Continuing north on Route 54D we come to a sign announcing we are about to cross the two-span Pialla Bridge. The Highway Department is justly proud of this large structure. The sign says the bridge is 158 m (520 ft) long and 89 m (292 ft) tall. It looks like another segmentally constructed cantilever structure, similar to Viaduct Beltran. We'll take a closer look at it tomorrow.
By the way, you may have noticed the steep cuts on the sides of the road, which are covered in wire mesh to protect motorists from falling rocks. My experience is that this mesh is quite effective for anything less than a large rockslide.
We are driving along the side of two large active volcanoes, and so the rock is probably cemented tuff or other pyroclastic material. Mexico's Pacific Coast is along a large subduction zone that causes many earthquakes and creates enough pressure underground to melt the underlying rock and cause active volcanoes.
Mexico's Bridges: Puente Pialla (1) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.