has a 50mph speed limit because the computer model said “Yes”.

Somewhat bizarre.

[Terminator X]

MarkwG said:

Good evening - I've done some searching but nothing definitive appears, so figured I might get something here. I was chatting to a friend who recently attended a speed awareness course, due to misreading a variable speed limit sign on a smart motorway. She was advised that reduced limits are sometimes applied when going through populated areas at night, to reduce noise & air pollution? Does that sound right? I vaguely recall some debate, but had no idea it had already been applied, or is this a case of the course deliverer making stuff up...? Thanks in advance!

Imho this is the main reason that variable / smart motorways are coming on strong, UK unable to meet emissions targets so must slow us all down to get emissions down.

Current locations and growing:



TX.

[anonymous-user]

Alucidnation said:

Makes sense if engines are at their most efficient at 50mph ish.

It would, but they’re not.

[MarkwG]

Thank all, very illuminating! It raises a number of curiosities in my mind - from the map above, the area she was referring to doesn't feature, so it is spreading but by stealth too. I'm also unconvinced by the "50=most efficient" debate - that's not the case for a number of vehicles I use, & how do electric cars feature, do they get let off???

[Pica-Pica]

REALIST123 said:

Alucidnation said:

Makes sense if engines are at their most efficient at 50mph ish.

It would, but they’re not.

Evidence?

[Mr Tidy]

To be fair if there is an issue with air quality on the Western side of the M25 then Heathrow Airport is probably at least as big a cause as the M25!

[Tony1963]

REALIST123 said:

It would, but they’re not.

They’re more efficient at 50 ish mph that they are at 70mph. And by quite a lot.

[grumbledoak]

It's "safety", then it's "CO2", then it's "noise", and then "pollution". Repeat.

If the problem changes but the answer is always the same, you can safely assume that the "reason" is a load of bollards.

[Shiv_P]

Tony1963 said:

REALIST123 said:

It would, but they’re not.

They’re more efficient at 50 ish mph that they are at 70mph. And by quite a lot.

Engines efficiencies are measured at RPMs, not speeds.

[Pica-Pica]

Shiv_P said:

Tony1963 said:

REALIST123 said:

It would, but they’re not.

They’re more efficient at 50 ish mph that they are at 70mph. And by quite a lot.

Engines efficiencies are measured at RPMs, not speeds.

But we are talking vehicle efficiencies, so distance covered per time must be relevant, especially with increasing drag at higher speeds.

[Ian Geary]

Shiv_P said:

Tony1963 said:

REALIST123 said:

It would, but they’re not.

They’re more efficient at 50 ish mph that they are at 70mph. And by quite a lot.

Engines efficiencies are measured at RPMs, not speeds.

Ok, so you're saying higher revs are more efficient? Or lower revs? Or both perhaps?


Or you just don't agree with equating speed and fuel efficiency ever, and everyone should stop talking about it right now?