When driving out to Hope from Hillsborough at the weekend I realised that only a few years ago, almost the entire route was subject to the national speed limit (60 mph), whereas now, most of it is speed limited lower than that. For example, the entire stretch of road between Ladybower and Hope is now subject to either 30, 40 or 50 mph. This seems to have happened gradually over the last two or three years - I don't know who decides which bits of road get speed limits, or whether there has been any public consultation or anything like that, it just seems to happen.

That made me think. My journey will be slower, so I'll take longer which means I'll use more fuel. And we all know that our car's fuel efficiency is greatest when cruising along the motorway at 70 mph rather than trundling along at 30 mph. While I'm sure Gordon Brown won't mind that (in fact you can almost say it's another stealth tax), I'd certainly have thought that the Green party would have been against the amount of extra pollution it entails.

We are often told that traffic congestion costs the country so many million or billion pounds. Has anyone estimated how much money the (ever increasing) presence of speed limits on our roads costs the country in terms of increased journey time, increased fuel usage, and increased pollution? A quick trawl of the web has so far failed to find such a figure - can anyone come up with any info?

Before everybody goes all hysterical and accuses me of wanting to abolish all speed limits and kill all pedestrians etc., that's not what I'm saying at all - I just want to know how much they cost the country as a whole.

Originally posted by metalman

That made me think. My journey will be slower, so I'll take longer which means I'll use more fuel. And we all know that our car's fuel efficiency is greatest when cruising along the motorway at 70 mph rather than trundling along at 30 mph.

For most vehicles, the figure is around 55mph IIRC. Still, it is an interesting question. On a (very) good day I average 16mph on my way to and from work. That can't be good!

Originally posted by metalman
That made me think. My journey will be slower, so I'll take longer which means I'll use more fuel. And we all know that our car's fuel efficiency is greatest when cruising along the motorway at 70 mph rather than trundling along at 30 mph.
Not sure about that logic, cruising at a set speed is more efficient yes...

...but is a vehicle more efficient at 70 than 30, or 50?

Figures would tend to suggest 50 is more efficient than 70, because wind resistance is exponential (squares with speed), so 30 may be the most economical speed.

MPG has no time element to consider, so MPH is irrelevant.

Check the figures and come back to us.

:)

Originally posted by metalman
I don't know who decides which bits of road get speed limits, or whether there has been any public consultation or anything like that, it just seems to happen.

I would imagine it's based on complaints from residents about speed in their particular village or the number of recorded accidents or a combination of the two.

With the former, a temporary "speed strip" is usually set up on the road to gauge an average and if it is above the average then a speed camera or restrictions are in place.

Even if I'm wrong about the exact speed at which a car's engine performs in the most fuel-efficient manner, the fact still remains that if your journey takes longer, you are pretty much bound to use more fuel and create more pollution. So I want to know to know how much that costs the country on an annual basis. Is it, for example, comparable to the amount quoted for congestion?

Originally posted by foo_fighter
Not sure about that logic, cruising at a set speed is more efficient yes...

...but is a vehicle more efficient at 70 than 30, or 50?

Figures would tend to suggest 50 is more efficient than 70, because wind resistance is exponential (squares with speed), so 30 may be the most economical speed.

MPG has no time element to consider, so MPH is irrelevant.

Check the figures and come back to us.

:)

it depends on the car, but generally around 55 is the most efficient.

It's not just down to wind resistance (most modern cars have very low drag cooefficients), it's also down to gearing. At 55 most engines will be at around 1500 to 2000 revs. At 30 the engine can't be in top gear, so the revs will be the same, using the same petrol then, but the car will be travelling slower.
At 70 the revs are higher, so using more fuel than 55, but probably still more efficient than 30.

Congestion is related to the average speed of travel, if we could move infinitely fast we would take 0 time to travel and thus our roads would all be empty.
As the average speed comes down (given a constant number of journeyers) the length of time on the road goes up and thus the number of cars on the road goes up.
As the number of cars passes a certain point, smooth traffic flow is interupted and people have to stop and start, this greatly increases the use of fuel.

Originally posted by metalman
...the fact still remains that if your journey takes longer, you are pretty much bound to use more fuel and create more pollution...
No, that's the point I'm making, that is not the case.

MPG is "distance" per "unit of fuel", there is no direct correlation to time taken, different vehicles are most efficient at different speeds, there is an optimum for each vehicle.

