The goal of improving roads is to increase the throughput of traffic. If it seems as though a problem could be solved by slowing down traffic, that's like solving a problem with a circuit by throwing a resistor into it. It may solve the immediate problem, but it will also reduce the overall efficiency of the circuit by converting energy into heat rather than usable work.
They could also put up signs that say 100% of the people who drive 40 mph reach their destinations 33% faster than those who drive 30 mph. It's just a distraction from the fact that solving the problem in a better way is more expensive than just blaming drivers and letting insurance handle the fallout.