I think that two subtly different measures are being discussed here: Road capacity, assuming no congestion should surely be a simple case of how many cars you can fit on a stretch of road whilst maintaining a safe distance between them. At present, safe distances are considered to be determined by stopping distances (though few people observe them, on motorways or elsewhere), and therefore the faster the traffic is travelling, the greater the spacing, and therefore the fewer cars you can fit in per mile of road. The second measure is that of congestion. Because cars are not all driven at the same speed, they will not remain evenly spaced. Where they can pass each other safely and easily this deosn't cause major problems, but where they can't (sometimes, but not always as a result of the weight of traffic) this results in congestion.

Although the road density is reduced when cars are travelling faster, assuming safe stopping distances are maintained, the time the cars are on the stretch of road is reduced, so it now free for more cars to come and use it. It means you can get more cars through a section of road per hour.

However - the crutch of the matter is we are really trying to reduce emmissions and not congestion (although that would be good too). Allowing people who can afford the lower fuel consumption obtained at high speed to whizz around the country is not a good idea. My car gets a nice 40ish mpg at a steay 70, increase that to 80 and is goes down to 30 mpg. Beyond that I have no idea how bad it gets ;-).

As an example of how well reduced speeds reduce congestion (and hence emissions) the M42 near Birminham also has active traffic management (and no hard shoulder during peak times). When I used to drive down it 5 years ago it was stop start all the way from J10 to J4. Now you from J7 where the traffic management starts you can keep a steady 40-50 most days. The bit by J10 is still a standstill for no reason other than 8 cars at a time try to join by forcing their way in.

A good system I saw in America to stop this is a "rally start" type traffic light on the top of the slip road, where only one car per green is allowed, with about 5 seconds in between each green. (It's great fun if you have a fast car as you can try and catch the car in front up ).

Coasting - known as Aberdeen Overdrive in the trucking community apparently, was told this some ten years ago, so nothing new there!

Green issues - I always thought buildings were part of life - you can't divorce life from the building can you? and anyway how do you keelhaul under a building?

As for how fast - anybody ever heard of a bus? Get this misguided government to introduce taxibuses, lots of mini buses etc. and get everybody that can away from private ownership!! Make the damn things a bit cheaper as well!! Lets face it most of the time a bus is perfectly adequate, especially if going the right way, at the right time! OH yes and reward those without cars with a tax break (don't drive - ain't helping to ruin the road, here's your road tax back!) after all they punish the bigger car for using more fuel, why not reward those who don't use any fuel!!

Posted By: arthurcoasting - but doesn't the argument depend on having fuel injection? Not all cars have this do they?

Any car that meets current emissions standards must have fuel injection - it is not possible to achieve this with a carburettor. Maybe some old cars do not have it, but they are a small minority (which sadly are responsible for most of the hydrocarbon and smoke pollution that causes smog) so their contribution to excess fuel usage is probably immeasurable.

Proper traffic design with appropriately synchronized traffic lights can go a large way towards encouraging people to drive at an appropriate speed without resort to speed bumps or cameras - but such design doesn't bring in any revenue for the local authority. Sadly, speed enforcement is becoming as much about a convenient source of tax revenue versus any proper environmental or other engineering concerns.

Paul in Montreal

Jamesingram- its nice to see someone considering the embodied energy of a new car versus the slightly higher emissions of an older car. Has anyone any experience of calculating the overall emissions of regularly buying new versus running and maintaining an older car?

Well - guess what - Spain has just announced plans to introduce a 50 mph (80 kph actually) limit!

http://www.guardian.co.uk/environment/2008/jul/30/energyefficiency.travelandtransport

Something like that might just about get the message across in this country that we don't have time to p around and this is a quick win with no cost (other than swapping 60 signs to national speed limit diagonal stripes)

i look forward to that over here.

