Following World War I, looking to improve access to Oakland, the city undertook to add a modern highway skirting the edge of the Bluff. The fast way to Oakland at that point was via Bigelow Boulevard, at the time Grant Boulevard, which looped to the north around the Hill District to reach Oakland from the northeast.
In its first phase, the Boulevard supplanted Second Avenue downtown, then took to a viaduct to reach the Bluff before petering out just past the Brady St. bridge on the south side of Forbes Avenue. (That's why, to this day, the road has an odd configuration just east of the Birmingham Bridge where the Boulevard passes overhead.)
In 1928, the road was extended on to Oakland, ending at Bates St. on the Zulema St. alignment. A large bridge over Forbes Avenue was erected by the McClintic-Marshall company, as could until about 10 years ago barely be read in painted letters on the girder over Forbes. Still later, the Charles Anderson bridge replaced the old Wilmot St. bridge and the Boulevard of the Allies reached its full extent, connecting to the broad road through Schenley Park.
Because Forbes Avenue hosted two way traffic in 1928, the bridge used to cross it featured an inbound ramp on the north side, and a second bridge to allow outbound traffic to enter Forbes from the right-hand side. When Forbes became a one way street, the inbound ramp became an odd, block-long one way street. The entrance to the Boulevard featured no acceleration lane, and the configuration made seeing approaching traffic difficult. As speeds increased, the narrow, curving Boulevard of the Allies bridge became a liability. Lack of maintenance didn't help, either.
In 2007, a project began to remove the old bridge and replace it with a modern span lacking the dogleg ramp to outbound Forbes Avenue (and forever locking Forbes into one-wayness). Here are a few pictures of the bridge before and during demolition, and today's reopening. (As a note, Historic Pittsburgh has shots of the original end of the Boulevard as well as the previous bridge just before its opening.)