Performance Analysis of the Dowling Multi-lane Roundabouts

AUTC project number: RR08.08

PI(s):

Ming Lee (UAF)

Funding:
  • US Department of Transportation (RITA)
  • Alaska Department of Transportation & Public Facilities
  • Start Date: Mar 3, 2008
  • End Date: Aug 31, 2009

Project Summary

The first multi-lane roundabouts in Alaska were constructed in 2004 at the ramps of the Dowling Road/Seward Highway interchange in Anchorage. These serve as junctions for commuters accessing the Seward Highway. As vehicle traffic in Anchorage continues to grow, however, use of the Dowling roundabouts also increases. The roundabouts are currently operating at or near capacity, with long vehicle queues at their entrances during peak traffic hours. This research project examined the performance of multi-lane roundabouts and how drivers use them. Analysis showed that extended queues were due to unbalanced flow patterns at the roundabouts, causing high circulating flow in front of one roundabout. This high circulating flow resulted in low-capacity, high-delay queue values. Researchers also found that accident rates and danger to pedestrians had increased in the past two years. Modeling traffic flow patterns for several possible alternatives suggested that reducing the eastbound flow rate "upstream" of the roundabout by 70% of the original flow could result in an acceptable level of delay and queue length at the eastbound approach of the west roundabout.