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Old January 29th, 2009, 12:24 AM   #61
andysimo123
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Originally Posted by Scazmattaz View Post
If i've read your comments correctly - these aren't narrow minded, they are infact fact-driven comments and opinions.

I think most motorway speed limits should be 50mph - this will reduce CO2 emissions, reduce noise pollution, reduce accidents, increase capacity (highest capacity is between 40 and 50mph i think) and reduce the number of AUDI/VW drivers who take the law into their own hands and zoom along at 100mph. Its irresponsible and careless and people have to be controlled if they encourage others to be lawless.
Simple logic tells me reducing the speed limit would decrease capacity. If a car if travelling at 50mph instead of 70mph its on the road longer therefore there are more cars on the road more of the time.

Reducing speed limits to 50mph on motorways would be total madness and totally unacceptable. It could nearly double journey times. Example up the limit to 80mph over 100 miles is 1 hour 15 minutes. 50mph over 100 miles is 2 hours. That 45 minutes extra.
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Old January 29th, 2009, 12:38 AM   #62
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Mass of emissitted CO2 is not proportional to time, but to fuel used. Because air resistance goes up with velocity squared, reducing speed is one of the best ways to reduce fuel consumption per mile travelled. That's why the limit in the US was reduced to 55 in the first 70s oil price shock.

As for speed limits - up to 80 on the motorway, but down to 20 on residential streets please.
Its going to depend on alot of things. Time is one, if I go out and drive for one hour at 70mph and then do the same thing for 2 hours am going to burn more fuel. I'd argue that increasing speeds from 70 mph to 80 mph with todays cars compared with cars from only afew years ago would make a massive difference.

The speed limit in the USA depends on the state. It ranges from 55mph to 80mph.

Today we need the speed limit upping to 80 mph but I'd say in 5 years time upto 85-90 mph. Engines and technology with have improved and made its way through the system. In 20 years if these new hydrogen cars come through. All that CO2 saving goes straight out of the window.
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Old January 29th, 2009, 02:08 AM   #63
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I always drive at around 60 on motorways as I'm always to poor to buy fuel! However on dual-carriage ways I find I'm always overtaking trucks while at the same time I hate the mere suggestion that I'm holding someone else behind me up so end up spending all my time at 80! I'm the same about holding people up on single-carriage ways and end up driving 70+. I like having the choice to drive 60 on the motorway and think others should have to choice to drive at whatever speed they like as long as it's suitable when accounting for road and weather conditions.
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Old January 29th, 2009, 09:33 AM   #64
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Originally Posted by andysimo123 View Post
Today we need the speed limit upping to 80 mph but I'd say in 5 years time upto 85-90 mph. Engines and technology with have improved and made its way through the system. In 20 years if these new hydrogen cars come through. All that CO2 saving goes straight out of the window.
I think 90mph should be the absolute maximum, anything over 100mph (given the limit+10mph allowance) without tracks are dangerous in my eyes.
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Old January 29th, 2009, 10:56 AM   #65
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Originally Posted by andysimo123 View Post
Its going to depend on alot of things.
Not really. It only depends on fuel burned per mile travelled.

Time taken is important when considering congestion, but energy efficiency is not dependent on time taken.
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Old January 29th, 2009, 12:15 PM   #66
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Quote:
Originally Posted by andysimo123 View Post
Simple logic tells me reducing the speed limit would decrease capacity. If a car if travelling at 50mph instead of 70mph its on the road longer therefore there are more cars on the road more of the time.

Reducing speed limits to 50mph on motorways would be total madness and totally unacceptable. It could nearly double journey times. Example up the limit to 80mph over 100 miles is 1 hour 15 minutes. 50mph over 100 miles is 2 hours. That 45 minutes extra.
But surely cars going faster need more road space in terms of safe stopping distance.

I don't know how it would even out though.
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Old January 29th, 2009, 02:16 PM   #67
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A conundrum!

Well, according to this site your stopping distance in feet is roughtly one and a half times your speed in miles per hour.

http://www.hintsandthings.co.uk/garage/stopmph.htm

So at 40 miles per hour you need 60 feet. At 80 miles per hour you need 120 feet.

