Posted speed limits tell you what the maximum speed is on that specific road. That is the speed that you are legally able to drive on the road.
Parts of the German Autobahn do not have posted speed limits.
For safety.
Roads are built for safety and speed limits are determined by the conditions of the road on which they are posted.
70
The distance is ~475 miles. Allow 8 or 9 hours for the trip if running at posted speed limits.
75 mph
Most interstate highways were designed for the posted speed limits on those interstates.
It is posted on all roadways - and if not, in order to get your license you are responsible to know what the prevailing speed limits are in your state. Obey them!
Yes, just follow posted speed limits.
about 45 hours
70
Yes, but they still have speed limits. Its law, the default for highways is 70 "unless otherwiese posted" 55 rural, 25 residential....