This bridge reminds me of a prefabricated bridge system like the old Bailey Bridges that could be easily transported and set up in the field. At Caltrans we often use the undercarriages of old railroad cars as temporary bridges. Maybe this system is similar. It looks like it could be used as falsework to support bridge forms when pouring concrete superstructures.
Note the PVC pipe sticking out of the concrete barrier rail. This must be the deck drainage system to prevent the bridge from carrying too much water and possibly hydroplaning vehicles. Usually, we drain the deck with ducts that carry the water through the columns, which is a pain since the columns are already stuffed with reinforcement. Also, the ducts are always getting filled with debris and are hard to clean.
The truck that we see crossing the bridge has a tandem trailer (illegal in California) and it's carrying what looks like an heavy load of bags of cement and rebar cages. This suggests that at least this single-span bridge with its odd understructure was well-designed.
Mexico's Bridges: Puente El Tapon by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.