The Government, through the Department of Transport, is responsible for setting the national speed limit. The national speed limit depends on the type of road - motorways being 70 mph. Local authorities are able to set local by-law speed restrictions.
They are not allowed to exceed speed limits
They are not allowed to exceed speed limits
They are not allowed to exceed speed limits
Speed limits are set by whatever level of government owns that particular roadway.
legislative
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.
They are not allowed to exceed speed limits
As a fuel saving measure during the oil embargo crisis.
states
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.
Train speed limits inside city limits vary depending on the specific city and the type of train. Typically, passenger trains are allowed to travel at speeds up to 79 mph in urban areas, while freight trains are limited to speeds around 40-50 mph. These speed limits are set to ensure safety for both passengers and pedestrians in densely populated areas.
Parts of the German Autobahn do not have posted speed limits.