4 ms·

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.