Faster is not necessarily more efficient, it might be up to a certain point, but then the opposite starts to be true.

The squarer the vehicle, the lower that point may be, and let's face it, big square vehicles are "generally" the most inefficient anyway.

Well OK, let's look at it another way. Suppose you have a 10-mile stretch of road with a speed limit of 60 mph. If a car goes along it at that speed limit it will take 10 minutes. If you then reduce the speed limit to 40 mph and a car goes along it at that speed limit, it will take 15 minutes, i.e. you have increased journey time by 50%. And if you've increased the amount of time your engine is running by 50%, I reckon you're bound to have increased the amount of fuel you use and thus the pollution you produce. Suppose that stretch of road is used by 500 cars a day, and it wouldn't have to be a very busy road for that to be the case (about 20 an hour say), and you've already got an extra 500 x 5 minutes = 2500 or over 40 hours of journey time. Multiply that by a year and I think you can see where I'm going - it's quite a lot.

So presumably somebody in the Department of Transport knows how many miles of road were changed from 60 to 40 mph limit last year, and roughly how many cars use those stretches of road, and could thus give us an estimate of how much increased journey time and fuel consumption that has produced. That's the sort of figure I had in mind.

Originally posted by metalman
Well OK, let's look at it another way. Suppose you have a 10-mile stretch of road with a speed limit of 60 mph. If a car goes along it at that speed limit it will take 10 minutes. If you then reduce the speed limit to 40 mph and a car goes along it at that speed limit, it will take 15 minutes, i.e. you have increased journey time by 50%. And if you've increased the amount of time your engine is running by 50%, I reckon you're bound to have increased the amount of fuel you use and thus the pollution you produce. Suppose that stretch of road is used by 500 cars a day, and it wouldn't have to be a very busy road for that to be the case (about 20 an hour say), and you've already got an extra 500 x 5 minutes = 2500 or over 40 hours of journey time. Multiply that by a year and I think you can see where I'm going - it's quite a lot.
"IF" a truck does 28 MPG @ 60 MPH, and 34 MPG @ 40 MPH, in 10 miles, 60 MPH will use 0.357 Gal' but 40 MPH will use 0.294 Gal'.

The time the engine runs is not relevant.

Your 500 vehicles will therefore use 178 Gal' (@ 60) of fuel , instead of 147 Gal' (@ 40).

Therefore, quicker a to b or not, they used 31 Gallons MORE fuel by going faster.

Originally posted by metalman
I want to know how much that costs the country on an annual basis. Is it, for example, comparable to the amount quoted for congestion?

It costs less than the revenue squandered in the NHS patching up RTA victims :mad:


And I agree with Foo :thumbsup: (see above) on the basis of basic physics, and ignoring my engineering degree ;)

Originally posted by foo_fighter
"IF" a truck does 28 MPG @ 60 MPH, and 34 MPG @ 40 MPH, in 10 miles, 60 MPH will use 0.357 Gal' but 40 MPH will use 0.294 Gal'.

The time the engine runs is not relevant.

Your 500 vehicles will therefore use 178 Gal' (@ 60) of fuel , instead of 147 Gal' (@ 40).

Therefore, quicker a to b or not, they used 31 Gallons MORE fuel by going faster.

Yes OK I can agree with that but I still think your average car engine is more likely to get more miles per gallon at the higher speed than the lower speed, rather than the other way round, in which case you would use more fuel. I suppose you're right in that the time your engine was running would only matter if you were actually stationary for some of that time, like in a traffic jam. In any case even you can't argue with the fact that your journey would take longer at the lower speed, and that's one of the things that's always factored in when working out the economic costs of congestion and so on.

Strix: I deliberately didn't want to introduce any sort of safety discussion into this thread, I did say that.

Originally posted by metalman
Strix: I deliberately didn't want to introduce any sort of safety discussion into this thread, I did say that.

As the two are not mutually exclusive, it is relevant.

Still - to contribute according to your rules - if you read your manual that the manufacturer supplies with the car, that will tell you the most efficient speed to travel at :thumbsup:

Our car spends most of it's time in traffic or hurtling down motorways 'at speed' ;)
As the petrol gauge is gammy, we use the trip counter to judge when we need to fill up. This is usually 300-320 miles.
When Brude was crippled with a broken back, we had to drive at much slower and more even speeds and were getting around 400 miles out of a tank :thumbsup:

It's accelerating and decelerating that's the biggest culprit, so travelling more slowly on the straights enables you to maintain speed for the bends, hence greater efficiency :thumbsup:

Originally posted by metalman
Yes OK I can agree with that but I still think your average car engine is more likely to get more miles per gallon at the higher speed than the lower speed, rather than the other way round, in which case you would use more fuel.
That's the sticky point isn't it, anybody got any definate figures on this.

