AASHTO's Policy on Geometric Design of Highways and Streets defines Route Continuity as providing a route on which changing lanes is not necessary to continue along the through route. Interchange designs that adhere to route continuity make the driving task simpler by reducing the need for through drivers to change lanes and typically result in simpler signing and improved traffic operations. AASHTO's Policy provides guidance in providing route continuity for typical interchanges. However, strategies for providing appropriate route continuity are less defined for complex interchanges in the Policy. Several key principles of lane continuity are as follows:
- The highway geometry should favor the through movement, not necessarily the heaviest movement.
- Highway geometry should communicate the through movement to the driver.
- Highway geometry should maintain its directional character throughout length of its route. Local variations due to combined routes or increased traffic can increase the number of basic lanes.
- A through movement should have at least as many lanes as the exiting movement.
Signing and Delineation, Division Traffic Engineers, GEC's and/or PEFs shall review proposed roadway plans for any deviations from the lane continuity key points described above. This should be done as soon as possible during a project so that the roadway can be changed to the best design possible while minimizing redesign costs and project delays.
Strategies to improve lane continuity include, but are not limited to:
- Establishment of the proper number of basic lanes for a route.
- Providing as many lanes for through movements as the exiting movement
- Providing auxiliary lanes between entering and exiting movements
- Providing lane balance (the number of lanes beyond the merging of two traffic streams should not be less than the sum of all traffic lanes on the merging roadways minus one.
- Providing collector distributor roads to eliminate weaving.
- Providing an additional through lane when an entering traffic stream has two or more lanes