The outcry from certain individuals will be as if Jeremy Clarkson has a free swinging Aston Martin suspended from his testicles by barbed wire.

i am as guilty as the next man of occasional acting a little heavy footed, but generally tootle about in a restrained manner.

Had to drive down the M74 and M6 to Yorkshire a week or two ago, and quietly sat at 60 - 65 all the way.
Very relaxed.
this despite the fact that the three lane, traffic free M74 cries out for daft speeds!

This all assumes that time has no value. Driving faster gets you there quicker. You can argue the toss as to how valuable the time you save actually is, but it does have some value. And yes, I know there are circumstances where you really don't save much time at all, in a real world situation

A couple of examples.

If I drive like a hooligan, I might get home on my commute all of 5 minutes earlier. So what - minimal value
Going to work, I gives me 5 minutes more in bed/asleep - rather more value - but still not much.
If I drive across France to go skiing (a once a year trip) it takes about 9 hours of actual driving from Calais at the French speed limit (82mph/130kph). If I slow to 60 it would genuinely take me a couple of hours longer (2 or 3 hours, I've not done it). That puts me in the resort really late - like 10pm, miss dinner etc, rather than 7.30pm, in time to wind down, have a meal fresh for the next day.
It costs me 560 miles at 9mpl vs 11mpl (40 vs 50 mpg). Thats 12 litres. Which is about £12. I'll pay that for 2-3 hours saved for a family of 5 thank you very much.cool:

Thats the reality of speed vs cost - the actual cash cost of the fuel saved is really quite small:

Well said Joe.e.

Going into neutral down hill to save fuel - it was something my father always did. He learnt to drive in the 1930s - I wonder if this was something that actually worked on that generation of car.

Back in the 1970s fuel crisis the Government cut the national speed limit to 50. I seem to recall that both fuel consumption and fatal accidents decreased during the period - that was from my parents talking.

It has always struck me as daft that the way the modern corporate job market works, you always seem to finish up with a long drive to work. My first job, I bought a house nearby. Then after some years I changed jobs, which changed a 10 minute drive to work, to a 10 minute drive to the station, then 1/2 to 3/4 hour on the train - didn't move house as liked where we were and didn't fancy moving to the city with the office in it. Then got fed-up with that job, and lucky we hadn't moved - I was now commuting in literally the opposite direction, a half to three quarter hour drive this time. Pity there isn't a vast amount more of teleworking that would save so much.

Then about three years ago we moved from commuting along the motorway, to westcountry lanes. Even the A roads around here wind a fair bit. You can do 60mph on most stretches (though not all corners) but the entertaining part is being caught behind a tractor and trailer (the more modern ones can do 30mph). The local practice around here is that you tend just to hang in behind the tractor, and every time the tractor reaches a lay-by it pulls in and lets the build up pass. You just have to factor in the tractor when planning your journey - especially at hay making time or muck spreading (roughly now). You can tell when the tourists have come - they are all the ones trying to roar past the tractors - and not always when the road is sensible for overtaking either. It took us a little while to relax into the local driving - but I'm much happier for it.

Reducing private cars is a good idea, but in any plan there does need to be recognition of people who have to have some form of minor haulage. We run an old diesel Volvo estate and use it to collect 25kg sacks of poultry feed and sheep nuts, move bales of hay, take sick sheep up to the vet etc. Minibuses would have to come with a trailer in our neighbourhood!

As I understand it, and I don't have any numbers, the biggest thing that would reduce fuel usage is grounding jet aircraft - including the freight aircraft bringing in cut flowers from the southern hemisphere not to mention all the fruit. All the other things we can do are worth doing, I just get a bit depressed by how much we are ignoring.

i think all cars should be limited to 80/85mph (allows a bit of leeway for overtaking etc)

i think rear spoilers on all cars should be illegal, even the little ones on rep-mobiles
it is a hideous affectation of the motor-trade
A spoiler spoils the air flow, and is designed to increase downforce.
converting power to downforce costs performance
loss of performance = increased fuel consumption

they have no effect on handling until near / well over the speed limit anyway.



