• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

Speed limits and traffic capacity

daenku32

Master Poster
D
Joined
Dec 27, 2002
Messages
2,189
Speed limits do not increase traffic capacity of a road.

Why? Because a 1 second interval between cars remains fixed, but the vehicle distance grows.

Discuss (unless you all agree with me, then answer with silence).
 
daenku32 said:
Speed limits do not increase traffic capacity of a road.

Why? Because a 1 second interval between cars remains fixed, but the vehicle distance grows.

Discuss (unless you all agree with me, then answer with silence).
Click to expand...

Just to take your statement, braking distance decreases with reduction in speed so the distance between vehicles decreases. Obvious really or we'd have massive if not infinite gaps when they stop.

But more generally, we have variable speed limits on some UK motorways to increase capacity/throughput. I've never seen any empirical data that shows this works. Anybody know of any published data on this?
 
I thought speed limits were there as a safety measure, not for increasing traffic flow.

ETA: But are you saying that if you took a highway with a speed limit of 110 km/h (68 mph) and reduced the limit to 40km/h (25 mph), that traffic capacity would be unaffected?
 
Last edited:
I disagree, as follows:

daenku32 said:
Speed limits do not increase traffic capacity of a road.
Click to expand...
This is a straw man. You're refuting here an argument nobody is making here. At least cite the argument you're attempting to refute, rather than launching non sequitur directly into your refutation.

Why? Because a 1 second interval between cars remains fixed, but the vehicle distance grows.
Click to expand...
Show your working. Why is the 1 second interval the relevant metric? How have you arrived at your conclusion.

Discuss (unless you all agree with me, then answer with silence).
Click to expand...
Your argument in its current form is too simplistic, and too lacking in citations and details, to be accepted as-is. Thus the default assumption is that you're wrong (and therefore silence signals dismissal, not acceptance).
 
It's true only if drivers adjust their following distance proportionally to speed. If a doubling of speed is associated with more than a doubling of following distance, then capacity decreases. If it's associated with less than a doubling of following distance, then capacity increases. I suspect the latter is the case in most places.
 
daenku32 said:
Speed limits do not increase traffic capacity of a road.

Why? Because a 1 second interval between cars remains fixed, but the vehicle distance grows.
Click to expand...
That would be true if cars had zero length, but they don't. At lower speed the time interval between cars must increase, because the required gap is between the front and rear of each vehicle, not just the front. If the speed limit is so low that it takes longer than 1 second for a single car to pass a particular point, it is impossible to have one car passing per second (even if they are bumper-to-bumper).

You are also assuming that traffic will be constant and perfectly smooth. In practice the number of cars trying to use a road varies, and they don't all maintain optimum distances and speeds, so overall throughput will be higher if the peak traffic is cleared faster (by allowing higher speeds).

theprestige said:
This is a straw man.
Click to expand...
Indeed. The main purpose of higher speed limits is to allow people to get to their destinations faster. Both high and low speed limits may be used to promote the smooth flow of traffic, but in general the limits are set as high as practicable to reduce the time that people spend driving. After all, getting there faster is what roads are made for!
 
Last edited:
Delvo said:
It's true only if drivers adjust their following distance proportionally to speed. If a doubling of speed is associated with more than a doubling of following distance, then capacity decreases. If it's associated with less than a doubling of following distance, then capacity increases. I suspect the latter is the case in most places.
Click to expand...

In practice, drivers tend to adopt unrealistically-close following distances in almost all conditions.
Anyone who's gone down a major highway at rush periods knows this....70mph with one-car-length distances is pretty common....And fantastically dangerous.

A one-second interval is too close by half..... The common measure is 2 seconds regardless of speed. When the car in front of you passes a given marker, it should take you two seconds to get to the same marker.

Another old measure is "one car-length per 10 miles an hour of speed. At 70, seven car lengths. I think this has been largely disregarded due to the fact that so few people can accurately gauge a "car-length".
 
daenku32 said:
Speed limits do not increase traffic capacity of a road.

Why? Because a 1 second interval between cars remains fixed, but the vehicle distance grows.
Click to expand...

Assume, just for fun, the following:
1) Car length 20 ft.
2) "Spacing" refers to distance from rear of leading car to front of traiing car,
3) Each driver takes his cues entirely from the driver immediately ahead,
4) Driver's reaction time is 0.2 sec (allowing for inattention), and
5) Braking acceleration is 1 g.

Then, for a 1 second vehicle spacing, a catastrophic crash (such as a vehicle crossing the median and colliding with an equivalent vehicle) at speeds above 35 mph will cause every vehicle on the road to crash into one another in a glorious chain reaction.

You might want to rethink the subject. The problem is that although the vehicle distance grows, the distance required to stop grows faster.

And consider the limiting case, in which vehicle separation is allowed to become arbitrarily small. By your logic, this should maximize traffic capacity.

