Lahaye Drive at Mount Support Road Intersection Improvements, Lebanon, New Hampshire

The intersection of Lahaye Drive and Mount Support Road is one of two main entrances to Dartmouth Hitchcock Medical Center, the largest hospital in New Hampshire and home to a Level I Trauma Center. As the hospital has grown and new residential communities have been built nearby, with additional housing proposed, traffic volumes at this intersection are expected to exceed its operational capacity. To address this, the city sought a long-term solution that could accommodate future traffic demand while improving safety, multimodal connectivity, and overall accessibility for all users. Several alternatives were evaluated; however, a hybrid two-lane roundabout emerged as the preferred long-term solution. This alternative best addressed existing traffic needs, provided sufficient capacity for future growth, and enhanced safety and connectivity for pedestrians, bicyclists, and vehicles.

The project faced several significant site constraints that required close collaboration with city staff, local agencies, and state regulators. Wetlands identified by GM2 were found in all four quadrants of the intersection and along both sides of Lahaye Drive east of the intersection. To minimize wetland impacts, lane and shoulder widths were reduced, and the roundabout’s footprint was carefully refined. Vehicle turning and traffic modeling showed that the compact roundabout design could safely accommodate all required vehicle movements while operating at an acceptable Level of Service*.

The project area includes Appalachian Barren Strawberry, a rare plant found in several spots along the roadway. While it was not possible to avoid all impacts on this species, further site investigations by GM2, in collaboration with the New Hampshire Natural Heritage Bureau, identified approximately 42,000 plants in the area. Because of this large number, the species was not considered rare at this site, so no mitigation was needed.

Managing stormwater management posed another key challenge due to the proximity of nearby wetlands, and the existing drainage infrastructure prevented the use of traditional stormwater ponds or infiltration systems. By working with the New Hampshire Department of Environmental Services, the team received approval to use hydrodynamic separators, which provide improved water-quality treatment.