Earlier quoted context omitted.
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.
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).