I have always felt that many people pay too little attention to the importance of speed limits. Speed limits are set not just for safety or the possibility of better fuel economy, although those are factors. With handling and stability of modern cars we could probably drive at much greater speeds if there were no other factors to consider. What is perhaps more important is traffic volume and flow. Speed limits are chosen to facility smooth traffic flow. Too many cars traveling too fast results in increased congestion which increases the chance of collisions.
It’s like pouring water down a drain. The faster the water is poured the more it tends to back up into the sink. If you pour it at a rate that is suitable for the size drain it will flow out with no problem.