So its a linear relationship. That means that speed doesnt affect road capacity. Double the speed and you double the traffic flow but halve the road capacity.

i.e. one mile is 5,280 feet. At 40 mph at a stopping distance of 60 feet you can fit 88 cars onto a mile of road. 40 mph means one car takes 1.5 minutes to travel the distance. Therefore at 88 cars every 1.5 minutes the road handles 3,520 cars per hour.

At 80mp at a stopping distance of 120 feet you can fit 44 cars onto a mile of road. 80 mph means one car takes about 45 seconds to travel the diatance. Thereore at 44 cars every 45 seconds the road handles 3,520 cars per hour.
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Old January 29th, 2009, 03:16 PM   #68
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don't forget that the car in front also has a stopping distance, it's a thinking distance that's needed as a gap. I believe the common rule is 2 seconds, which happens to be lampposts on roads that still have the same speed limit as their lighting. There's those chevrons that are 1 second apart, and you keep two between you.

Cars per mile is a silly way of measuring flow - it should be cars passing past a point. In which case, you get 1800 an hour per lane if everyone obeys the 2 second rule and there are no other factors.
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Old January 29th, 2009, 03:21 PM   #69
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Originally Posted by sotonsi View Post
Cars per mile is a silly way of measuring flow - it should be cars passing past a point
Its the same thing isnt it? At least you get the same result as above if you calculated it that way.

Thinking time is also linear. If the time to react is static at two seconds then the distance covered while reacting would double if the speed was doubled. It wouldnt effect the calculatiion above other than reducing the predicted road capacity by the same amount in both scenarios.

Which I think is the point you were making in the last sentence!
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Old January 29th, 2009, 03:28 PM   #70
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It isn't a linear relationship.
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Old January 29th, 2009, 03:42 PM   #71
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One variable increases by 1, the other by 1.5. Thats a straight line which is neither horizontal or vertical. To me that is a linear relationship. Unless you are saying a linear relationship only exists if the graph is at an angle of 45 degrees?
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Old January 29th, 2009, 04:41 PM   #72
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Its the same thing isnt it? At least you get the same result as above if you calculated it that way.
But you are going a long way around. Cars/mile * miles/hour = cars/hour. However to work out cars/mile, given that distance between is a time distance, involves going cars/hour * hour/mile (ie dividing by mph). You end up starting with the right unit, converting and then converting back. It's just dumb.
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It isn't a linear relationship.
Stopping distance indeed isn't, however thinking distance is (as it's just speed times a set time). Of course, the more attentive you are, the shorter the time in reality, therefore too-low speeds would increase the thinking distance.

s=u+1/2*a*t^2 - note the squared factor wrt time. (s is stopping displacement, u is initial velocity)
t=(u-v)/a; v=0 the final velocity; a is a constant.

s=u+1/2*a*(u/a)^2
s=u+1/2*a*(u/a)*(u/a)
s=u+1/2*(u)*(u/a)
s=u+1/(2a)*u^2

so stopping distance takes the form of constant*speed squared + speed.

However, as I've explained, you do NOT need to keep a stopping distance away, just a thinking distance (plus a smidge), given that if the car in front needs to break suddenly, it breaks in the stopping distance. The thinking distance is the distance travelled in a fixed time, therefore keeping a fixed time away, 2 seconds, is the official line in the Highway Code.
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Old January 29th, 2009, 04:44 PM   #73
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Originally Posted by sotonsi View Post
But you are going a long way around. Cars/mile * miles/hour = cars/hour. However to work out cars/mile, given that distance between is a time distance, involves going cars/hour * hour/mile (ie dividing by mph). You end up starting with the right unit, converting and then converting back. It's just dumb.
Yes, I know its a bit circular. It made the point though so
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Old January 29th, 2009, 04:54 PM   #74
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Yeh I was meaning stopping distance to speed isnt a linear relationship.

The 2 second rule is essentially a human form of moving block signalling. Some day soon we will actually have moving block signalling on our roads too which will further increase capacity. In fact i think high end cars already have auto pilots pretty much with front sensors.

In reality on the roads though, really high speeds reduce effective capacity, just as low ones do to, but it would be way too complicated to work out the optimum speed.
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Old January 29th, 2009, 09:24 PM   #75
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I studied highway engineering a long time back at university but one thing I do remember is that our motorway network has a design speed of 80mph.

