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>> results in accidents that are made much worse by excessive speed Sure but there's no free lunch here. If you trade off speed in the non-accident cases, whic

>> results in accidents that are made much worse by excessive speed Sure but there's no free lunch here. If you trade off speed in the non-accident cases, which is by far the majority of all miles driven, then there's a cost to that. It happens to be a really hard to measure cost so it's always completely ignored. How would you even begin to measure the cost to the nation of traffic arriving 5 minutes later? Not all 5 minute delays are equal, e.g the cost of delay to an ambulance rushing to hospital is different to the increased cost of delivering packages. >> the reaction time available In fairness, you could make the reaction time measurable in hours and it'd be insufficient to prevent some inattention caused road incidents. Another problem with only looking at speed is the unintended consequence of drivers de-rating the risks. E.g. if a vehicle was limited to 5mph, this as you know is slow enough to be fairly unlikely to cause a pedestrian death even if there is an impact, it would have the un-wanted effect that some drivers would view the risks as lower and would be inclined to engage in more risky behaviours, e.g. taking their eyes off the road to turn around and argue with a child in the back seat where in 20mph traffic they might choose instead to just speak to the child. >> So the overall probability of accidents No that doesn't hold. We don't have more accidents on the fastest roads in the UK.

yholio 6y ago
> the cost to the nation of traffic arriving 5 minutes later? Traffic delays are dominated by congestion and road capacity does not increase with higher speeds. Once you hit the peak for that road section, trafic grinds. If, for example, you have a 30 minute comute that spends 15 minutes in congestion-limited trafic and 15 minutes on a speed limited empty highway, you would need to drive 50% faster on those sections to recoup a full 5 minutes, so something like 105 mph on a 70 mph section for a full 10 minutes. That doesn't sound realistic. There is a cost to low speed limits, clearly, but it is usually marginal - and when it's not, you should address that specific problem of a too low speed limit instead of making speed limits unenforceable throughout. If a section generates lots of speed tickets and no even minor incidents, it's clearly a sign to increase speed. > drivers de-rating the risks. True, but the risk rating they assigned to higher speeds was probably not proportional to the actual exponential increase to begin with. > We don't have more accidents on the fastest roads in the UK. That's moving the goal posts. The increase in risk is exponential on any given single road. Some roads are exceptionally safe at low speeds so you can increase the speed quite a bit and keep the risks low, hence the higher limits.
- CraigJPerry 6y ago
  >> road capacity does not increase with higher speeds Fundamentally untrue. If you reduce the average speed of a section of road, say for example in heavy rain, the capacity or throughput of that section of road is reduced since less vehicles can pass through in the same time. If you can keep drivers attentive enough to maintain safe distance between vehicles, you can increase the speed of a section of road. >> Once you hit the peak for that road section You can increase the practical peak in an imperfect world by increasing attentiveness of drivers. For example, you could defer the onset of gridlock by X minutes by making more efficient use of the available road space. In some cases it will be possible to increase X such that gridlock is never encountered for the same traffic volume. >> Some roads are exceptionally safe at low speeds And yet the focus on speed means that roads which are not exceptionally safe at low speed have reduced incentive to be improved. Why improve a road (hard in some cases) when the impact is not measurable and speed is an easy band aid. Better to pretend the immeasurable doesn’t exist and simply focus on speed. Even if other approaches could have a greater impact on road safety.
  - 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.

yholio 6y ago

> the cost to the nation of traffic arriving 5 minutes later? Traffic delays are dominated by congestion and road capacity does not increase with higher speeds. Once you hit the peak for that road section, trafic grinds. If, for example, you have a 30 minute comute that spends 15 minutes in congestion-limited trafic and 15 minutes on a speed limited empty highway, you would need to drive 50% faster on those sections to recoup a full 5 minutes, so something like 105 mph on a 70 mph section for a full 10 minutes. That doesn't sound realistic. There is a cost to low speed limits, clearly, but it is usually marginal - and when it's not, you should address that specific problem of a too low speed limit instead of making speed limits unenforceable throughout. If a section generates lots of speed tickets and no even minor incidents, it's clearly a sign to increase speed. > drivers de-rating the risks. True, but the risk rating they assigned to higher speeds was probably not proportional to the actual exponential increase to begin with. > We don't have more accidents on the fastest roads in the UK. That's moving the goal posts. The increase in risk is exponential on any given single road. Some roads are exceptionally safe at low speeds so you can increase the speed quite a bit and keep the risks low, hence the higher limits.

CraigJPerry 6y ago
>> road capacity does not increase with higher speeds Fundamentally untrue. If you reduce the average speed of a section of road, say for example in heavy rain, the capacity or throughput of that section of road is reduced since less vehicles can pass through in the same time. If you can keep drivers attentive enough to maintain safe distance between vehicles, you can increase the speed of a section of road. >> Once you hit the peak for that road section You can increase the practical peak in an imperfect world by increasing attentiveness of drivers. For example, you could defer the onset of gridlock by X minutes by making more efficient use of the available road space. In some cases it will be possible to increase X such that gridlock is never encountered for the same traffic volume. >> Some roads are exceptionally safe at low speeds And yet the focus on speed means that roads which are not exceptionally safe at low speed have reduced incentive to be improved. Why improve a road (hard in some cases) when the impact is not measurable and speed is an easy band aid. Better to pretend the immeasurable doesn’t exist and simply focus on speed. Even if other approaches could have a greater impact on road safety.
- 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.