As Cyclone said, 56 MPH is (or was) a target for good MPG, above that things went down hill fast.

C'mon, give us some figures (all I can find are Urban/Extra Urban type figures, which are all about stop start motoring).

:)

Originally posted by Strix
As the two are not mutually exclusive, it is relevant.

Still - to contribute according to your rules...

I'm not making any rules - it's just that the subject of speed in relation to accidents and injuries has been covered many times before, and I didn't want to turn this into another re-run of all that. I'm sure that by abolishing all speed limits we could turn the roads into a bloodbath, in the same way that by making every road a 10 mph limit we could probably cut road deaths by a very large percentage, but nobody's suggesting either of those. All I wanted to find out was whether there was an associated economic cost in reducing the speed limit on a stretch of road, and if so, what sort of figure that amounted to per annum over the whole country. So far we seem to have established that it's going to be very difficult to tell whether there's any associated cost in terms of fuel used because of all the complex factors involved, but what about the increased journey times?

Originally posted by metalman
When driving out to Hope from Hillsborough at the weekend I realised that only a few years ago, almost the entire route was subject to the national speed limit (60 mph), whereas now, most of it is speed limited lower than that. For example, the entire stretch of road between Ladybower and Hope is now subject to either 30, 40 or 50 mph. This seems to have happened gradually over the last two or three years - I don't know who decides which bits of road get speed limits, or whether there has been any public consultation or anything like that, it just seems to happen.

That made me think. My journey will be slower, so I'll take longer which means I'll use more fuel. And we all know that our car's fuel efficiency is greatest when cruising along the motorway at 70 mph rather than trundling along at 30 mph. While I'm sure Gordon Brown won't mind that (in fact you can almost say it's another stealth tax), I'd certainly have thought that the Green party would have been against the amount of extra pollution it entails.

We are often told that traffic congestion costs the country so many million or billion pounds. Has anyone estimated how much money the (ever increasing) presence of speed limits on our roads costs the country in terms of increased journey time, increased fuel usage, and increased pollution? A quick trawl of the web has so far failed to find such a figure - can anyone come up with any info?

Before everybody goes all hysterical and accuses me of wanting to abolish all speed limits and kill all pedestrians etc., that's not what I'm saying at all - I just want to know how much they cost the country as a whole.

A very interesting post. Of course, it can be added to as well. Councils (such as Sheffield...) introduce traffic "calming" (they don't make me very calm) measures such as speed bumps, chicanes, and my personal worst - bus stops being forced into the middle of the road so the bus has to hold up traffic as it is unable to pull in. All these measures seem to increase congestion, journey times, likelihood of a bump/damaging suspension, etc etc, so add to the environmental effect as well as the personal costs of more time wasted in a car and more money wasted on repairs.

Originally posted by metalman
but what about the increased journey times?
As a contractor on an hourly rate... It's cherfin expensive :rant: but that's nothing to do with fuel prices, that's to do with lost time ;)

Originally posted by Strix
As a contractor on an hourly rate... It's cherfin expensive :rant: but that's nothing to do with fuel prices, that's to do with lost time ;)

You more than make up for it, I'm sure, by spending half of your contracted hours on the forum. :hihi:

Originally posted by t020
You more than make up for it, I'm sure, by spending half of your contracted hours on the forum. :hihi:

Ahem *cough* you noticed too?

I'm 'resting' :hihi:

Perhaps i should go and cut the grass?

Originally posted by Strix
As a contractor on an hourly rate... It's cherfin expensive :rant: but that's nothing to do with fuel prices, that's to do with lost time ;)

That was my point - there is an associated time penalty for introducing yet another speed limit. If goods or services take longer to reach their destinations then distributors either need to hire more lorries to get round the same number of destinations in the same time, or else they just deliver things more slowly. In either case the end result is that the prices of those goods and services go up.

I can't speak for all cars but I have some rough estimates for my car.

Given a journey where I do 90 miles on the motorway and only about 10 not we can pretend that it's done at a constant speed, it's close enough.

