5 ms·

It’s not the average drive. The p95 will be a lot higher in North America than China or Europe. Theres also elevation, winters and our obsession with going 70-8

It’s not the average drive. The p95 will be a lot higher in North America than China or Europe. Theres also elevation, winters and our obsession with going 70-80mph on highways.

akira2501 4y ago
Obsession? The higher the speed the greater the capacity of the road, and when freight has to travel 2000 miles by road, the 10mph/h difference makes a considerable impact on the time. Isn't it the case that Germany completely removes speed limits for some portions of it's "Autobahns?" What "obsession" drives that decision? There's this assumption that all the negative qualities of road are down to malignable user behavior issues. It's an unusually hostile way to view the problem.
- tuatoru 4y ago
  > The higher the speed the greater the capacity of the road This is false[1]. Stopping distance increases as the square of speed, and therefore safe following distance also. This and other effects mean that 80 mph is well down on 30 mph. Theoretically capacity should be independent of speed[2], but in practise it seems to decrease at high speeds because most drivers are careful. 1. Plot of safe capacity vs speed: https://www.researchgate.net/figure/Safe-road-capacity-for-cars-as-a-function-of-speed_fig1_285199854 https://www.researchgate.net/figure/Safe-road-capacity-for-c... 2. https://civilengineering-softstudies.com/57-traffic-capacity-studies-highway-engineering.html https://civilengineering-softstudies.com/57-traffic-capacity...
  - akira2501 4y ago
    The first plot is of a study particular to specific roads and their current configuration in Bangkok. The second arrives at an equality that I don't fully understand, but applying it's formula shows about a 2.6x greater spacing requirement when moving from 30mph (8.5 ft) to 80mph (22.8 ft). Which is close to what the two second rule would tell you when going from 30mph (88 ft) to 80mph (234 ft). Interestingly, if you look at actual braking distances, then for 30mph (45 ft) to 80mph (320 ft) shows a 7x change in distances. So, the above advice relies on the fact there will be no fixed obstructions in the road and that anything in front of you will have to brake as hard as you do. Perhaps this is why pedestrians still do so poorly on the roads, but it also makes me suspicious that you can just apply this formula equally to 30mph traffic and 80mph traffic. The hazards are different, the road duty cycle is different, traffic controls exist on one and barely on the other. Even so.. using the basic 2.6x capacity reduction factor going from 30mph to 80mph, you're getting a 2.6x capacity increase from the change in speed. As soon as you add _one_ additional lane, the 80mph road makes a bunch of sense.

It’s not the average drive. The p95 will be a lot higher in North America than China or Europe. Theres also elevation, winters and our obsession with going 70-80mph on highways.

akira2501 4y ago

Obsession? The higher the speed the greater the capacity of the road, and when freight has to travel 2000 miles by road, the 10mph/h difference makes a considerable impact on the time. Isn't it the case that Germany completely removes speed limits for some portions of it's "Autobahns?" What "obsession" drives that decision? There's this assumption that all the negative qualities of road are down to malignable user behavior issues. It's an unusually hostile way to view the problem.

tuatoru 4y ago
> The higher the speed the greater the capacity of the road This is false[1]. Stopping distance increases as the square of speed, and therefore safe following distance also. This and other effects mean that 80 mph is well down on 30 mph. Theoretically capacity should be independent of speed[2], but in practise it seems to decrease at high speeds because most drivers are careful. 1. Plot of safe capacity vs speed: https://www.researchgate.net/figure/Safe-road-capacity-for-cars-as-a-function-of-speed_fig1_285199854 https://www.researchgate.net/figure/Safe-road-capacity-for-c... 2. https://civilengineering-softstudies.com/57-traffic-capacity-studies-highway-engineering.html https://civilengineering-softstudies.com/57-traffic-capacity...
- akira2501 4y ago
  The first plot is of a study particular to specific roads and their current configuration in Bangkok. The second arrives at an equality that I don't fully understand, but applying it's formula shows about a 2.6x greater spacing requirement when moving from 30mph (8.5 ft) to 80mph (22.8 ft). Which is close to what the two second rule would tell you when going from 30mph (88 ft) to 80mph (234 ft). Interestingly, if you look at actual braking distances, then for 30mph (45 ft) to 80mph (320 ft) shows a 7x change in distances. So, the above advice relies on the fact there will be no fixed obstructions in the road and that anything in front of you will have to brake as hard as you do. Perhaps this is why pedestrians still do so poorly on the roads, but it also makes me suspicious that you can just apply this formula equally to 30mph traffic and 80mph traffic. The hazards are different, the road duty cycle is different, traffic controls exist on one and barely on the other. Even so.. using the basic 2.6x capacity reduction factor going from 30mph to 80mph, you're getting a 2.6x capacity increase from the change in speed. As soon as you add _one_ additional lane, the 80mph road makes a bunch of sense.