5 ms·

I wonder if 40mph is a magic number that is a good tradeoff between speed and survivability (and injury-related out-of-court settlements)

by daenz 4y ago

sjcoles 4y ago
Has to do with the limitations of radar. Radar will fail >40mph for stopped vehicles or detect them too late to stop. Honda and Tesla are working to remove radars and go vision only for speed control. If you have a car with a AEB or CMBS (automatic emergency braking, crash mitigation braking system) the owners manual has a ton of fine print about it only making crashes less severe, not preventing them outright.
- t0mas88 4y ago
  The detection range for radar is more than 200m, you can stop from speeds above 100mph in that distance. And the radar sensor automatically includes relative speed information on the target (doppler shift) which vision doesn't so it's easier to figure out whether something is stationary or not. So I don't believe this "vision only will fix it all" story.
rootusrootus 4y ago
I imagine there are a couple reasons that boil down at some point to kinetic energy. Stopping distance goes up pretty quickly, there's a huge difference between, say, 40mph and 60mph.
bryanlarsen 4y ago
That threshold is generally considered to be 20mph. Below that speed pedestrian injuries are unlikely to be fatal. It's what urban speed limits are in Sweden, and they average about one pedestrian death a year in the entire country.
- Closi 4y ago
  20mph is considered to be good for survivability, but is not really good from a speed/travel perspective. I believe OP meant maybe 40mph is a good balance between the two factors of speed (higher is better) and survivability (higher is better) from a corporate risk / insurance perspective. Raising one of these lowers the other - so the actual threshold depends on how you value speed / convenience vs accident survivability / risk, and what you want to market your car as able to do (few consumers will see a 20mph self-drive speed limit as a viable self driving system).
  - zip1234 4y ago
    20mph is within populated areas and not on a main road. In the US, the norm is 25mph, which isn't that far off.
    - hgomersall 4y ago
      25mph is >150% of the energy at 20mph. It's a big increase.
      - FabHK 4y ago
        And not only that. In a situation where, with 20 mph, you can just come to a complete stop before the child, with 25 mph initially you'd hit it with 15 mph. vr = (c^2 - 1)^0.5 v1 where v1 is the original slow speed with which you'd come to a standstill just before the obstacle, and v2 = c*v1 is the faster speed, and vr the speed with which you hit the obstacle coming in with the faster speed (assuming you hit the brakes at the same spot, and constant deceleration).
        hgomersall 4y ago
        Indeed, as an interesting aside it's a 3-4-5 triangle. 25^2 - 20^2 = 15^2
    - Closi 4y ago
      That comes with it’s own challenges - ie limiting speed to 20mph in the UK would realistically mean it can only be used in areas where there are schools and lots of pedestrians - ie less chance of an incident being fatal but possibly more chance of an incident.
- judge2020 4y ago
  It's limited to highways as well (hence the geofence), so they probably don't expect VRU incidents to happen much, if at all.
- easytiger 4y ago
  The reduction from 30mph to 20mph in London is political CV padding and has had no real effect on outcomes. All based on Chinese whispers. Wouldn't be surprised if your comment ended up as an original source in Islington's justification for some thing or other. It's that flimsy
  - stefan_ 4y ago
    Average speeds in London are below 20mph. Accelerating absurdly overweight, overpowered cars to anything over 20mph in between stops at lights is just a moral hazard.
    - easytiger 4y ago
      > Accelerating absurdly overweight, overpowered cars to anything over 20mph in between stops at lights is just a moral hazard. The level of absurdity in suggesting a moral element to driving a car at below 30mph just shows how fundamentally biased people have become about a normal part of everyday life. For whatever reason
      - stefan_ 4y ago
        I think you just don't know what that term means.
        easytiger 4y ago
        Absurd
  - hgomersall 4y ago
    If it doesn't it's because it's not enforced. It's irrefutable that vehicles moving at 20mph are less dangerous than vehicles moving at 30mph.
    - easytiger 4y ago
      > If it doesn't it's because it's not enforced. It's irrefutable that vehicles moving at 20mph are less dangerous than vehicles moving at 30mph. Then make them drive at 1mph
- dwighttk 4y ago
  not gonna be a lot of pedestrians on the highway, are there?
- zaroth 4y ago
  This is only on highways. Survivability is concerned only with passengers not pedestrians.
fweimer 4y ago
It's translated from 60 kph. Apparently, the current system is restricted to German highways. Those are limited to vehicles which can go at least as fast as 60 kph, and this might be where this number comes from. It's basically the slowest highway speed at which there is no controversy about legality. Targeting highways first makes sense from a complexity and data quality perspective. Although driving at 60 kph on German highways seems quite risky; at that speed, most trucks will overtake you.
- IfOnlyYouKnew 4y ago
  I doubt this reasoning. While it's true that 60 kph is the minimum for Autobahn, it's the minimum in the sense that the vehicle must be capable of that speed. There is no requirement to actually drive 60 kph or faster. On the other side of the equation, nobody would feel comfortable cruising along at that speed, since you'd be continuously overtaken by trucks, which drive a little more than the allowed 80 kph unless it's physically impossible.
  - jhugo 4y ago
    The vehicle is not capable of more than 60 km/h when used in self driving mode, so the reasoning seems quite logical. I wouldn't want to use this on the Autobahn though; everyone would be overtaking constantly which introduces additional risk, and the speed is so low that journey time would be considerably inflated.
Dave_Rosenthal 4y ago
I don't think it's magic, but: 1) Stopping distances go up as the square of speed, so 40mph has huge advantages over, say 70mph. 2) Survivability of accidents at 30-40 are way higher 70 (unless pedestrians are involved, in which case 30-40 is already too high.) 3) Radars and Lidars really can't see/classify things very well at the distances required to do comfortably stop for obstacles at highway speeds.

