Why aren't speed limits set at the 85th percentile any more..

[unrealrocks]

Interesting article and totally agree that blanket limits are idiotic. I totally agree that 8-9am outside a School - even 20mph is probably too fast. However even that same road could often be perfectly safe at 30, or even 40 at quieter times. By the time a system is being policed using controls that basically remove the decision making process it is already too late to go back to what we had before unfortunately I believe

 

[Mirage]

watch out guys, we got a child killer over here.

I reckon we should copy Gibbo's post over on to mumsnet.

 

[Fuzz]

Main reason is emissions.

/thread

"Question 4 of the consultation asked consultees whether they agreed
that compliance with air quality limits could be a factor in the choice of
speed made by local traffic authorities and if not, to explain their
reasons."

TfL commented that the impact of speed limits, and consequently
vehicle speeds, on air quality is complex. Not only is the air quality
metric under consideration relevant (PM, NOx, CO2 etc), with speed
changes affecting different metrics in different ways, but many other
factors are also relevant including congestion levels, whether speeds
are moving towards or away from emission minima in vehicle engine
cycles and the potential of speed limit changes to deliver modal shift.
Additionally, future changes in vehicle engine design, vehicle fleet
make up (e.g. diesel versus petrol proportions of the fleet) and
emission reduction targets will impact the decision making process
should air quality compliance be a driving factor in speed limit
decisions.
73.
Given this complex landscape, TfL encourages DfT to demonstrate
evidence on the impact of changes to speed limits in different
scenarios in order to ensure that the guidance facilitates improvements
to air quality and not the reverse.

Therefore, when such limits are
implemented they will need to comply with the normal
implementation criteria to ensure mean speeds match the
relevant speed limit; this may mean physical engineering
measures may be required. Air quality limits should not be the
18
sole deciding factor in the setting of a speed limit, other gravity
factors of a road safety nature should also be present to ensure
a consistent application nationally

 

[CaptainRAVE]

I find 50 stretches on motorways to be the most challenging. The cars are usually bunched up, I am constantly monitoring my speed and am not concentrating anywhere near as much on the road as I would be on any other stretch. There are also so many roads where the limit also changes for no apparent reason.

I agree, I will drive to the road conditions. The cars on the road that are well under the speed limit are more dangerous imo.

 

[Haggisman]

"Question 4 of the consultation asked consultees whether they agreed
that compliance with air quality limits could be a factor in the choice of
speed made by local traffic authorities and if not, to explain their
reasons."

TfL commented that the impact of speed limits, and consequently
vehicle speeds, on air quality is complex. Not only is the air quality
metric under consideration relevant (PM, NOx, CO2 etc), with speed
changes affecting different metrics in different ways, but many other
factors are also relevant including congestion levels, whether speeds
are moving towards or away from emission minima in vehicle engine
cycles and the potential of speed limit changes to deliver modal shift.
Additionally, future changes in vehicle engine design, vehicle fleet
make up (e.g. diesel versus petrol proportions of the fleet) and
emission reduction targets will impact the decision making process
should air quality compliance be a driving factor in speed limit
decisions.
73.
Given this complex landscape, TfL encourages DfT to demonstrate
evidence on the impact of changes to speed limits in different
scenarios in order to ensure that the guidance facilitates improvements
to air quality and not the reverse.

Therefore, when such limits are
implemented they will need to comply with the normal
implementation criteria to ensure mean speeds match the
relevant speed limit; this may mean physical engineering
measures may be required. Air quality limits should not be the
18
sole deciding factor in the setting of a speed limit, other gravity
factors of a road safety nature should also be present to ensure
a consistent application nationally

I make it my personal mission to ensure emissions, noise levels, and speed are adequately maintained at all times. That means in 30mph zones with speed bumps, I'm forces to rev the nuts off my car after each bump to maintain the equilibrium*




*This may not be true, but sometimes is very tempting, childish as it may be

 

[Dr Who]

measures may be required. Air quality limits should not be the
18
sole deciding factor in the setting of a speed limit, other gravity
factors of a road safety nature should also be present to ensure
a consistent application nationally

It's never the sole factor, the other factors are accident history, traffic calming etc, but the effect of reducing the speed limits has a measurable effect that the UK can use to meet it's emission limits, or at least show they are attempting to meet them rather than trade them on the carbon market.

 

[Janesy B]

I find 50 stretches on motorways to be the most challenging. The cars are usually bunched up, I am constantly monitoring my speed and am not concentrating anywhere near as much on the road as I would be on any other stretch. There are also so many roads where the limit also changes for no apparent reason.

I agree, I will drive to the road conditions. The cars on the road that are well under the speed limit are more dangerous imo.

In all honesty I find some sections of motorway work better at 50MPH than 70.

It's never the sole factor, the other factors are accident history, traffic calming etc, but the effect of reducing the speed limits has a measurable effect that the UK can use to meet it's emission limits, or at least show they are attempting to meet them rather than trade them on the carbon market.

