Speed is rarely the only factor in a fatal crash. Alcohol and drugs are the more pertinent issues. Aside from that, pedestrians are 1/6 of all fatalities, as are motorcycles. Most remaining fatalities are single vehicle accidents. Hard collisions with solid objects, ejections from vehicles and bodies of water are fairly common modes. The rear seat in vehicles is noticeably less safe than the front seats. The time of day in some states shows massive swings in the rate of fatalities, and not always correlated with rush hour traffic patterns. The state you are in also seems to be a large factor, and again, not correlated with overall population density.

There's a huge number of data points, and speed does seem like a relatively unimportant one. If you do want to have an impact there, then I suspect that simply redesigning your roads with good considerations for pedestrians would be a better use of your budget.