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.

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.

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

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

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

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

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

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.

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

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