495/90 Interchange Reconstruction Project

Use the interactive map to the right to view the project in relation to the natural resources delineated by the VHB team.

The I-495/I-90 interchange redevelopment project is situated among hundreds of acres of environmentally sensitive areas that include the Sudbury River and its watershed, vernal pools, and many acres of Atlantic White Cedar swamp—an endangered wetland habitat that is a haven for rare plant, animal, and insect species.

VHB began by physically mapping these areas using 3,600 wetland flags, then used HydroCAD and HEC-RAS technologies to model how the areas would be affected by changes to the interchange during and after construction. VHB’s design work comprised four major environmental considerations:

• Complying with environmental regulations and permit requirements
• Preserving wetland resources
• Managing stormwater
• Designing hydraulic systems

Our scientists and technicians monitored impacts on environmental areas through numerous final design iterations and development of construction staging packages, closely coordinating with MassDEP, MassDOT, and the construction team throughout. VHB’s extensive knowledge of Massachusetts’ environmental regulations, experience designing solutions to complex environmental challenges, and access to innovative technological tools equipped us to keep the project on track by complying with state and local environmental regulations while improving runoff conditions for more than 650 acres of roadway.

Our designers ultimately devised solutions that not only preserve the existing ecology but also establish new wetlands in some areas that had previously been paved—all while maximizing driver safety during and after storm events.

The BTC for the ramps between I-495 northbound and I-90 eastbound bordered sensitive environmental resource areas, including an archaeological resource area, wetlands, an intermittent stream, and land that falls under Massachusetts Department of Conservation and Recreation Article 97.

VHB redesigned these ramps with an ATC that leaves more space between vulnerable areas and structures, allowing for over six acres of wetland restoration.

Because our ATC was only considered a minor change to the BTC, it did not require extensive review by regulatory agencies or permit amendments under the Massachusetts or National Environmental Policy Act (NEPA), minimizing the potential for project delays.

The ATC mitigates stormwater by reducing impervious cover (the result of eliminating three structures in the BTC) and meets Massachusetts environmental performance standards for bordering lands subject to flooding and stream crossings. It also pulls the proposed infrastructure further away from the archaeological resource areas.

Shown right: A 3D model of the proposed interchange, showing both overhead and driver viewpoints.

In the proposal phase, VHB developed a 3D model of the project to map proposed alignments and evaluate their temporary and permanent impacts on the surrounding area and its environmental resources. Using model-based design software, we created a 3D rendering of the interchange, allowing VHB and MassDOT to digitally evaluate and visualize alternatives.

Ultimately, VHB’s design included 13 total alignments—encompassing I-90 eastbound and westbound, I-495 northbound and southbound, Fruit Street (which runs over I-495 adjacent to the interchange), and eight ramps in between the two highways—to realize the final interchange configuration envisioned by the BTC and our added-value ATC.

Our designs, including our approved ATC, improved the alignments, reduced profiles and wall heights where necessary, optimized structure depth, and minimized impacts to wetland resource areas while still maintaining minimum vertical clearances between roadways and railroads.

Highway designs also included guardrail, retaining wall profiles, side slope grading, drainage, and grass swales.

Shown right: Initial Traffic Staging Plans.

To minimize the project’s impacts on drivers traveling through the interchange, MassDOT required the team to maintain three lanes in each direction on the I-495 and I-90 mainlines during peak travel hours, while also maintaining all ramp connections between I-495 and I-90 throughout construction. This presented the design-build team and VHB’s traffic engineers with the complex task of facilitating travel and construction simultaneously throughout the project’s five major construction stages.

To do so, VHB’s traffic engineers collaborated with our construction partners and designed temporary lane shifts, a temporary crossover of I-495 southbound, temporary ramp alignments, temporary barriers between travel lanes and the work zone, and new signage and pavement markings for shifted mainline lanes and ramps to help maneuver traffic safely through the project area.

We also planned access and egress points for construction staff, equipment, and materials to safely access and egress the work zone. All temporary traffic controls are supported by a Real-Time Traffic Monitoring (RTTM) system that provides travel time and speed information to the traveling public.