Interesting problem. Kinetic energy is E~k~ = kv^2^, where k = 1/2 for Newtonian physics. To double your speed, you need to apply four times the energy. Momentum is p = mv, so doubling your speed doubles your momentum. When you're in a vehicle collision, it's your momentum that hurts you as you collide with the car frame around you. Small particles don't have much mass, but at relativistic speeds they have a lot of momentum.