Right now there are 3 run speeds for survivors based on their health.
40-100% - full speed (220)
2-39% - slower (150, ~68% of full speed)
1% - very slow (85, ~39% of full speed)
A better system might be to have more speed steps (eg 75-100, 50-74, 25-49, 2-24,1) or some sort of linear equation. Maybe something similar to:
Velocity = 1.2 x Health + 100
When Health = 100, the survivor would run at 220
75 Health = 190
50 Health = 160
25 Health = 130
2 Health = 102
... and everything linearly in between.
This could even be applied to the tank to slow him down somewhat after taking damage. It doesn't make much sense for any other infected. I hope I explained this idea well enough.