The original girder bridge was, of course, a railroad bridge which was built: 1908 - 1912. In 1931 the Florida East Coas Railroad went bankrupt and in 1935 the line was damaged in a hurricane and service was abandoned.
The State of Florida bought the roadbed from Flagler's widow and converted the bridges into highway bridges for US. Route 1. They did this by decking over the top of the railroad bridge to make it JUST barely wide enough for two cars to pass. (Trucks had to fold in their driver's side mirrors or they would get broken.) They used the rails as railings. The picture shows the amount of distance that the highway hung over the basic railroad track structure.
In ~1980 The Seven Mile Bridge was closed entirely, and some sections were demolished. This made the middle of the old bridge inaccessible and also allowed sailboats to get through Moser channel.