IN 2012, LANDSCAPE ARCHITECT Guy Michaelsen found himself strolling the Alaskan Way Viaduct as part of a Seattle Department of Transportation team inspecting the roadway’s structural integrity. Just below Pike Place Market, he fell in love.

A gray-green sign bridge — beneath which he had driven hundreds “if not thousands” of times — stopped him cold. “It was a treasure hiding in plain sight,” he says. “Once I got to see it at walking speed, I realized it was remarkable.”

The curving steel legs, 33 feet tall yet showing only 16 above the deck, bore art deco ornamentation and oval cutouts — infrastructure from another era, made, as Michaelsen puts it, “with a sense of design, not just utilitarianism.”

The viaduct itself long divided Seattle. Opened in 1953, the double-decker highway offered automotive commuters a free, front-row view of Elliott Bay. But concerns about seismic safety — and the noisy barrier separating downtown from its waterfront — eventually won the day.…