New Concepts for Urban Freeway Interchanges

Since approximately 1945, the number of highway agencies having experience with the location, design, and operation of urban freeways has increased from few to many. Until recently (as of 1962), comparatively few agencies have had an opportunity to observe operations on their own completed facilities in order to perfect the development of the total design. Although basic concepts of location and design on urban freeways remain unchanged, new ideas concerning design details are evolving as experience is gained in freeway-type operations, particularly experience in the combination of relatively high speeds and high volumes. These developments are centered in the three-dimensional design of areas in which traffic streams diverge and merge-that is, the detailed design of interchanges, including ramps. Proper interchange spacing is receiving much attention. Long distances between interchanges enhance the operation on the through lanes, but they may accumulate more traffic than the cross facility is able to accommodate. Closely spaced interchanges provide a better distribution of traffic and increase the level of local service, but with consequent interference to through movements. A logical balance between the two extremes is required. Downtown distributor sections frequently require special treatment.