Hannes, it varies by state. Most are :-
* 20 mph by schools when they are open
* 35 mph in town
* major roads not motorways 45 or 55 mph and a lot of the original route 66 are these roads
* Motorways (Interstates) 70 or 75. Most were 75 but I think it was Missouri that was 70. Around big cities like LA, St Louis & Chicago they reduce to 65 or even 45 but the drivers ignored that.
