According to the United States 2014 National Bridge Inventory the number of concrete bridges far outstrips the number of steel bridges.
Reinforced Concrete: 253,336
Prestressed Concrete: 148,333
TOTAL CONCRETE: 401,669
TOTAL STEEL: 181,095
Why would concrete bridges outnumber steel bridges?
Basis for Categorizing By Material Type
The concrete and steel totals are based on the "Main Structure Type" inventory code from the NBI Recording and Coding Guide. This is a three digit code defined as shown below.
Motivation for Question
In the state where I work we often design steel and concrete "alternates" for each bridge. However, we consistently see the concrete alternate (usually prestressed) selected instead of the steel. I'm trying to understand why this would be since steel seems to offer quite a few benefits...quick construction in the field, predictable material properties, often lighter than concrete girders, damage is usually readily inspectable, etc. etc. I've always chalked it up to some sort of inertial regional preference (when I was in Chicago we saw tons of steel, but in California and Nebraska it's mostly been concrete) and thought I'd see if there were some more substantive reasons.