Professional Football
A mechanism designed to insert unbalanced individuals into fast cars at the lowest age possible.

Posted By: TheDoctor
i think rear spoilers on all cars should be illegal, even the little ones on rep-mobiles
it is a hideous affectation of the motor-trade
A spoiler spoils the air flow, and is designed to increase downforce.
converting power to downforce costs performance
loss of performance = increased fuel consumption

they have no effect on handling until near / well over the speed limit anyway.

Surely spoilers on the back of a car only improve handling and performance if it's a rear wheel drive? Which makes the modded boy racer cars even more ridiculous.

aerodynamic aids improve efficiency to a point.

some aerodynamic aids are designed to improve performance at high speeds.
Stephen, you say that they 'only improve'
until you are doing 60 or so, you may as well just open the window and stick your hands out.

a spoiler does nothing to the handling of your average car at legal speeds other than reduce fuel economy, increase tyre wear etc etc etc.
add to this the fact (as pointed out above) that most spoilers are fitted to front wheel drive cars.......

You can't take spoilers away from chavs, it keeps a whole industry of spoiler makers in work.

With the current economic climate, everyone needs a job!

;)

Timber

p.s. I have a huge 'birdbath' spoiler on the back of one of my cars!

Posted By: TheDoctorOlly, if you can do 45 at 85, the question is, why are you not doing 50+ at 70 odd?

I do sometimes and I can average 50mpg, however driving economically sitting in the slow lane on the motorway gets quite boring after a while, I'd rather put my foot down a little if the road's clear. Oddly, from an economy point of view, 70mph is not an ideal speed for my car, it's too slow for 6th (1600rpm) and a little fast for cruising in 5th (>2000rpm).

If I really wanted to improve my average MPG I think my best option would be to commute outside rush hour and slash my 40 minute commute to a 15 minute commute by cutting out all the stop-start traffic.

Posted By: JoinerVisiting my brother in Montreal I was impressed by the road discipline of Canadian drivers,

Wow - Montreal drivers have a reputation in Canada as being aggressive - it's nice to hear an external viewpoint that refutes this. I get frustrated when I leave Montreal and the drivers are so timid and clueless about lane discipline etc. Sure, Montreal drivers tend not to indicate as much as one would like, but at least they don't stick in the overtaking lane thinking they have a God-given right to do the speed limit and not a bit more.

Posted By: JoinerDriving to Ottowa to visit relatives I was almost screaming to be let out of the car from the sheer boredom.

It is pretty dull - once you've done it a few times you get to learn where the speed traps are and can therefore cruise at closer to the design-speed of the road rather than the "local tax collector imposed limit". And even if you get caught for speeding, you can plead that you were driving within the safe limits of the conditions and road layout and get your ticket reduced to something reasonable that carries no penalty points (I did this after getting stopped for doing 144km/h and had the ticket written up as 115km/h - official limit is 100km/h but the "cop threshold" is around 120-125km/h depending which local tax collector you're dealing with. In the Greater Toronto Area the threshold is closer to 135km/h (and this was the limit the police were pushing for to be posted on the signs, rather than 100)).

Posted By: Joinerthe totally nonsensical rule that you must give way to SIDE ROADS in suburban areas had both my wife and I dreading the thought of going into town and suffering from another session of nodding-dog syndrome. Please don't try and convince me that that continual stop/start is good for fuel consumption.

This is a pet peeve of mine and I continually bring it up at our local council meetings. They're obsessed with impeding traffic flow as much as possible - despite having anti-idling bylaws in place as an environmental initiative. I keep pointing out that this is contradictory and the interminable processing of stop signs is tantamount to forcing people to idle unnecessarily. One of these days I'm going to go and chop them all down and paint mini-roundabouts in their place.

Paul in Montreal