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The capacity of a sufficiently long road section does not increase with speed because the safety distance must also increase with speed. A good rule of thumb is
by yholio 6y ago
The capacity of a sufficiently long road section does not increase with speed because the safety distance must also increase with speed. A good rule of thumb is to keep a 2 second (or 3 second) distance between vehicles, and that means that the peak throughput of any single lane is 0.33-0.5 vehicles per second regardless of speed.
The total time spent on the road increases and the utility of the road decreases since it gets you to your destination slower, but the capacity does not increase, it's the same capacity with an added delay line. In fact, the safety distance increases more than linearly with speed, precisely because the combined effect of reaction time, braking distance and severity of incidents is more than linear. The 2 second rule is just a rough approximation.
These are really "fundamentally true" issues that are well known in traffic engineering so there is nothing much to debate. The rain example is disingenuous since in rain the braking distance increases dramatically, drivers instinctively know this and maintain larger clearances. And the demand of the car traffic increases too in bad weather, for example people avoid using bicycles or walking and opt for a car, which injects additional vehicles into a road network and pushes it over into disequilibrium and gridlock.
- CraigJPerry 6y ago>> The capacity of a sufficiently long road section does not increase with speed Most roads aren't sufficiently long and there's plenty of other factors and practical road usage is nowhere near this perfect maximum of safe usage but to point out your mathematical error: For 10 minutes a section of road 1 mile long has traffic pass through it. The cars have 3 second gaps between them. At 30 mph: 160 cars max throughput At 60 mph: 180 cars max throughput In the 30mph example, for the first 2 minutes, 0 cars have completed the 1 mile stretch, for the period 2-10 mins, cars pass the finishing line every 3 seconds. 160 cars complete the 1 mile stretch in 10 mins. In the 60mph example, only the 1st minute is spent without traffic completing the throughput analysis. For the period 1-10 mins, cars pass the finishing line every 3 seconds. 180 cars complete the 1 mile stretch in 10 mins.
- yholio 6y agoYes, you need to double the speed and construct an artificial trafic burst to notice a marginal increase in capacity that is strictly due to the increased delay (and storage capacity) of the slower road. That was my point. In practice trafic ramps up slow enough so that we can ignore the lost capacity due to empty roads in the early hours. After that, it is steady state for both scenarios. > there's plenty of other factors and practical road usage is nowhere near this perfect maximum of safe usage Precisely, but this even further argues against high speed limits. If the road tends to be congested, windy, has trafic lights and is poorly maintained, the areas where you can increase the speeds are very limited and the total impact in trip time is low. So high speeds not only not deliver higher throughputs but they fail to even deliver speed.