I would far rather ondergo a 60 MPH deceleration across the ground over many feet, perhaps hundreds if I do it right, than a vertical deceleration at 20 MPH in no more than about 4? feet!
We can compare them. The vertical 20mph to zero in 4 feet is 3.36G. The horizontal 60mph to zero at 3.36G takes 36 feet. 60mph to zero in 4ft (stone wall scenario) is 30G.
speed squared = 2 times 32 times G times distance
e.g. (20x5280/3600)sqd = 2x32xGx4
As said already, tolerance of vertical and horizontal G will vary.