Actually, most states have a reasonable & prudent speed limit. Only it is in conjuction with a posted speed limit. So you could actually get a speeding ticket for going below the speed limit if your speed is too high for conditions.
Most of the time, however, the real "reasonable & prudent" speed is that of the traffic, no matter what the speed limit. Yes, if you collide with a vehicle head-on, you are more likely to die if you go 70 rather than 55. However, on a divided highway, that type of accident will rarely, if ever, happen. Most of the accidents are going to be with other cars going in the same direction as you. Therefore, it is the relative speed that will determine the severity of the accident. If you are going within 5 MPH of the rest of the traffic, your bumper will be able to absorb the impact with no damage. If you are going 55 while everyone else is going 75, the result will be sort of like those crash tests...