tinyurl.com/la3pa9h
(Telegraph)
I hope not. It takes long enough to get anywhere as it is!
(My average speed, with about 70% motorway driving is 41 MPH per my trip computer. I don't want it to get any slower!)
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>> I hope not. It takes long enough to get anywhere as it is!
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It is often argued that traffic moves more efficiently at a lower speed, at more cars per minute.
Faster speeds require more space, and speeding merely takes cars faster to the next bottleneck.
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>> It is often argued that traffic moves more efficiently at a lower speed, at more
>> cars per minute.
>> Faster speeds require more space, and speeding merely takes cars faster to the next bottleneck.
Yes, perhaps in certain circumstances with multiple junctions and vehicles leaving and joining constantly, though it would be ridiculous if the long open stretches between major cities were reduced to 60, I would have thought that there is a strong argument to increase the limit for cars to speed up journey times and avoid clusters of vehicles all travelling at the same speed and all arriving at a bottleneck together.
In general if journey times can be reduced then congestion is also reduced as a result as it minimises the number of vehicles on the road at any time.
Last edited by: Hard Cheese on Sat 6 May 17 at 08:40
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Stretches of motorways with the highest emissions are inevitably those on which 40 mph is not a feasible speed never mind 60 mph.
That however will not stop somebody in an office who thinks it's a good idea, implementing it.
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Like many drivers of a certain age, I spent my early driving life going about the place as fast as the conditions and the vehicle would allow, and then some.
But, now the traffic volumes are so high, and cameras are so commonplace, I just bumble along mostly complying with the often puzzlingly low limits that are set on vast stretches of the motorway system.
At one level it is frustrating, often deeply boring, but on another, it is at least a low stress way of getting around especially in a modern car with an automatic gearbox, speed limiters, cruise control etc.
The "system" has sort of won.
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Anyone remember the last oil crisis in the '70s? Motorway speed limits were reduced to 50MPH. One regular 120 mile journey I used to make took ten minutes longer and used much less petrol.
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A regular trip I make is 180 miles of mainly motorway, with a final stretch of 20 miles of A-road cross country. If I "try" really hard, I can do it traffic permitting, in 3 hours 15 minutes. If I just relax into it, keep to all the rules etc, it takes 3 hours 30 minutes. Kind of illustrates the futility of risking your licence given the number of cameras there are now.
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I look forward to doing 60mph on a British motorway on my next Blighty run. Last year my average between Portsmouth and Winchester was 24kph or less and that was before I started approaching Oxford. As for the M5 in the Westcountry, I gave up hope on that years ago.
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I wonder what the tailgating, dangerous overtaking and general d**kead drivers do with the few minutes or often seconds they save having risked their and others lives to acquire them.
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>> Anyone remember the last oil crisis in the '70s? Motorway speed limits were reduced to
>> 50MPH. One regular 120 mile journey I used to make took ten minutes longer and
>> used much less petrol.
>>
Back in those days, you probably would save a noticeable amount of fuel. Those cars which had a fifth gear, or more normally an overdrive box, tended to be those of a larger engine displacement which would benefit most from mimsing at that sort of speed. Modern cars are designed to cruise most effectively at higher speeds than was the case then.
It should be noted that many hauliers who limited their vehicles to 50 or 52 mph in recent years, largely due to the spike in fuel prices, have since given it up as they've found that the saving was negligable when set against the longer journey times and consequent increased wage costs.
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>> Back in those days, you probably would save a noticeable amount of fuel. Those cars
>> which had a fifth gear, or more normally an overdrive box, tended to be those
>> of a larger engine displacement which would benefit most from mimsing at that sort of
>> speed. Modern cars are designed to cruise most effectively at higher speeds than was the
>> case then.
No amount of design can overcome the laws of physics; more speed requires more energy.
Keeping to around 60 my Roomster will get well over 50mpg on open roads. Pushing it to 70 and consumption is down into forties. With Berlingo + caravan the difference is much bigger. At 60+ barely hits 30mpg, keep to 50 and see 38-40.
