30kmh – making streets liveable

[Vorpal]

I'm not doubting what happens in Norway. I'm asking for evidence it works, or would work, in the UK, and what kind of journeys by bicycle we are talking about (from cul-de-sac conversions) that would otherwise be in cars.

You dismissed the only evidence that we have available. The only other thing to do is try it. It doesn't cost much.

[Geriatrix]

As I said in my previous post, the problem with a 20mph limit is that it leads to higher, not lower, emissions.

How is that being measured?
I suspect the chief problem is not the limit, but driver attitude. Many roads have implemented the limit along with traffic calming speed humps. Now as a cyclist I can keep a constant speed of 20 mph on the road and over the bumps, but my momentum is frequently destroyed by motorists who accelerate past me between the bumps, and then brake before the next bump. This is the least fuel efficient way to drive and it's unnecessary. I know that as a motorist because I can maintain a constant speed of 20 mph over the bumps in my car.

[Mark1978]

As cyclists we know how much more energy is required to move through air at 30 than 20.

I suggest that the petrol engine isn't actually *that* variable in it's efficiency to make up for that difference. I rather think that people are inept at maintaining lower speeds without the "burn money" pedal in the middle...

In general yes, faster == more fuel burned, but it's not as simple as that, there are a lot of factors between the simple fuel burning and traction being put down to the road. The fact that I get better MPG out of my car at 50mph rather than 30mph suggests that. That's not to say that my car couldn't be made to be more efficient at 30mph than 50mph, but that's the way it's built, as are most.

[jezer]

Well, I'm no engineer, and all this technical stuff passes over my head. Perhaps car producers should be forced to make their products more efficient at lower speeds. I can hear the moans from Audi/BMW/Mercedes drivers already

[reohn2]

In general yes, faster == more fuel burned, but it's not as simple as that, there are a lot of factors between the simple fuel burning and traction being put down to the road. The fact that I get better MPG out of my car at 50mph rather than 30mph suggests that. That's not to say that my car couldn't be made to be more efficient at 30mph than 50mph, but that's the way it's built, as are most.

But that's not true,if you could,but can't practically,drive at 30mph on the m/way you'd without doubt get more MPG than at 50mph.
It's the changes of speed and sitting in traffic jams that eat fuel,that's if,and it's a big if,the engine ever gets upto something resembling optimum temperature on urban roads,bearing in mind that most car journey are under 5miles.
The right pedal always uses more juice the further down it's pressed.
Add to that the stupid way people accelerate then brake eg;when running upto traffic jams,by their inability to read the road ahead and what Geriatix posted above about motorists and speed humps,it's clear that drivers actually believe they're getting somewhere quicker by what used to be termed as "clog and anchor" driving.These drivers wouldn't know smoootthhh if it caressed their face with silk scarf
The number of times I've been overtaken when cycling ,by cars as I approach a traffic jam which is clearly visible,is times out of number,only for me to then sail past them and said jam on the outside.
The problem is a psychological one actually the dim wits believe they've gained something.
Like they believe they gain something by driving at 35 or 40 in a 30 limit,it's a fallacy they buy into.In the urban enviroment they only get to the next traffic light quicker and when everyone else is doing the same thing a jam is formed,which slows traffic even more and so it goes.

[Mark1978]

It's true. I do find the best way to save fuel is never to brake and so anticipate what the traffic ahead is doing such that I never catch the up if I can help it.

[Mark1978]

It's also notable that on terrain that isn't entirely flat, then slow doesn't always equal most efficient either. I know because I've done many experiments with this in my car, on a long flat section followed by a slight downhill. If I proceed along at 30mph I don't have enough momentum to get all the way down the hill, so have to get on the throttle before the bottom, whereas if I'm travelling at 45mph (indicated) then I save the most amount of fuel as I have the momentum to get down the hill entirely off the throttle - and no throttle rolling down hill in gear is the absolute key to saving fuel when driving. 73mpg this morning, bit disappointing.

You can show this is true on a bicycle too, when you have a sudden dip in the road and then a rise, it's most energy efficient to accelerate before the hill so you carry maximum speed down, and then you can save energy getting up the other side, as opposed to just rolling down and then having to climb.

[reohn2]

You can of course give it a squirt down the hill so it'll roll up the other side
Seriously I'm not buying the 50mph uses less fuel than 30.

[Vorpal]

In general yes, faster == more fuel burned, but it's not as simple as that, there are a lot of factors between the simple fuel burning and traction being put down to the road. The fact that I get better MPG out of my car at 50mph rather than 30mph suggests that. That's not to say that my car couldn't be made to be more efficient at 30mph than 50mph, but that's the way it's built, as are most.

But that's not true,if you could,but can't practically,drive at 30mph on the m/way you'd without doubt get more MPG than at 50mph.
It's the changes of speed and sitting in traffic jams that eat fuel,that's if,and it's a big if,the engine ever gets upto something resembling optimum temperature on urban roads,bearing in mind that most car journey are under 5miles.