I wonder if 40mph is a magic number that is a good tradeoff between speed and survivability (and injury-related out-of-court settlements)

by daenz 4y ago

I wonder if 40mph is a magic number that is a good tradeoff between speed and survivability (and injury-related out-of-court settlements)

sjcoles 4y ago

Has to do with the limitations of radar. Radar will fail >40mph for stopped vehicles or detect them too late to stop. Honda and Tesla are working to remove radars and go vision only for speed control. If you have a car with a AEB or CMBS (automatic emergency braking, crash mitigation braking system) the owners manual has a ton of fine print about it only making crashes less severe, not preventing them outright.

t0mas88 4y ago
The detection range for radar is more than 200m, you can stop from speeds above 100mph in that distance. And the radar sensor automatically includes relative speed information on the target (doppler shift) which vision doesn't so it's easier to figure out whether something is stationary or not. So I don't believe this "vision only will fix it all" story.

rootusrootus 4y ago

I imagine there are a couple reasons that boil down at some point to kinetic energy. Stopping distance goes up pretty quickly, there's a huge difference between, say, 40mph and 60mph.

bryanlarsen 4y ago

That threshold is generally considered to be 20mph. Below that speed pedestrian injuries are unlikely to be fatal. It's what urban speed limits are in Sweden, and they average about one pedestrian death a year in the entire country.

