I-66 / US Route 15 Diverging Diamond Interchange (DDI)

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Project Location: Prince William County and Town of Haymarket, VA

Client: RDA and VDOT

Description of Project: VDOT issued a design-build request for proposal (RFP) on September 24, 2013 (and Addendum No. 1 on December 20, 2013) to reconstruct the I-66/US Route 15 interchange, including adding auxiliary lanes in the vicinity of the I-66/US Route 15 interchange on US Route 15 (James Madison Highway), VA Route 55 (John Marshall Highway/Washington Street), construction of a new service road, and replacement of northbound and southbound bridges carrying US Route 15 traffic over I-66 with a single bridge.

T3D’s Role in Project: T3 is part of the design-build (D-B) team that proposed a diverging-diamond interchange (DDI) concept on U.S. 15 at Interstate 66 as an alternative concept in response to VDOT’s initial design-build RFP. As part of the D-B team’s response to the RFP, T3 conducted an initial analysis that demonstrated that the DDI, which shifts vehicles to the opposite side of the road and eliminates left turns that cross oncoming traffic thus improving safety, was feasible.

Recently VDOT accepted this concept and awarded a $36.2 million contract to design and build the DDI. Under this newly awarded contract, T3 is performing the following work in support of the D-B team:

  • Traffic Analysis: T3 is preparing an update to the traffic analysis for the project using VISSIM modeling software. All traffic modeling will adhere to VDOT and FHWA requirements and will be used to validate that the DDI concept operates at an “equal or better” level than the original proposed alternative.
  • IMR Update: T3 is preparing a supplement to the previously approved VDOT Interchange Modification Report (IMR), analyzing the now-preferred DDI alternative.  The supplement details operational and safety benefits of the DDI design. T3 is participating in coordination meetings with VDOT and FHWA to gain final approval of the IMR update.
  • Traffic Signals: T3 Design is preparing traffic signal plans for full signal construction or modification of existing signal equipment to accommodate the DDI and modifications at the following adjacent signalized intersections: Route 15 at Heathcote Boulevard; Route 15 DDI crossover at I-66 off-ramps D2 and A2; and Route 15 at Route 55 (John Marshall Highway/‌Washington Street). These traffic signal plans will include foundations, traffic signal poles, signal heads, conduit system, junction boxes, circuitry, video detection devices, emergency preemption, signal cabinets, control equipment, uninterruptable power supply (UPS), terminus lighting, signal interconnect conduit and wiring, and signal-related signing. 
  • Temporary Traffic Signals: T3 is preparing temporary traffic signal plans for the four signalized intersections to provide traffic control as needed during each phase of construction.  Temporary signal requirements will be coordinated with the traffic control plans to achieve the established project construction sequence.
  • Transportation Management Plan (TMP): T3 is assisting in development of a TMP, which will document the sequence of construction to show how traffic will be managed during each of the multiple construction phases. 
  • Work Zone Traffic Analyses: T3 is preparing a work zone traffic analysis as necessary to support the temporary traffic control plans and includes work zone speed analyses necessary to support speed reduction requests by the design-builder.
  • Traffic Operations Strategies: T3 is preparing transportation operation strategies including incident management plans.

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