Those roads and bridges were built with bonds and debt. That entire business model requires future profits from taxation to eventually pay the bonds or debt. So in areas where the local tax base has collapsed (Detroit, California Cities (4), etc.) the municipalities that issued the bonds and acquired the debt have gone bankrupt. Since some of the places that went bust, had decent roads and bridges, do not think the roads and bridges are the main issue.
Those roads and bridges were built with bonds and debt. That entire business model requires future profits from taxation to eventually pay the bonds or debt.
Not only that, but most of the actual roadbuilding was done by private contractors. In my work at Mississippi DOT (over 20 years), even the shadiest, corner-cutting contractor did a better job than your average MDOT roadbuilding crew. Now, with some jobs becoming "Design-Build" (where the Contractor actually designs the road and builds it, instead of taking the plans DOT gives them and having DOT inspectors looking over their shoulders at everything), the government has even LESS involvement in building roads and bridges.