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Comment by daenz | Hacker News Reader

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

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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.

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).

20mph is within populated areas and not on a main road. In the US, the norm is 25mph, which isn't that far off.

25mph is >150% of the energy at 20mph. It's a big increase.

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).

Indeed, as an interesting aside it's a 3-4-5 triangle.

25^2 - 20^2 = 15^2

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.

It's limited to highways as well (hence the geofence), so they probably don't expect VRU incidents to happen much, if at all.

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

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.

> 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

I think you just don't know what that term means.

Absurd

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.

> 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

The effect is huge. Energy goes up quadratically, so here (4->9) you have more than doubling. Stopping distance goes up quadratically (well, due to reaction time there is a linear component in there as well, but still).

And if there is a sudden obstacle that you can just avoid hitting going 20mph initially, you'd hit it with 22 mph if you're going 30 mph initially (assuming no reaction time and constant deceleration).

I submit that that's a massive difference: hitting a child not at all or with 22 mph.

Things can be quantifiably described as "doubled" without that having any real world impact.

In my experience the people supporting these measures are adopting them not because they care about safety at all, but have another agenda.

not gonna be a lot of pedestrians on the highway, are there?

This is only on highways. Survivability is concerned only with passengers not pedestrians.

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.

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.

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.

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.

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.

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 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.

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