Closi 4y ago
20mph is considered to be good for survivability, but is not really good from a speed/travel perspective. I believe OP meant maybe 40mph is a good balance between the two factors of speed (higher is better) and survivability (higher is better) from a corporate risk / insurance perspective. Raising one of these lowers the other - so the actual threshold depends on how you value speed / convenience vs accident survivability / risk, and what you want to market your car as able to do (few consumers will see a 20mph self-drive speed limit as a viable self driving system).
- zip1234 4y ago
  20mph is within populated areas and not on a main road. In the US, the norm is 25mph, which isn't that far off.
  - hgomersall 4y ago
    25mph is >150% of the energy at 20mph. It's a big increase.
    - FabHK 4y ago
      And not only that. In a situation where, with 20 mph, you can just come to a complete stop before the child, with 25 mph initially you'd hit it with 15 mph. vr = (c^2 - 1)^0.5 v1 where v1 is the original slow speed with which you'd come to a standstill just before the obstacle, and v2 = c*v1 is the faster speed, and vr the speed with which you hit the obstacle coming in with the faster speed (assuming you hit the brakes at the same spot, and constant deceleration).
      - hgomersall 4y ago
        Indeed, as an interesting aside it's a 3-4-5 triangle. 25^2 - 20^2 = 15^2
  - Closi 4y ago
    That comes with it’s own challenges - ie limiting speed to 20mph in the UK would realistically mean it can only be used in areas where there are schools and lots of pedestrians - ie less chance of an incident being fatal but possibly more chance of an incident.
judge2020 4y ago
It's limited to highways as well (hence the geofence), so they probably don't expect VRU incidents to happen much, if at all.
easytiger 4y ago
The reduction from 30mph to 20mph in London is political CV padding and has had no real effect on outcomes. All based on Chinese whispers. Wouldn't be surprised if your comment ended up as an original source in Islington's justification for some thing or other. It's that flimsy
- stefan_ 4y ago
  Average speeds in London are below 20mph. Accelerating absurdly overweight, overpowered cars to anything over 20mph in between stops at lights is just a moral hazard.
  - easytiger 4y ago
    > Accelerating absurdly overweight, overpowered cars to anything over 20mph in between stops at lights is just a moral hazard. The level of absurdity in suggesting a moral element to driving a car at below 30mph just shows how fundamentally biased people have become about a normal part of everyday life. For whatever reason
    - stefan_ 4y ago
      I think you just don't know what that term means.
      - easytiger 4y ago
        Absurd
- hgomersall 4y ago
  If it doesn't it's because it's not enforced. It's irrefutable that vehicles moving at 20mph are less dangerous than vehicles moving at 30mph.
  - easytiger 4y ago
    > If it doesn't it's because it's not enforced. It's irrefutable that vehicles moving at 20mph are less dangerous than vehicles moving at 30mph. Then make them drive at 1mph
dwighttk 4y ago
not gonna be a lot of pedestrians on the highway, are there?
zaroth 4y ago
This is only on highways. Survivability is concerned only with passengers not pedestrians.

fweimer 4y ago

It's translated from 60 kph. Apparently, the current system is restricted to German highways. Those are limited to vehicles which can go at least as fast as 60 kph, and this might be where this number comes from. It's basically the slowest highway speed at which there is no controversy about legality. Targeting highways first makes sense from a complexity and data quality perspective. Although driving at 60 kph on German highways seems quite risky; at that speed, most trucks will overtake you.

IfOnlyYouKnew 4y ago
I doubt this reasoning. While it's true that 60 kph is the minimum for Autobahn, it's the minimum in the sense that the vehicle must be capable of that speed. There is no requirement to actually drive 60 kph or faster. On the other side of the equation, nobody would feel comfortable cruising along at that speed, since you'd be continuously overtaken by trucks, which drive a little more than the allowed 80 kph unless it's physically impossible.
- jhugo 4y ago
  The vehicle is not capable of more than 60 km/h when used in self driving mode, so the reasoning seems quite logical. I wouldn't want to use this on the Autobahn though; everyone would be overtaking constantly which introduces additional risk, and the speed is so low that journey time would be considerably inflated.

Dave_Rosenthal 4y ago

I don't think it's magic, but: 1) Stopping distances go up as the square of speed, so 40mph has huge advantages over, say 70mph. 2) Survivability of accidents at 30-40 are way higher 70 (unless pedestrians are involved, in which case 30-40 is already too high.) 3) Radars and Lidars really can't see/classify things very well at the distances required to do comfortably stop for obstacles at highway speeds.