Seems so miniscule. This is what I thot was most interesting: Construction proceeded using mining techniques rather than traditional tunneling techniques, starting from a stope and working outward to the portals.

In the 1920s this end of the canyon appeared to be a dead end, an impassable barrier to transportation. To highway engineers the toughest challenge was the cliff above. Their solution: a one-mile tunnel behind the cliff face.
The highway features a 5,613-foot tunnel that follows the profile of the Pine Creek Canyon wall at a consistent distance of 21 feet from the outside face of the rock to the centerline of the tunnel.