It's done this way in downtown because when the areas are congested, averaging 35mph will be impossible.. Instead, they average the speed from a complete stop to the speed limit with heavy traffic considerations and it comes out usually to slightly less then the posted speed limit.
Most modern traffic systems can dynamically change this to match the flow of traffic. The set timeings are holdovers from the old more difficult to change systems. Los Vegas Nevada actually sends monitors out on foot and to drive along the routes in order to direct the changing of these timeings to increase the flow of traffic. Of course they are of the impression that they make their money from successful businesses paying taxes not from penalizing people trying to patronize those businesses.