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>> Anyone remember the last oil crisis in the '70s? Motorway speed limits were reduced to
>> 50MPH. One regular 120 mile journey I used to make took ten minutes longer and
>> used much less petrol.
>>
10 minutes a day over a working year is 40 hours, equivalent to a an extra week of work! I work long enough already!
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>> In general if journey times can be reduced then congestion is also reduced as a
>> result as it minimises the number of vehicles on the road at any time.
>>
I think that's putting the cart before the horse. You state the objective (reduced journey times) as if that can be a given, and then the precondition (reduced congestion) would follow as a consequence. It's the other way round.
Congestion = too many vehicles at one particular point. It's not the same as total number of vehicles on the road at that moment.
It's like getting people through an emergency exit. If they run, there is a jam at the exit. If they walk briskly, the overall flow is faster and people get out sooner.
Also cars travelling at 70 mph need more road space than at 60. there's obviously an optimum speed - 20 mph would be absurd, but on balance I'd go along with 60. But there's no point unless it is enforced, otherwise the speeders just jam the system for everyone else.
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Door to door it takes me 2 hrs to a son's house (115 miles). He drives faster than me / speeds & takes about 10+ minutes less - it is motorway/dual carriageway for the whole journey bar 1 miles @ either end.
Reducing the max to 60mph would probably add 10 mins #~ neither here there IMHO.
A few winters back it was extremely icy roads on the way home for roughly 60 miles - it was 30/40mph max - the on-board computer started to "fill the tank" as the further I drove the more miles were in the tank!
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This is quite interesting. In round terms, the capacity of a road lane depends on the separation time of the cars. If cars took up not space, and were separated by two seconds, the capacity of a lane would be 1800 cars per hour regardless of the speed they were travelling at. Obviously journey time would vary.
Allow for the length of vehicles and the theoretical capacity reduces a bit with speed, but not much within reasonable limits.
Lowering the speed of all vehicles, because the capacity of the road (vehicles passing a point each hour) remains more or less constant, would not materially change the numbers of vehicles arriving at a pinch point.
That's a bit ideal anyway, but then it gets even more complicated, because very fast traffic tends to develop waves of bunched traffic - a dab of brakes in a line of cars doing 80 mph can set up a cascade of braking which, if it doesn't end with a crash, will see cars at the back virtually stopping - this "shockwave jam", without visible cause, then "moves" backwards down the road.
www.newscientist.com/article/dn13402-shockwave-traffic-jam-recreated-for-first-time/
Lower speed limits tend to reduce shockwave jams.
I don't mind lower speed limits on busy roads - I used to spend hours on the M1 in the 80s and 90s alternating between 90mph blasts and clumps of congestion that seemed to srise from nothing. Much less aggro in recent years to trundle at 50 for 20 miles or more at a time through the roadworks, and probably just as quick overall.
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>> This is quite interesting. In round terms, the capacity of a road lane depends on
>> the separation time of the cars. If cars took up not space, and were separated
>> by two seconds, the capacity of a lane would be 1800 cars per hour regardless
>> of the speed they were travelling at. Obviously journey time would vary.
>>
>> Allow for the length of vehicles and the theoretical capacity reduces a bit with speed,
>> but not much within reasonable limits.
>>
That broadly makes good sense, if you then add the irrefutable fact that traffic travelling at higher speeds reduces journey times and therefore reduces the number of vehicles on the road at any one time (or conversely that traffic travelling at lower speeds increases journey times and therefore increases the number of vehicles on the road at any one time); you can see how lowering speed limits can be counter productive as more vehicles on the road generally means congestion is more likely.
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This, or something similar, has been posted before, but still makes fascinating viewing and playing. Very useful for demonstrating to people who just do not get it!
www.traffic-simulation.de/
worth changing some of the variables.
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>> This, or something similar, has been posted before, but still makes fascinating viewing and playing.
>> Very useful for demonstrating to people who just do not get it!
>>
>> www.traffic-simulation.de/
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>> worth changing some of the variables.
