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Inattention is hard to measure and results in accidents that are made much worse by excessive speed, which is easy to measure. If the goal is to reduce overall

by yholio 6y ago

Inattention is hard to measure and results in accidents that are made much worse by excessive speed, which is easy to measure. If the goal is to reduce overall traffic victims, then investing in speed control seems like the low hanging fruit. The energy of vehicle (thus, the energy absorbed during impact by every object contained or hit) is proportinal with the square of the speed. Meanwhile, the reaction time available to avoid the collision drops linearly with speed, while the braking or turn distance increases more than linearly. So the overall probability of accidents and their severity increases exponentially with speed.

Silhouette 6y ago
The problem is that naive arguments about speed and laws based on fixed numbers don't really help anything except ease of prosecution. Inappropriate choice of speed can obviously be dangerous, but what is appropriate to the conditions and what the fixed numerical speed limits dictate can be wildly different. For example, the default speed limit in the UK on most residential roads is 30mph, but if you drive past a school at 30mph at the end of the afternoon when the kids are coming out and there are lots parked cars obscuring your view, that is still inappropriately fast for the conditions. However, slower isn't always safer. On a motorway, the speed limit is usually 70mph, but in poor weather conditions and heavy traffic, everyone travelling at lower and similar speeds may be the safest choice. If you're the one driver insisting on unnecessarily doing 20mph when everyone else is doing a steady 40mph, you're also increasing the danger to yourself and others. Another example is that the speed limit for heavy goods vehicles on many of our major roads was increased by 10mph in 2015, having previously been 20mph below what cars were allowed to do on the same roads. The results were notable for several reasons, among them that the actual average speed of those heavy vehicles only increased a little but the safety of the roads appears to have increased. This was attributed primarily to closing the gap in speeds between different types of vehicle and thus, among other things, reducing instances of queues behind and overtaking of the slower vehicles, both of which increase the risk of an accident. Finally, we have the classic motorway speeding situation, where on an open road in good conditions, a competent driver with a suitable vehicle could do far more than the legal limit of 70mph with negligible increase in risk. Moreover, statistically our motorways are both our fastest roads and yet also among our safest. You could certainly print money by setting up a covert speed camera watching some stretches of motorway, but whether it is a good use of police resources if your objective is to increase road safety is a different question.
CraigJPerry 6y ago
>> 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.

by yholio 6y ago

Silhouette 6y ago

The problem is that naive arguments about speed and laws based on fixed numbers don't really help anything except ease of prosecution. Inappropriate choice of speed can obviously be dangerous, but what is appropriate to the conditions and what the fixed numerical speed limits dictate can be wildly different. For example, the default speed limit in the UK on most residential roads is 30mph, but if you drive past a school at 30mph at the end of the afternoon when the kids are coming out and there are lots parked cars obscuring your view, that is still inappropriately fast for the conditions. However, slower isn't always safer. On a motorway, the speed limit is usually 70mph, but in poor weather conditions and heavy traffic, everyone travelling at lower and similar speeds may be the safest choice. If you're the one driver insisting on unnecessarily doing 20mph when everyone else is doing a steady 40mph, you're also increasing the danger to yourself and others. Another example is that the speed limit for heavy goods vehicles on many of our major roads was increased by 10mph in 2015, having previously been 20mph below what cars were allowed to do on the same roads. The results were notable for several reasons, among them that the actual average speed of those heavy vehicles only increased a little but the safety of the roads appears to have increased. This was attributed primarily to closing the gap in speeds between different types of vehicle and thus, among other things, reducing instances of queues behind and overtaking of the slower vehicles, both of which increase the risk of an accident. Finally, we have the classic motorway speeding situation, where on an open road in good conditions, a competent driver with a suitable vehicle could do far more than the legal limit of 70mph with negligible increase in risk. Moreover, statistically our motorways are both our fastest roads and yet also among our safest. You could certainly print money by setting up a covert speed camera watching some stretches of motorway, but whether it is a good use of police resources if your objective is to increase road safety is a different question.

CraigJPerry 6y ago

>> 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.