If the law set the speed limit as the hard boundary, with hard punishments for driving at 70, everyone else would drive at 65, occasionally getting close to 68-69.

If the reasonable speed for a road is 70, the legal hard limit could be at 75 to be considerate of this effect. But make 75 the actual hard limit with $200 fines for hitting 75, $1000 fines for hitting 80, and jail time for hitting 85. Make it almost certain to be caught if you hit these hard limits.

I guarantee you fewer people would die on the roads if this is the way things worked.