The curving white roadway beside the Bay Bridge is a detour, which will be connected to the cantilever section over Labor Day weekend.
A Caltrans animation shows how an old section of bridge will be cut out, rolled off, and a new double-decker section will slide in to connect the detour. All of this will occur 150 feet in the air.
Motorists will have quite an adjustment to a new winding section of bridge. The half-mile detour is needed to allow demolition of the old bridge and completion of the new one over the next four years.
"The new eastern span of the Bay Bridge has traffic side by side, but as soon as you get to Yerba Buena Island Tunnel, that traffic has to transition into a double-deck configuration and that will be the next structure we have to build right in this location," says Bart Ney, from Caltrans.
Ironically, the section of the bridge that will be cut out Labor Day weekend was not made to be moved, so workers are now strengthening it for removal.
Resident engineer Bill Casey calls the elaborate plan "extreme engineering."
"We have an hour by hour schedule, redundancy on equipment, men and materials," Casey.
The only weather condition they're worried about possibly slowing down operations on Labor Day weekend is wind. The structures and equipment can handle high winds, but the human workers cannot.
"I don't think there's an absolute line that says it reaches this wind speed and it's done, but it's fair to say that around 30 miles an hour we're going to be checking workers safety and how everyone's feeling out there," says Ney.
Another unforeseen potential obstacle to staying on schedule is man-made. The furlough Fridays created by the state budget crisis mean Caltrans staff, like lead engineers, have to take three days a month off, but the contractors are not affected; so Caltrans bridge staff are off on days that least affect progress. The target date for completion is 2013.