The bridge exists because a town had to drown. When the New Don Pedro Dam was authorized in the 1960s, its reservoir would submerge the old highway alignment through the Tuolumne River canyon near Chinese Camp. A replacement crossing was needed, and it had to be finished before the water rose. The stakes sharpened every decision. With the tallest column standing roughly 230 feet above the canyon floor, conventional falsework for a concrete superstructure was impractical. The engineers turned to steel: painted curved girders fabricated by the San Jose Steel Company and trucked to the site. Peter Kiewit and Sons, the general contractor, carved a gravel access road with switchbacks into the canyon walls to reach the work zone. Steel girders were erected from the canyon floor, then lifted and connected from the cantilevered tips of the north and south spans. The bridge opened to traffic in 1971, ahead of the rising reservoir.

The highway planners of the 1960s assumed that traffic through the Sierra foothills would eventually demand a four-lane highway. They asked Roberts to design a bridge that could be widened from two lanes to four. He obliged with characteristic thoroughness. The columns and foundations were engineered to support a second superstructure. The top 70 feet of each column taper into an equilateral triangular cross-section, shaped specifically to carry the additional lanes. The initial two-lane deck was placed on the outside of the curve, which required eccentric loading analysis. The foundations, excavated from bedrock, are essentially hexagonal to stabilize the columns against the asymmetric weight. More than fifty years later, the second superstructure has never been needed. North-south traffic in central California flows through the flat Central Valley on Interstate 5 and Highway 99, leaving Highway 49 as a winding, scenic route through the foothills. The bridge's overbuilt bones remain a monument to cautious planning.

For thirty-six years after the bridge opened, it was simply the Tuolumne River Bridge. In 2007, the California legislature renamed it for the man whose first big project it had been. By then, James E. Roberts had become one of the most consequential bridge engineers in American history. As California's Chief Bridge Engineer, he led the state's response to the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake, directing the reconstruction of damaged structures and spearheading Caltrans' 4.5-billion-dollar seismic retrofit program. Under his leadership, Caltrans became a world leader in seismic research. He was elected to the National Academy of Engineering in 1996 and received the Roebling Medal for lifetime achievement in bridge engineering. Roberts retired in 2001 after more than fifty years of public service, including time in the U.S. Army and Army Reserves. He died in 2006, a year before the bridge received his name.

Today the bridge carries two lanes of traffic between Sonora and Groveland, serving as a gateway to Yosemite from the west. Drivers crossing it may not realize they are on a curve, the radius is gentle enough that the experience feels natural. But from below, or from the air, the geometry reveals itself: the arc of the deck, the triangular columns rising from the canyon, the hexagonal foundations planted in bedrock. The AISC jurors who awarded the bridge their Medium Span Steel Bridge prize in 1972 noted "the clean curving superstructure and the sculptured piers." It remains a working piece of infrastructure on a quiet highway, carrying modest traffic through Gold Country. The canyon it spans now holds the northern arm of Don Pedro Reservoir, the same body of water that necessitated its construction. Lightweight concrete on the deck, normal-weight concrete in the piers, steel girders that follow the curve of the road: every material choice reflects an engineer thinking through a problem that had no standard solution.