Extending from historic downtown Mesa to Gilbert Road, the $117-million, 1.9-mile light rail extension called for a significant engineering innovation—the construction of a four-way roundabout featuring an automobile interface with the rail line.

The arrangement, said to be the first of its kind in the U.S., involved complex train control and traffic signaling during design and construction as well as extensive testing and safety certification before completion.

During preconstruction, project team members, including traffic engineers, collaborated to produce a roundabout design with the proper radii to ensure the intersection could handle the traffic load. The roundabout needed to stack a sufficient number of vehicles in the queue for northbound and southbound traffic when a train was present while still providing free-flowing movement for eastbound and westbound traffic.

The project’s negotiated contract value exceeded the original advertised estimate, so the team performed constructibility reviews and value engineering, identifying and mitigating project risks and bringing estimates within budget. The focus was on meeting budget and schedule while maintaining quality and safety.

However, the budget cuts reduced right-of-way acquisitions, requiring late design revisions. To minimize schedule delays, the team worked with the city to estimate how long it would take to acquire each parcel and then built that information directly into the schedule.

The completed extension included roadway work to make room for the center-running track as well as construction of two new rail stations, traction power substations and a park-and-ride lot. Crews also replaced signals and street lighting. New storm runoff detention basins provide better rainwater storage capacity.

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