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ADOT&PF Launches Seward Highway Differential Speed Limit Study

July 8, 2016

The Alaska Department of Transportation & Public Facilities (ADOT&PF) is launching a pilot program to improve passing lanes through the use of “differential” speed limits on the Seward Highway Mile 60-65 (Hope Junction to the top of Turnagain Pass). Differential speed limits maintain the normal speed limit in the left (fast) lane, while reducing the speed of the right (slow) lane to allow more vehicles to pass at reasonable speeds.

The first phase of this pilot project used low-cost driving simulators to test alternatives that modeled Alaska’s passing lane geometry. The most effective alternative to get drivers to slow down in the outside lanes was to simply add passing lane differential speed limits.

Passing lanes are less effective when vehicles in the right (slow) lane are traveling near the existing 65 mph speed limit. Merge points at the end of passing lanes can be areas of conflict when drivers are traveling at a higher rate of speed while trying to pass fast drivers in the right (slow) lane. With a lower posted speed limit in the right (slow) lane, passing speeds can be reduced while allowing more vehicles to pass within designated passing lanes. Additionally, aggressive behaviors can be reduced when drivers feel like they have time to pass. Efficient use of passing lanes is critical in summer months when traffic is at its peak and recreational drivers are on the road.

Highway engineers continue to look for cost-effective solutions to relieve congestion along the Seward Highway, which goes from an average daily traffic of 9,000 to peaks of 20,000 vehicles for several days in July each year. This is a cost-effective option to maximize existing infrastructure while improving safety.

The Seward Highway passing lanes between mileposts 60-65 are an ideal section of highway to test the program because it is a newer section of highway with good geometry and few destinations located along the route. The right hand (slow) lane will be marked for 55 mph, and the left hand (fast) lane will be marked for 65 mph. The public is encouraged to observe the regulatory speed limits and allow others to pass when driving in the right lane.