8.1.8 Speed Change Lanes

A speed change lane or auxiliary lane is defined as the portion of roadway adjoining the through lanes for speed change, turning, storage for turning, weaving, truck climbing and other purposes that supplement through-traffic movement (e.g., maintaining appropriate lane balance between ramp terminals). Auxiliary lanes are typically provided on freeways to:
  • Comply with the concept of lane balance;
  • Comply with capacity needs;
  • Accommodate speed changes;
  • Accommodate weaving;
  • Maneuvering of entering and exiting traffic; and
  • Simplify traffic operations by reducing the number of lane changes.
The width of an auxiliary lane should be equal to or greater than the adjacent through lane.
When auxiliary lanes are provided along freeway mainlanes, the shoulder width should be 10-ft. However a 6-ft shoulder can be considered in weaving areas of LOS A or LOS B. A design waiver is not required for this condition.
The length of a weaving auxiliary lane should be determined through the procedures outlined in the
HCM
which is applicable to all freeway contexts. See
Section 15.6.3
or refer to
AASHTO’s A Policy on Geometric Design of Highways and Streets
for additional guidance on the use of auxiliary lanes.
Figure 8-5
shows an example of a freeway auxiliary lane.
Example of Freeway Auxiliary Lane ( click in image to see full-size image)
Figure 8-5 Example of Freeway Auxiliary Lane