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.
Ejection seat - Wikipedia, the free encyclopedia
"The pilot typically experiences an acceleration of about 12–14 g (117–137 m/s²)"
- and the purpose is for them to walk away from the scene, so 3g is more like the forces experienced on a half decent rollercoaster!