When I drive to Brum outside of rush hour, say on a Sunday, I maintain an even 90. It takes me about 1hr 15, and my mpg (according to the onboard computer) is around 27/28 mpg.
When I do it in moderate traffic and my speed is forced to hold steady at about 60, it takes nearly 2 hrs and the mpg is closer to 34/35
So, we can see a 25% increase in fuel use for a 50% decrease in time.

Originally posted by metalman
If goods or services take longer to reach their destinations then distributors either need to hire more lorries to get round the same number of destinations in the same time, or else they just deliver things more slowly. In either case the end result is that the prices of those goods and services go up.

Or get goods back on the trains?

Originally posted by alchresearch
Or get goods back on the trains?

A sensible suggestion if ever I heard one.

Originally posted by alchresearch
Or get goods back on the trains?
Can we have a poll?

surely everyone will agree.
Lorries are the largest source of congestion on our roads.

Originally posted by Cyclone
surely everyone will agree.
Lorries are the largest source of congestion on our roads.
On "open" roads, yes (with doddering pensioners a close second), in cities tho'...

...surely that honour falls to the humble bus (again, with doddering pensioners a close second), no?

:D

Originally posted by foo_fighter
On "open" roads, yes (with doddering pensioners a close second), in cities tho'...

...surely that honour falls to the humble bus (again, with doddering pensioners a close second), no?

:D

i rarely have a problem driving through the city, if i'm held up it's always on the motorway.

Might be interesting to note that one of the highway federations is proposing to reduce A and B road speed limits to 40mph.

The source was Autocar magazine :gag:

Bring on that man with the red flag all over again...

Originally posted by t020
A very interesting post. Of course, it can be added to as well. Councils (such as Sheffield...) introduce traffic "calming" (they don't make me very calm) measures such as speed bumps, chicanes, and my personal worst - bus stops being forced into the middle of the road so the bus has to hold up traffic as it is unable to pull in. All these measures seem to increase congestion, journey times, likelihood of a bump/damaging suspension, etc etc, so add to the environmental effect as well as the personal costs of more time wasted in a car and more money wasted on repairs.
Interesting. But what about the city that has taken Traffic Calming to its most extreme so far - London. Congestion charging in many instances is reducing costs to businesses in that they soen less time travelling.

There is also an issue about the marginal cost of road space - the faster vehicles travel the more road space they occupy - so the cost to society of each stretch of raod could be said to be more with higher speeds, as there is less road available for everyone to use.

Also the cost to London society in general is reduced in terms of pollution and there is a saving made in terms of social accessibility - a greater number of buses and more reliable journeys means a greater number of people (those without cars) are able to travel to the places they wish to go.

Interesting question. We have to pay the police to enforce speed limits as well. Does the DfT have any answer?

Originally posted by metalman
That was my point - there is an associated time penalty for introducing yet another speed limit. If goods or services take longer to reach their destinations then distributors either need to hire more lorries to get round the same number of destinations in the same time, or else they just deliver things more slowly. In either case the end result is that the prices of those goods and services go up.

Then the answer is to produce, distribute and consume goods more locally, not build more roads to transport stuff up and down the country.

Originally posted by theflyingfish
Interesting. But what about the city that has taken Traffic Calming to its most extreme so far - London. Congestion charging in many instances is reducing costs to businesses in that they soen less time travelling.

There is also an issue about the marginal cost of road space - the faster vehicles travel the more road space they occupy - so the cost to society of each stretch of raod could be said to be more with higher speeds, as there is less road available for everyone to use.

Also the cost to London society in general is reduced in terms of pollution and there is a saving made in terms of social accessibility - a greater number of buses and more reliable journeys means a greater number of people (those without cars) are able to travel to the places they wish to go.

Interesting question. We have to pay the police to enforce speed limits as well. Does the DfT have any answer?

I really don't think congestion charging should be necessary in Sheffield. Most of the congestion I come across seems to be self imposed congestion through supposed "traffic management" schemes. My solution would be to scrap speed bumps, chicanes and the mid-way across the road bus stops, reduce the amount of crossings in particular areas (e.g. Eyre Street), roll-back the 'growing pavement' syndrome that reduced perfectly good dual carriageways to bottle-necked single lane roads (e.g. Charter Row, Eyre Street) and commission a review into the need for bus lanes, with possibly a week of no bus lanes to monitor the effects.