Almost all of our motorways have curved alignments with short stretches of straight. That is for a number of reasons. Curvature allows the road to follow river valleys, skirt round obstructions and fit into the landscape better but it also removes the hazard of long straight motorways that bedevils the early German Autobahns. The problem with dead straight roads is that the monotony sends drivers to sleep and many crashes have resulted.

However, an effect of curvature is that sighting distances and lateral forces come into play and so the radius of curves, lengths of transition curves and amount of superelevation are all determined for a design speed of 80mph.

As the general speed observed by vehicles on uncongested motorways is around 10mph above the speed limit, it is probably just right to set it to 70mph.

Increasing the speed limit would enable people to travel more quickly on uncongested motorways but there are few other benefits.

Certainly the faster you go, the less safe you are for obvious reasons - not only do you need a greater distance to react to any problem but the amount of energy stored in your speeding car is proportional to the square of the speed so, in the event of a crash, at 80 mph you would only be travelling 14% faster than at 70 mph but the energy to be dissipated in the crash would be 31% greater.

I have read Sotonsi's oft-repeated argument that someone speeding is more alert than someone obeying the speed limit but I don't find that too convincing as, if true, boy racers would be the safest drivers on the road (although I bet they think they are).

Certainly braking systems have improved greatly over the last few decades but then road congestion has increased exponentially and so people are less inclined to keep the required distance to the car in front.

Increasing speed limits certainly does not increase fuel efficiency or reduce pollution. That is why car manufacturer's often quote fuel consumption at a speed of 56 mph, which is around the optimum. Certainly increasing speed reduces the time over which fuel is consumed and pollution emitted but, as one forummer noted, the air resistance increases with the square of the speed and so the amount of energy required increases proportionately. Also, in real life traffic situations, a speeding car will always be braking and accelerating to get around slower vehicles and that increases energy consumption.

Another myth is that increasing speed increases capacity. Up to a certain speed that is true but that will vary with the amount of congestion and it is why the variable speed limits were introduced on congested parts of the M25.

As a car travelling at higher speed needs a greater spacing from the one in front, the amount of road space taken up increases but also, as speed increases, the more weaving across traffic lanes occurs, which tends to reduce the overall speed of vehicles.

I think the real argument for raising speed limits is that people just want to go faster and all the rather dubious technical arguments arise from that.

I know it is not as sexy but wouldn't it be better to campaign for minimum speed limits on motorways and trunk roads? It is too easy to confuse the top speed that you do with your average speed and minimum speed limits do help to increase capacity by eliminating weaving between lanes - something which always increases maximum speeds. Say if our motorway minimum speed limit was 50mph, pollution and energy consumption would be reduced (as vehicles would be travelling closer to optimum speed), roads would be safer as there would be reduced weaving and average speed would be increased for everybody for the same reason.
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Old January 30th, 2009, 02:21 AM   #76
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A conundrum!

Well, according to this site your stopping distance in feet is roughtly one and a half times your speed in miles per hour.

http://www.hintsandthings.co.uk/garage/stopmph.htm

So at 40 miles per hour you need 60 feet. At 80 miles per hour you need 120 feet.

So its a linear relationship. That means that speed doesnt affect road capacity. Double the speed and you double the traffic flow but halve the road capacity.

i.e. one mile is 5,280 feet. At 40 mph at a stopping distance of 60 feet you can fit 88 cars onto a mile of road. 40 mph means one car takes 1.5 minutes to travel the distance. Therefore at 88 cars every 1.5 minutes the road handles 3,520 cars per hour.

At 80mp at a stopping distance of 120 feet you can fit 44 cars onto a mile of road. 80 mph means one car takes about 45 seconds to travel the diatance. Thereore at 44 cars every 45 seconds the road handles 3,520 cars per hour.
It is a conundrum because there are alot more factors than this is how fast your are traveling this is when you will stop.

Am going to put a big flaw in 'this is the capacity at 80mph or 40mph'. Every car on a piece of motorway doesn't travel at the same speed, they also don't stop at the end of each mile from 80mph to 0mph. The UK government say you need how ever many feet to do this but that depends on the car, the breaks, the speed and weight of the vehicle. If your driving a van and the guy behind is driving a brand new BMW, hes going to have no problem stopping. The van on other hand is going to have a totally different breaking distance. My view is on a motorway you shouldn't really push your breaks unless you really have to. Basically that means the breaking distance doesn't mean you have to be 120 foot behind the other vehicle. Technically a better equipped vehicle behind a van could sit 40-50 foot behind and have no problem stopping. Some cars as I said can go from 125mph to 0mph in 70 odd foot. So 80mph in 120 foot will be a doddle.

