Pollution and M'way speed limit

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Sun 13 Sep 2020 18:06
Pollution and M'way speed limit - Andrew-T

Saturday's paper announced that on four shortish stretches of M'way the limit would be reduced semi-permanently from 70 to 60, to reduce local pollution. That started me wondering how great the effect would be. I know this has been done before for various reasons.

Making various simplifying assumptions: that the same traffic continues to use those roads obeying the limits (as they do now :-) ) probably in the same top gear - would mean the motors would each perform the same no. of revs, generating a similar quantity of exhaust, but over about 15% longer time. The only obvious change will be a drop in wind and rolling resistance, reducing the power needed. Are those the only factors which come into play ?

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Sun 13 Sep 2020 18:51
Pollution and M'way speed limit - Sofa Spud

There would be the small amount of energy saved by not accelerating the car by that last 10 mph from 60 to 70.

More fuel would probably be saved by keeping the 70 mph limit while enforcing it more rigorously, though.

Edited by Sofa Spud on 13/09/2020 at 18:52

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Sun 13 Sep 2020 19:36
Pollution and M'way speed limit - bathtub tom

More fuel would probably be saved by keeping the 70 mph limit while enforcing it more rigorously, though.

I doubt it. IIRC wind resistance is proportional to the square of the velocity.

The limit's only between adjacent junctions (except in one case) and I suspect will only make seconds difference to journey times.

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Sun 13 Sep 2020 19:44
Pollution and M'way speed limit - focussed

"That started me wondering how great the effect would be. I know this has been done before for various reasons."

Presumably as the new limit will not affect HGV's at their limited speed of 60 mph, I think the proposed reduction in pollution will be practically very little, but of course will be trotted out as a major step forward in health terms.

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Sun 13 Sep 2020 19:53
Pollution and M'way speed limit - gordonbennet

Slowing the traffic to 60mph, given typical speedometer error, will actually be the 55/56mph that HGV's are limited to as maximum speed, so as on so called smart motorways should result in a steady flow of traffic through the section with minimum braking and acceleration needed to maintain speed, result lowering of all emissions.

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Mon 14 Sep 2020 09:17
Pollution and M'way speed limit - Andrew-T

Slowing the traffic to 60mph, given typical speedometer error, will actually be the 55/56mph that HGV's are limited to as maximum speed, so as on so called smart motorways should result in a steady flow of traffic through the section with minimum braking and acceleration needed to maintain speed, result lowering of all emissions.

Perhaps the most noticeable effect will be more rolling roadblocks, because drivers will take much longer to pass HGVs at a legal speed ?

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Sun 13 Sep 2020 19:50
Pollution and M'way speed limit - badbusdriver

There would be the small amount of energy saved by not accelerating the car by that last 10 mph from 60 to 70.

More fuel would probably be saved by keeping the 70 mph limit while enforcing it more rigorously, though.

Not sure i understand either of these comments, it seems to suggest you think travelling at 70mph requires no more energy than travelling at 60mph?. As Scotty says, "Ye canna change the laws of physics"!. It isn't just accelerating from 60-70mph which requires more energy, but actually maintaining it (i.e, you are constantly using more fuel). Just how much fuel you will save doing 60mph rather than 70mph will depend mainly on aerodynamics, but also gearing.

But given most motorway drivers are doing at least 80mph, (unless the 60mph limit is policed effectively with average speeds cameras) all that will happen is that cars will be doing 70mph rather than 80+mph

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Sun 13 Sep 2020 20:50
Pollution and M'way speed limit - RT

This "experiment" is a farce - the areas concerned have high levels of pollutants at peak times when traffic probably can't exceed 60 anyway - off-peak speeds are higher but pollutants are lower as traffic is lower.

Of course traffic is low because of covid restrictions so any comparison will be apples and pears.

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Sun 13 Sep 2020 21:11
Pollution and M'way speed limit - Zippy123

I recall an article a few years ago re the M25 near Heathrow.

Basically a scientific type confirmed the 50 limit was kept in place not to reduce congestion but pollution.

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Sun 13 Sep 2020 22:35
Pollution and M'way speed limit - bathtub tom

Anyone else recall the blanket 50MPH motorway speed limit in the '70s during the Suez crisis?

I was doing a regular 120 mile journey. It would take around ten minutes longer. The fuel saving was incredible. If I was saving that much fuel, imagine how much less was coming out of my exhaust!

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