My opinion as a poor track racer / car enthusiast with a very high performance daily driver. I speed daily. Btw.
We need to set speed limits to balance the safety of everyone in the road because we can't control all variables at play. If we all rode trains where all of these variables can be calculated, then the higher speed limits are warranted.
There simply are too many risks to justify it.
Experience levels of drivers. Some are new at the wheel. Most will panic in a crisis. I've trained so many people and you would shit your pants to see how people react to a breaking distance test (hint: Every one fails) or how they react to a car losing traction.
Capabilities of cars on the road (Grandmas 77 Corolla, Jimmys 4x4 with a lift kit, Rocco's beater that hasn't been inspected in 25 years).
Elderly and disabled drivers. Some who can't react quickly, can't hear well, can't see well).
Performance training of drivers. Meaning experiencing pushing their cars to the limits (hard turns, corrections, slide recovery, fast maneuvers, understands physics).
Tire variations (some may have off road tireds, snow tires, bald tires that all can lose traction with a light mash of the pedal).
Breaking capabilities - some cars do not have abs, heck breaking mechanisms can fail, some have economy disc based breaking systems. It can take one car 25+ feet further to stop than the next.
Driving hazards: someone smoking weed, texting, getting a bj, distracted, fighting with his girlfriend, someone rushing to the hospital, on medication, on sleep aids, etc).
Road hazards: animals, pot holes, unexpected turns, lighting conditions, careless drivers, blind sides etc.
Bottom line is - rarely is a car in a perfect scenario to optimize local and highway speed limits above 75mph. We all don't check our cars in the morning, make sure our lugs are tight, balance tire pressures, etc. We are more often than not in our physical prime, with excellent driving training, diving a BMW optimized for precision under high speeds.
The majority of America is not only fucked individually, but drive beater cars that they could care less about.
Setting a speed limit for the perfect scenario makes absolutely no sense, and the limits are there for a reason. Especially on local streets. If you're going 10+ over this dramatically reduces your chances of breaking in time especially if your car isn't equipped for it or if you weren't paying full attention.
If folks want to speed then they shod face the risk of paying for that privilege. There are far too many deaths per year to justify the need for it. Give people an apple they will take 2 - and endanger everyone else at the same time.
I know that I can recover from a tire blowout. That I can gain control of my car if it loses traction. That I know exactly where my car will stop far in advance. MOST DONT.