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Improving Connectivity: The Custer Interchange Project

Appendix C Roadway/Geometric Design Skill Set

Idea Name Detailed Description Implementation Details (barriers, skills set coordination, etc.)
Roundabouts
  • 2 lane roundabouts.
  • Roundabouts at both sides at the ramp termini on Custer as well as major intersections Washington and Montana.
  • Phase sections from ramp termini to ramp termini.
  • 5 roundabouts.
  • Benefits:
    • Safety.
    • Efficiency.
    • Air quality.
    • Mobility.
    • Less R/W.
    • Improved Access Control.
    • Narrow up structure by eliminating median turn lane.
  • Implementation Details:
    • Acceptance from the Chief Engineer.
    • Public Buy In.
    • Traffic Modeling.
  • Barriers:
    • R/W might be needed at Montana & Washington.
    • EIS commitments.
    • Capacity Concerns.
    • Close Proximity to Adjacent intersections.
  • Skill Sets Involved:
    • Traffic.
    • Public Relations.
    • Roadway Design.
Offset Alignment
  • Build new bridge to N.; keep traffic on existing.
  • Eliminate need for Cedar detour.
  • Allows phasing under funding constraints.
  • Implementation Details:
    • Connection to existing road.
    • Traffic Control.
  • Barriers:
    • MDT and Local agencies desire to widen Cedar.
    • Introducing curves on Custer.
  • Skill Sets Involved:
    • All.
    • Public Relations.
    • Structures.
Project Phasing (when funding is available for each phase)
  • Phase 1: Construct a two lane bridge to the north of the existing bridge and utilize bridge for westbound traffic. Use existing bridge for eastbound traffic. Widen Custer to north.
  • Phase 2: Remove existing structure and build second half of ultimate bridge. Use first phase bridge as detour.
  • Phase 3: Build interchange ramps and complete Custer. Allows phasing under funding constraints.
  • Implementation Details:
    • Bridge Design.
    • Connecting to existing alignment.
  • Barriers:
    • Structural condition of existing bridge.
    • Seismic condition.
    • Difference in deck elevations.
  • Skill Sets Involved:
    • All.
Diamond Interchange
  • Eliminate the C2 loop ramp.
  • Traffic conflicts eliminated with C2 and C3 merges.
  • Implementation Details:
    • Excess R/W.
  • Barriers:
    • Structural cost.
    • Left turn phasing will require additional storage Impact traffic operations.
    • Additional R/W in southwest quadrant.
  • Skill Sets Involved:
    • All.
Compressed Diamond
  • Eliminate the C2 loop ramp.
  • Traffic conflicts eliminated with C2 and C3 merges.
  • Eliminates left turn storage between ramp terminals. (Single controller runs both ramp signals.)
  • Implementation Details:
    • Excess R/W.
    • Retaining walls.
  • Barriers:
    • Structural cost.
    • Left turn phasing will require additional storage Impact traffic operations.
    • Additional R/W in southwest quadrant.
  • Skill Sets Involved:
    • All.
Three Level Diamond Multiple Bridges and single point roundabout. Takes all through movements out of the intersections.
  • Implementation Details:
    • Design.
  • Barriers:
    • Cost.
    • Additional bridges.
  • Skill Sets Involved:
    • All.
Custer Avenue Widening
  • Widen Custer put ramps in later.
  • Immediate reduction in congestion on Custer.
  • Allows phasing under funding constraints.
  • Allows time to secure additional funding and partnerships.
  • Implementation Details:
    • Department and local agencies buy in.
  • Barriers:
    • Public expectations.
    • Developer expectation.
  • Skill Sets Involved:
    • All.
Move I-15 Interchange 1500 ft. North Keep Custer overpass but relocate interchange north of Lowes between Target and Hastings.
  • Implementation Details:
    • Complete New Design Studies.
    • Environmental Studies.
  • Barriers:
    • Canal.
    • Utility.
    • Cemetery.
    • Reopening EIS.
    • Not a direct connection to Custer.
  • Skill Sets Involved:
    • All.
Sanders to Montana
  • Consider reduced width section, Sanders to Montana.
  • Share sidewalk path on north side only.
  • Implementation Details:
    • Acceptable widths according to AASHTO guidelines.
  • Barriers:
    • Safety.
    • Bicyclists.
    • Pedestrians.
    • Opposing traffic for bicyclists.
  • Skill Sets Involved:
    • All.
Reduced Vertical Clearance
  • Lower bridge to 16 ft. plus future overlay allowance to reduce adjacent fill expense.
  • 16 ft. is the AASHTO Guidance.
  • Allowance is included for future overlay.
  • Implementation Details:
    • Bridge.
  • Barriers:
    • MDT policy 17ft.
  • Skill Sets Involved:
    • Bridge.
Fill Slopes
  • Reduce fill slopes to 2:1 from Sta. 18+25 to 19+25.
  • Save fill costs.
  • Reduce R/W impacts.
  • Guardrail will be required.
  • Implementation Details:
    • Design Exception.
  • Barriers:
    • None.
  • Skill Sets Involved:
    • None.
Lower Interstate Lower the interstate three ft. to match existing bridge deck on Custer.
  • Implementation Details:
    • Interstate Design.
  • Barriers:
    • Drainage Issues.
    • Utility.
    • Interstate traffic control.
  • Skill Sets Involved:
    • All.
Single Point Urban Interchange (SPUI) Custer Rd. at grade; I-15 crosses over top. SPUI on Custer. Minimize R/W requirements and intersections along the corridor. Increase the capacity on the ramp terminals. Enable left turns to interstate.
  • Implementation Details:
    • Interstate Design.
  • Barriers:
    • Drainage Issues.
    • Utility.
    • Interstate traffic control.
    • Difficult pedestrian accommodation.
  • Skill Sets Involved:
    • All.
Investigate the Weave from Custer to Cedar Interchange
  • Use ramp C2 to develop 3rd full lane on I-15 South.
  • Ramp C3 will need acceleration lane.
  • May increase capacity of the weave.
  • Implementation Details:
    • Design.
  • Barriers:
    • Final Bridge Length.
  • Skill Sets Involved:
    • Road Design.
    • Traffic.
Sanders Street
  • Build Ramp C1 from southbound I-15 to meet with Sanders St.
  • Reduces one traffic signal on Custer.
  • Implementation Details:
    • Requires change to EIS.
  • Barriers:
    • R/W
    • Access Approval.
  • Skill Sets Involved:
    • Road Design.
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Updated: 10/31/2013
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000