What am saying is in the real world people driver closer to one another and then aim on slowing down to say 40mph or 50mph rather than stopping dead. If you've ever driven or been on a motorway, there are certain habits and judgements each driver can make.
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Old January 30th, 2009, 08:30 PM   #77
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I think this stopping distance issue is a case not just of the ability of vehicles to stop but the psychology of the driver.

When you read the stopping distances recommended in the highway code, what do they mean? If you were travelling at the recommended stopping distance behind a car in front that instantly stopped dead and you applied the brakes immediately, would you end up with your front bumper just touching the car in front as you stopped? I doubt it. There is a lot more to be considered.

First, there is thinking distance. I'm sure we don't all drive with our eyes glued on the car in front - we may be looking for a road sign, changing a CD admiring the countryside or the driver of the car we've just overtaken. Then, when we do see that the car ahead is stopping, we do need a little time to register that fact and take appropriate action.

Braking distance isn't a constant either - it depends on factors such as load in the car, wear on the tyres, wear on brake pads etc. We are meant to double the braking distance when the road is wet - how many of us do?

There are other factors. We may all know the recommended braking distance for the speed we are travelling at but do we always observe that distance? I always think that I'm pretty good at keeping my distance to the car in front but when I get to one of those 'keep apart two chevrons' stretches I often have to pull back some distance. Again, many (probably most) drivers in the middle and fast lanes on motorways will be driving 5 or 10mph above the speed limit, sometimes much more and so braking distances will be increased considerably (does anyone know the safe braking distance at 85mph?).

There is understood to be a trade off in safety so that, for example, people with a high performance car with good braking will drive closer to the car in front - so negating the safety benefit.

All these factors, and many more besides, have to be taken into account when advising on minimum stopping distances.
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Old January 31st, 2009, 12:39 PM   #78
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I studied highway engineering a long time back at university but one thing I do remember is that our motorway network has a design speed of 80mph.
Close - when the design standards were last substantially re-written, in the 1970s, the motorway design speed was actually 120 km/h. New rural single carriageway roads are designed to a design speed of 100 km/h, urban roads 50 km/h, and home zones to 30 km/h.

Of course the speed limits didn't go metric then as was expected but the standards haven't changed so motorways built since the 70s are actually designed to 120 km/h, or about 75mph.

In Ireland - which essentially uses the UK design standards for motorways - the limit was traditionally 70mph as in the UK, but in 2005, when they changed from mph to km/h, they raised the motorway limit to 120 km/h.

If the UK's limits went metric, we could change to 120km/h as a decent halfway house between 70 and 80mph (and then foreign vehicles would then actually be able to read and observe our speed limits, which would be a massive bonus for road safety by itself).
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Old January 31st, 2009, 01:56 PM   #79
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How, after 4 pages of posts, has no one mentioned that the petition is wrong anyway? The national speed limit is 60mph, not 70mph. The 70mph speed limit is for certain vehicles on dual carriageways and most vehicles on motorways.

Anyway, I think a 80mph speed limit would be fine for many motorways. I find that driving at 70mph in a modern car, on a straight stretch of good quality motorway is a bit slow.
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Old January 31st, 2009, 02:14 PM   #80
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How, after 4 pages of posts, has no one mentioned that the petition is wrong anyway? The national speed limit is 60mph, not 70mph. The 70mph speed limit is for certain vehicles on dual carriageways and most vehicles on motorways.
because you are wrong?

NSL *is 70mph*, but (as you say) for certain vehicles on certain roads. Other roads and other vehicles have different - single carriageways and minibuses have 60mph NSL.

NSL is commonly used to refer to the top limit, to the 70mph. Of course, that's wrong, as NSL can be as low as 40mph (lorries on single carriageway)
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Anyway, I think a 80mph speed limit would be fine for many motorways. I find that driving at 70mph in a modern car, on a straight stretch of good quality motorway is a bit slow.
Yes, I agree. A bit slow is also a bit dangerous, due to the inattentiveness. Drive to conditions, not slowly or zooming around too fast for the conditions (boy racers). Speed limits shouldn't be a target - if it's unsafe to reach them, don't.
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