I suspect that once you have enough mass though then it is velocity that determines the impact on safety. Getting hit at 50 mph by a Ford F-150 that has a total mass (truck plus cargo) of 9000 pounds is not going to be much more harmful to a pedestrian than getting hit by a similar shaped truck at the same speed that has half that mass.

Similarly on the other end if velocity is low enough mass won't matter. Consider for example getting hit by a freight train at 1 mph. Unless the collision knocks you over and you fall under the train it probably won't be fatal (and likely won't even injure you). Even though that freight train would have 300 times the momentum and 6 times the kinetic energy of the 50 mph 9000 pound truck it would be fine.