>>
That looks cleverer than it is I feel. Too many built in assumptions.
Yes, controlling flow makes sense around junctions etc though reducing speeds on open sections would be counterproductive.
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In general if journey times can be reduced then congestion is also reduced as a result as it minimises the number of vehicles on the road at any time.
>>
>>
>> I think that's putting the cart before the horse. You state the objective (reduced journey
>> times) as if that can be a given, and then the precondition (reduced congestion) would
>> follow as a consequence. It's the other way round.
>>
It is that way around - if you reduce journey times then correspondingly there will be, on average, fewer vehicles on the road at any one time - simple maths.
>> Congestion = too many vehicles at one particular point. It's not the same as total
>> number of vehicles on the road at that moment.
>>
Yes, that's true therefore there are situations where controlling flow is beneficial.
>> It's like getting people through an emergency exit. If they run, there is a jam
>> at the exit. If they walk briskly, the overall flow is faster and people get
>> out sooner.>>
I don't think that analogy works, it's more like 20 vehicles leaving a row of toll booths at the same time, if they all travel at the same speed they will collide as the 20 lanes funnel down to two; whereas if thye travel at varying speeds they will have much more chance of merging safely at the bottleneck.
>> otherwise the speeders just jam the system for everyone else. >>
It's not speed that jams the system any more than speed kills (speed can be safe), as per above traffic travelling at varying speeds is less likely to jam than if it is all travelling at the same speed.
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>> It's not speed that jams the system any more than speed kills (speed can be
>> safe), as per above traffic travelling at varying speeds is less likely to jam than
>> if it is all travelling at the same speed.
Understand the theory but observation says otherwise. M42 managed m/way moves far better than before scheme.
The M1 and M6 are both currently 50 limits between their respective junctions 16 and 19 pending conversion to 'smart' motorway. Both were previously subject to constant shockwave stops and now, excepting the near daily shunts/breakdowns, move smoothly.
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How would variable speed motorways be? - for example lane1 max 60mph, lane 2 max 80mph, lane 3 100 mph. Would it make much difference to how things are now seeing as plenty of folk seem to exceed 70 most of the time.
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>> How would variable speed motorways be? - for example lane1 max 60mph, lane 2 max
>> 80mph, lane 3 100 mph. Would it make much difference to how things are now
>> seeing as plenty of folk seem to exceed 70 most of the time.
>>
Most drivers have trouble handling the single speed system of all lanes at the same limit. Different speeds for each lane would be chaotic. Think of the 100+ driver tailgating in lane three when a sudden slowdown occurs and he avoids an impact by swerving into slower lanes.
Last edited by: Old Navy on Sat 6 May 17 at 15:50
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Missed the edit -
Lanes two and three are overtaking lanes.
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>> Lanes two and three are overtaking lanes.>>
Absolutely correct.
One point about a reduced speed limit that's been overlooked is that a road or motorway can carry more vehicles per hour.
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>> One point about a reduced speed limit that's been overlooked is that a road or
>> motorway can carry more vehicles per hour.
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Really?
See Manatee's post above.
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>> I don't think that analogy works, it's more like 20 vehicles leaving a row of
>> toll booths at the same time, if they all travel at the same speed they
>> will collide as the 20 lanes funnel down to two; whereas if thye travel at
>> varying speeds they will have much more chance of merging safely at the bottleneck.
>>
That's true in the rather special case you select, with just 20 cars timed to arrive at a particular point staggered so that they don't congest. But in reality there would be thousands of cars starting from innumerable different points, and as we know it takes very little to suddenly cause the ripple bunching to turn a flow into a jam. I would suggest that three causes of that are
a) cars travelling faster need more road space
b) faster cars often in groups catching up slower cars and braking
c) slowing faster cars trying to move to other lanes to secure perceived advantage.
If they all moved constantly at 60 wouldn't this effect be lessened, and overall flow faster rather than slower?
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Motorway Speed Limits to Reduce to 60MPH? - Cliff Pope
