Although uncommon on urban streets, grade separations and interchanges may sometimes be the only means available for providing sufficient capacity at critical intersections. However, single, or multiple grade separations do not usually improve the throughput of a roadway when other intersections are signalized along the street. Except for grade separations at railroads,

grade separations on urban streets are usually diamond interchanges with four legs.

Locations considered include high-volume intersections and where terrain conditions favor separation of grades. See Chapter 10 of the and for additional information on optimal geometric configurations and traffic control solutions for interchanges.

Where applicable, the width of the entire roadway approach, including parking lanes or shoulders, should be carried across or under the separation. Interchange design elements may have slightly lower dimensional values as compared to freeways due to the lower speeds involved. For example, the length of diamond ramps may be controlled by the minimum distance to overcome the elevation difference at suitable gradients.

In some instances, it may be feasible to provide grade separations or interchanges at all major crossings for a long section of arterial street. The urban street then assumes the operating characteristics and appearance of a freeway. In these instances, it may be appropriate to eliminate the remaining at-grade crossings and control access by design (i.e., provide continuous frontage roads) where ROW availability permits. It is not desirable to intermix facility types by providing intermittent sections of fully controlled and non-controlled access facilities