Then why do they always stick in slaloms/islands, silly mini roundabouts, speed humps and other traffic calming measures that then increase emissions because you have to waste energy braking or slowing down and then accelerating again?

 

[snips86x]

In all honesty I find some sections of motorway work better at 50MPH than 70.

Then why do they always stick in slaloms/islands, silly mini roundabouts, speed humps and other traffic calming measures that then increase emissions because you have to waste energy braking or slowing down and then accelerating again?

In general or due to traffic?

 

[theone8181]

In general or due to traffic?

Both i imagine, how many times does the person infront brake at a roundabout then look, rather than looking to see if its clear before they get there.

 

[snips86x]

Both i imagine, how many times does the person infront brake at a roundabout then look, rather than looking to see if its clear before they get there.

Fair point but that's something I've often found to be strange. Surely traveling at a reasonably low speed allows the driver to check approaching vehicles to the right before breaking? Something which I do quite easily!

 

[lemonade]

Heres one that relates to this: http://www.bbc.co.uk/news/uk-wales-south-west-wales-30378801

 

[[TW]Fox]

If speed killed then the autobahn would be like something out of carmageddon.

I drove a couple of hundred miles in Germany where allowed at constant 210km/h without issue and the only reason I did not go faster because if a vehicle is fitted with Winter tyres that is the safe maximum speed, otherwise I'd have gone quicker.

It makes travelling far easier and in my view safer.......

Why you ask?

Well 3hr journey at 70mph or less than 90 minutes at 150mph, for me it is far safer to only be travelling for 90 minutes versus twice that amount of time, you will tire and your period of risk is doubled (Based on a car easily capable and stopping at such speeds).

It's almost impossible to cruise at a sustained 150mph+ on the Autobahn though - the traffic levels are too high and it's too risky be cruising at 150+ when you've got bunched up traffic on the inside lane as they can and do lunge out with little warning.

The most I managed today (Cologne to the Netherlands) was touching 140mph for a few seconds, generally it was difficult (from a safety perspective) to cruise much above 120mph.

 

[Janesy B]

The logic is flawed because you have to concentrate harder and end up arriving more tired or tiring much sooner. It's like walking a mile or running it, you get there quicker (in theory you should be less tired because you've spent less time travelling) but really you've exerted more energy doing so.

 

[Dr Who]

Then why do they always stick in slaloms/islands, silly mini roundabouts, speed humps and other traffic calming measures that then increase emissions because you have to waste energy braking or slowing down and then accelerating again?

They only use those in 30/20mph zones where traffic calming is a greater priority than emissions. There is no argument for 85th percentile on these roads so your point is not relevant.

Like he said, emissions is not a sole factor.

 

[Dr Who]

It's almost impossible to cruise at a sustained 150mph+ on the Autobahn though - the traffic levels are too high and it's too risky be cruising at 150+ when you've got bunched up traffic on the inside lane as they can and do lunge out with little warning.

The most I managed today (Cologne to the Netherlands) was touching 140mph for a few seconds, generally it was difficult (from a safety perspective) to cruise much above 120mph.

Indeed, the way some people talk about autobahns you would think they are empty race tracks you can do what you like on. The reality as you say is very different. It takes a brave (read stupid) man to do 150mph on a 2 lane road with a busy nearside lane. Over here that would be insane with mirror use as it stands...

 

[ChrisD.]

I've lived in Germany for a good part of my life, the only times I've managed to do more than 130mph constant is in the night time and non working days. Apart from that, 120 is about the most I used to dare go.

Night time as well is interesting due to the lack of cat eyes. I used to sit in the middle lane if nothing else was around in case of any corners coming up.

Managed 156 gps or so in the Evo before I bottled it due to traffic ahead, and that was on a quiet day. I fail to see how you could cruise at 150 unless it's the middle of the night with excellent headlights.

What I always found with Germany is if you were sat doing 120+ for a long period and then need to come off, almost all exits seem to double back on themselves, so you'd have to slow down a fair bit more than you'd think.

 

[[TW]Fox]

And the German's dont bomb everywhere at 150+ either. The overwhelmingly majority of cars are doing far less than that.

 

[Clarkey]

Fuel consumption becomes pretty tragic when you're well into 3 figures. I've driven a sustained 150 indicated in the 328 and IIRC that equated to 9mpg, compared to 37 at 70

 

[muon]

So many red light cameras are being switched to dual purpose. Again on roads that should never be 30mph.

 

[Meridian]

It takes a brave (read stupid) man to do 150mph on a 2 lane road with a busy nearside lane.

Have the autobahns changed then? It used to be that the "any speed" bits only applied to 3-lane sections, and all the 2-lane bits had 150kph limits (or lower).

That said, yes, the danger in the unrestricted bits lies in the speed difference between lane 2 (usually about 80mph) and lane 3 (anything goes). You can legitimately look in your mirror, see an empty road for half a mile back, pull out, and find a Porsche stuck to your back bumper flashing its lights.