Why do the speed limits vary on different parts of the freeway system?

With the exception of a limited number of urban freeway miles, State law sets maximum speed limits on "US," "I" and "M" routes at 70 mph. In densely populated (urban) areas where there are many exit and entrance ramps constructed close together, the speed limit is held to 55 mph. The speed limit for commercial trucks on all highways is never higher than 55 mph.
For more on speed limits, visit the Office of Highway Safety Planning(OHSP).