The problem with speed limits in general is that they're not universally applicable. Darkness, fog, rain, snow, etc. can all change what the actual safe maximum speed is. So even with a posted 55mph speed limit, the maximum safe speed at a particular time might be lower (even considerably lower), and a LEO could cite you for going too fast even if you're driving under the posted limit. (I've been on the interstate in the snow where you'd be likely to get pulled over if you were going much over 25mph, even with a posted 65mph limit.)

Driver skill and reaction time also plays a factor, but of course people are not so great at judging what their own specific safe speed is all the time. And all other things being equal, you're more likely to get into a crash if you're driving faster rather than slower, and the injuries you sustain will be worse at a higher speed.

IIRC speed limits are often set at some percentile (85th?) of what all drivers would (theoretically) "naturally" drive if there was no posted limit. And, on highways, cops will often not pull people over for exceeding the speed limit by a moderate amount. Once, long ago, a cop told me that, absent adverse conditions or other unsafe behavior, he usually will not stop anyone unless they're going more than 10mph over the highway speed limit. And I expect if he were hiding in a speed trap that no one could actually see driving by, and everyone was going 70mph on your 55mph road, he'd probably just sit there and not bother anyone, unless they were doing something else that was unsafe.

I guess this is a long winded way to say that there really is no single safe speed that applies to everyone, in every road condition. The law acknowledges this, and police often let you do your thing unless they believe you're actually doing something unsafe. The discretion and judgment calls can be a problem (biases, etc.), but I don't think a society where unavoidably "fuzzy" laws were always prosecuted would be a great society either.

A lot of departments have policies about not interrupting thr normal flow of traffic.

When I was a kid, the argument for lower speeds on expressways was fuel efficiency.