Some of the western US states used to have no upper speed limits, but they no longer do. In order to accept Federal Highway Funds they had to come into compliance and establish upper limits to the speed allowed on their highways.
No, not all 50 states raised the speed limit in 1987. The increase in speed limits was the result of the National Maximum Speed Law being repealed in that year, allowing states to set their own speed limits. Some states did choose to raise their speed limits, but not all 50 states implemented higher speed limits in 1987.
Local, state and Federal. Federal is the highest, but all of them have their own powers reserved to them. Ex: States can decide whether they'll allow gay marriage, speed limits, ect.
states
The federal government sets national speed limits by promising monetary incentives to states that cooperate with their legislature. The states can set their own limits but if they want aid from the federal government they must comply with their mandates.
Interstates and other main roads of the like which have the highest posted speed limits.
The average speed from New York to Washington DC is 43MPH which hasn't change in the last 25 years. That should give you a idea of the Interstate avg speeds. One must account for traffic, road condition, normal congestion in high traveled areas. If you mean the average speed limit, it's probably 65 MPH. In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits. About 30 states have limits of 70 MPH or higher. A few Western states have no speed limits on certain stretches of highway.
Speed limits aren't in the Constitution. If you wanted to really reach you could say the 10th--any powers not reserved to the federal government devolve to the states--but that would be a big stretch.
In the United States their is no limit, but in other countries their are limits on speed and power.
Parts of the German Autobahn do not have posted speed limits.
That varies between states, who have jurisdiction to set speed limits in their respective state.
Yes Germany does have speed limits but on a lot of motorways they dont :)
Yes it is "Legal". Road signs and thus local road rules such as speed limits are detirmined by counties. There is not a law which states that counties cannot detirmine their own speed limits. There are numerous reasons why speed limits might be different on either side of the road such as corners or schools etc.