Dowling Rd. & Seward Hwy.
Anchorage, AK
BACKGROUND
The existing roundabouts built in 2004 at the Dowling Road/Seward Hwy. interchange were experiencing excessive congestion and delay.
Part of the National Highway System, the Seward Highway is an urban principal arterial interstate located within the Municipality of Anchorage, AK. The Seward Hwy./Dowling Rd. interchange is a diamond form interchange with roundabouts at the ramp terminals. These were Alaska’s first multi-lane roundabouts and were designed to accommodate projected 2020 traffic. They fit within the highly constrained existing infrastructure and replaced poorly operating signals. The roundabouts have now exceeded the 2020 traffic projections and are experiencing substantial congestion in peak hours.
SUMMARY
MTJ’s roundabout analysis and design work as part of the Lounsbury project team supported a Preliminary Engineering Report (PEF) to re-evaluate and update a ‘Major Investment Study’ (MIS) and an ‘Environmental Assessment’ (EA) to determine the preferred interchange type.
MTJ provided the roundabout-specific design and analysis component for this study that included seven different interchange forms. An initial screening effort narrowed the seven initial alternatives down to two viable options; tight diamond and roundabouts. These were carried forward to preliminary design for a more detailed comparison.
The roundabout alternative was selected due to:
- Minimized ROW and environmental impacts
- Ability to meet traffic planning objectives to 2040 traffic operational objectives (for all modes)
- Superior business access circulation with u-turn capabilities at each roundabout
- Increased capacity achieved with larger ICD, no new laneage necessary
MTJ’s extensive roundabout design experience played a pivotal role in achieving the optimal solution for this interchange by delivering designs that ensured accurate evaluation of feasible alternatives and high levels of confidence in the decision-making process.
MTJ is again teaming with Lounsbury and Associates on the final design. Construction is planned for 2020.
HIGHLIGHTS
Intersection control evaluation/feasibility study evaluated six interchange designs to replace existing over-capacity roundabouts built in 2004
Study recommended new roundabouts for the ramp terminals
Operational objectives met with modified roundabout geometrics using Rodel Geometric Capacity Model —
- Increased capacity with larger ICD
- No new lanes necessary
Roundabouts provided lower cost with superior business access circulation
Improved safety for all modes
Tight Diamond Alternative: $30 – $40 million
Roundabout Alternative: $25 – $35 million
