Is it me?
But I thought cars were more fuel efficient at higher speeds in high gears (looking at the urban and extra urban consumption figures). So 20mph limits surely make cars perform worse? Especially those with speed bumps as slowing down and speeding up all the time uses more fuel than keeping at a constant speed. I may be wrong on this, please correct me if so.

And personally anyone stupid enough to mill around in the road deserves to get run over. I believe this should come under 'natural selection'.

And yes! Let's melt all the Chelsea tractors down! (Unless you are a farmer or such like that actually does need a 4x4). Force all the mums to drive their offspring to school in metros (hey, it worked for my mum and dad) or matizs or something else hugely underpowered.
( VandenPlas can't think of anything clever - no change there!, Mon 13 Oct 2008, 9:48, 1 reply)

Re: car engines...
I believe so. They have a sort of "optimum" speed, which is usually around motorway speeds, where the engine runs most efficiently. So it is actually less efficient to drive at 20mph.

However, I don't know how much difference there is in efficiency between driving at 20mph and driving at 30mph. There's probably a way of working it out but that would require me to start trawling wikipedia.

I'm also with y'all on melting down the Chelsea tractors (vulgar, fuel-hungry totems to narcissistic excess that they are). But let's force the flabby little to walk to school. Or, if it's really that far, would it kill them to have to share a bus with all those "common" folk?

Grrrrr
(LongJohnBaldry, Mon 13 Oct 2008, 11:29, closed)

Good point
I hasten to add, I used to walk to school. The metro was for long journeys (the joys of a family of four plus a sodding huge tent going to Norfolk in a tiny car ^_^)

Can you imagine all those little kids trying to *walk* to school? Surely their little limbs will be atrophied with all the carting around and no exercise.
( VandenPlas can't think of anything clever - no change there!, Mon 13 Oct 2008, 12:55, closed)

It's not so much
a difference in efficiency between 30mph and 20mph- that's probably negligable.

It's between doing 20 and having to slow to 5mph EVERY FREAKING 50 FEET! You can't sit at 5mph or you get honked at- and don't really get anywhere anyway.

Increasing 1 tonne (1000kg) from 5 to 20mph means 20kJ of energy is burnt. Now given a 20% efficiency of engine that means about 100kJ of energy is wasted for every speed bump. So on a 10 speed bump road, each trip of each car wastes 0.02kWh of energy. Think how many cars go down your road, multiply the 0.02 by that number, then double that as they'll come home at the end of the day too. The speed bumps on a long residential street could waste as much energy just in use- never mind extra maintanence etc- as a whole other car doing 20.
( uberdeity Wondered why Owen Lars knew of the term "hell" on, Mon 13 Oct 2008, 15:47, closed)