SR-18 Bluff Street and Red Hills Parkway Interchange

Wilson & Company worked with the City of St. George and the Utah Department of Transportation (UDOT) to evaluate three design alternatives: no-build, Tight Diamond Interchange (TDI), and a Center-exit Interchange at the intersection of Bluff Street and Red Hills Parkway. Using traffic analysis software (i.e., Synchro and SimTraffic), a grade-separated Center-exit Interchange was selected as the best alternative. This configuration improves traffic operations over the TDI, similar to a Single Point Urban Interchange (SPUI), lowers the project costs by utilizing existing pavement, and increases safety for pedestrians and trail users.

Bluff Street and Red Hills Parkway are key routes in St. George. The existing at-grade intersection needed to be improved to accommodate the projected travel demand. The new configuration operates similarly to a SPUI; however, the through lanes are to the outside, and center exits onto Red Hills Parkway and Snow Canyon Parkway with on-ramps that merge onto Bluff Street. The project consists of reconstructing the existing intersection with a bicycle/pedestrian trail system that includes five grade separations, allowing pedestrians and bicyclists to get through the interchange without having to cross traffic on Bluff Street.

Additional project elements include:

  • Structures: Preliminary design included a structure type study and a cost comparison of different retaining walls. The final design includes two single-span bridges, five MSE retaining walls, three overhead sign structures, a concrete box culvert pedestrian underpass, and a drainage diversion box. Load ratings were performed on both bridges and the concrete box culvert.
  • Traffic Analysis and Design: Using traffic analysis software helped determine the required auxiliary storage lane lengths for the interchange and assessed pedestrian crossing and traffic signal phasing options.
  • Environmental: Amended the Red Hills Parkway EA documents and coordinated a revised Biological Opinion for impacts to the Desert Tortoise preserve.
  • Other Design Elements: Roadway and intersection, safety improvements, drainage (storm drain system and hydraulics), electrical, lighting, survey and ROW plans, utility design and coordination, MOT, roadway modeling, trail options, landscape and aesthetics, geotechnical, and SUE.