Different engines have different points at which they are the most efficient. Car/engine manufactures tend to aim for a best efficiency point that is aligned with the usage profile. There are also reflections of this efficiency point in other gears. For some cars, the efficiency will be nearly as good in one or two other gears as at the best point. For others, that is sacrificed to achieve a sales figure (test) fuel economy.

Fuel economy at steady speeds is best at around 50 - 60 mph for most cars. However, the difference between 20 mph, 30 mph, and 50 mph is not huge for most cars. It is huge for comparisons between 50 and 75 mph because fuel economy drops off rapidly as speeds increase above 65ish mph.

It is true that driving at steady speeds will make the biggest difference. I would estimate that for most cars, the difference between driving in urban traffic at 30 mph and driving at steady speed of 30 mph will increase fuel economy by 30 or 40%. Driving at 50 mph rather than 30 mph, all other factors being equal, will make something more like a 5% difference. Let me ephasize that different engines have different points at which they are most efficient, so this applies to typical or representative vehicles.

[reohn2]

Then there's wind drag,which increases dramatically as speed increases and more so for the 2ton square box type 4x4's that litter our roads any society in it's right mind would slap 100% increase in VED on such polluters unless for necessary business use such as farmers etc.
I take the point about engine power characteristics,I'm still not convinced of only a 5% difference in fuel consumption between 30 and 50mph.
The problem of course is keeping a constant speed which as I said upthread,you'd struggle to driver at 30mph constantly on a motorway,whereas 50mph could be achieved on such a road.
One thing is for sure though,if everyone on urban roads drove at 20mph and left their ego at home,the roads would be much safer and less stressful and no one would lose anymore time on their (short) journeys.
Anyway the debate is purely academic,the point was whether cars use more fuel and cause more pollution at 20mph compared to 30mph which is a nonsense.

[gordy]

I note with interest, that even on a cycling forum, the debate about "making streets livable" revolves around whether 30 kph would be convenient, safe or economic for the driver. I fail to see why engine efficiency should be a factor in planning urban roads and local travel networks.

The point of livable streets, IMHO, is to change the lives of people who are frightened by the speed and noise of vehicles in their neighbourhood. Urban space that is currently jammed with parked vehicles and congested traffic is shared more equitably. People become more relaxed, communities more sociable, local businesses busier. There are long term strategic advantages too - modal shifts if you will - in improving health and subtly encouraging people to make different decisions about where to live and where to work.

Making big political decisions about infrastructure can have other benefits too:






Could we keep drivers, engines, gears and fuel out of this debate?

[meic]

Not really as it is their speed which we are seeking to restrict.

[reohn2]

I note with interest, that even on a cycling forum, the debate about "making streets livable" revolves around whether 30 kph would be convenient, safe or economic for the driver. I fail to see why engine efficiency should be a factor in planning urban roads and local travel networks.

The point of livable streets, IMHO, is to change the lives of people who are frightened by the speed and noise of vehicles in their neighbourhood. Urban space that is currently jammed with parked vehicles and congested traffic is shared more equitably. People become more relaxed, communities more sociable, local businesses busier. There are long term strategic advantages too - modal shifts if you will - in improving health and subtly encouraging people to make different decisions about where to live and where to work.

Making big political decisions about infrastructure can have other benefits too:

Well said that man!

Could we keep drivers, engines, gears and fuel out of this debate?

I'd love to,too.I was responding to others' posts who seem to think lowering speed is a problem not a solution.

[CJ]

You can show this is true on a bicycle too, when you have a sudden dip in the road and then a rise, it's most energy efficient to accelerate before the hill so you carry maximum speed down, and then you can save energy getting up the other side, as opposed to just rolling down and then having to climb.

It's very difficult to measure energy efficiency and show any such thing on a bicycle, but by calculation one can prove it is not so.

Provided the bicycle has a sufficient range of gears at close intervals to avoid gross variations in cadence, it uses least energy if one does not deliberately increase one's speed above that which one intends to maintain on a flat road. It is more satisfying and saves time to give it a bit of welly on the downhill, thus to crest the following rise at something close to cruising speed, plus it avoids having to shift as many gears, but it certainly does use a bit more energy.

That's because aerodynamic energy transfer over any given distance increases with the square of speed, so any stratagem involving more speed requires more input energy.

In a motor car there will be artefacts of gearing and rpm that favour certain power levels and speeds, which if maintained can yield savings that exceed the extra energy demanded by that speed. But this does not apply to a bicycle with a wide range of umpteen gears, since the human engine has a relatively smooth rpm-efficiency curve.

Both vehicles are nevertheless affected by a psychological aversion to going awfully slow, that makes crawling in low gear feel like harder work than it is, whether uphill or on the flat.

[karlt]

That's something you just have to get used to around here - and I know there are people in hillier places.