I believe swing bridges were more popular before WWII when they were thought to be economical and there was less objection to having a pier in the middle of the waterway. After the war, the US government would help pay the cost of replacing or removing obstructions in waterways. Most of these obstructions were old swing bridges.
Sutter Slough Bridge was built in 1939 in Courtland California. The swing mechanism sits on a three column bent. The bridge carries two-way traffic (on two lanes), it has a total length of 397 ft, and it carries an average of 6020 vehicles a day.
Movable Bridges - Sutter Slough Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.