Wanna bet?
 
ctamblyn said:
There's this:

http://en.wikipedia.org/wiki/Fundamental_diagram_of_traffic_flow
Click to expand...

That is a lovely piece of maths and I appreciate the reference.

What I was wondering is how well this works in practice with mixtures of cars and HGV, with different characteristics of size, acceleration and braking, and drivers with different skills, objectives and styles and finally of road conditions varying with surface, weather, curvature and visibility.

This interest was triggered after a discussion with a civil engineer who was 'tuning' a sequence of traffic lights. This was an iterative procedure of changing the timings and observing the flow and took some months. I asked how robust would be the final configuration to future changes. The answer was 'we don't know', and the impression was of disinterest. A peculiar response after the large effort involved.
 
Last edited:
Acleron said:
Just to take your statement, braking distance decreases with reduction in speed so the distance between vehicles decreases. Obvious really or we'd have massive if not infinite gaps when they stop.

But more generally, we have variable speed limits on some UK motorways to increase capacity/throughput. I've never seen any empirical data that shows this works. Anybody know of any published data on this?
Click to expand...

I was at a seminar at the NEC as part of Sustainability Live a few years ago, where a chap argued that lowering the speed limit from 70 to 55 would not only save carbon emissions, but increase traffic flow.

It can be seen on the M42, where the speed limit is lowered during certain times, that the raod is on average, less congested. And he used these situations as examples, referring to a study which found just that. Couldn't point to the study, but I do know that slowing down long before you come to stationary traffic can clear traffic jams sooner, and keeps the traffic moving.

I would argue that this points to a better flow, and thereby increasing the capacity of the road.

One of the major causes of the congestion though is vehicles (mainly lorries) who are moving considerably slower that most cars. The relative speed is the key, as fast cars in the outer lanes are an inherently unstable flow.

A good link I usually point to at this point is here:
http://trafficwaves.org/

And a link on that site is here:
http://iopscience.iop.org/1367-2630/10/3/033001/fulltext/
In conclusion, jam formation is an effect of the collective motion in the physics of a non-equilibrium phase transition of a many-particle system
Click to expand...

It is the high speed that causes the instability initially, but a lorry moving say 15mph less than the cars will increase this instability.

So you increase the stability of the flow, and you get less traffic jams. If you lower the speed limit to that of most lorries, you increase the stability of the flow of traffic, and get less jams. Thus, you effectively increase the capacity of the road.
 
Last edited:
One computer model has had an interesting side effect:
As the model runs, it moves vehicles according to rules that embody realistic rates of acceleration and deceleration. No infinite decelerations are allowed, for instance. The result is a software model that combines realistic driver behaviour with realistic physics.

The model is already being used to forecast traffic on the autobahn network around the city of Cologne, based on traffic data gathered in real time from sensors buried in the road. Its forecasts, which predict conditions up to an hour ahead, are displayed on the web at www.autobahn.nrw.de. More than 90 per cent of time, it correctly predicts traffic density.

But the website has already become a victim of its own success, admits Schreckenberg. Some of the 300,000 people a day who are visiting the site are replanning their journeys on the basis of its forecasts, and this is beginning to make the forecasts themselves less accurate. And soon it could get even worse when the website becomes available on 3G cellphones, he says.

So the researchers are now trying to adjust the way the traffic information is provided to drivers to take this destructive effect into account. One idea, says Schreckenberg, might actually be to provide less complete traffic information to encourage drivers to adopt more varied strategies for evading congestion, so they do not all flock to the same exits.
Click to expand...
http://www.innovations-report.com/html/reports/logistics/report-30794.html
 
theprestige said:
This is a straw man. You're refuting here an argument nobody is making here. At least cite the argument you're attempting to refute, rather than launching non sequitur directly into your refutation.
Click to expand...
Huh? You're quoting the OP - how can his post be a straw man? How can an OP be refuting an argument that hasn't begun yet?
 
SezMe said:
I find this to be fascinating. It takes only 40 seconds for traffic to come to a halt.

Click to expand...

That beautifully illustrates why variable speed limits improved traffic flow on London's M25 orbital motorway. By reducing the limit on a busy section from 70mph to, say, 40mph, all of the traffic tended to travel at the same speed, which is the key to preventing that kind of turbulent stop/start flow.
 
Bikewer said:
Another old measure is "one car-length per 10 miles an hour of speed. At 70, seven car lengths. I think this has been largely disregarded due to the fact that so few people can accurately gauge a "car-length".
Click to expand...

Yes people are poor at judging (or don't care) given the cars right up my jacksie sometimes. I've seen chevrons painted on the road to demonstrate a safe distance.

A while ago in the UK it was suggested that 2 seconds was a safe gap. We were told to recite "only a fool breaks the two second rule" as the car ahead passes a point and when we pass that same point. If you only leave half the gap you end up reminding yourself that you are "only a fool" :)
 
You must log in or register to reply here.

Back
Top Bottom