One substantial problem in transportation policy is simply that the institutional culture at many state departments of transportation is rotten. Basically, they exist to find ways to make the case for building more highways, and perhaps to make decisions about where new and wider highways should go. Other considerations often aren’t on the table at all.
Thus, Dave Alpert’s flow chart:
Things are changing, though often not fast enough. In my neighborhood, a dense walkable area, we had a movement a little while back to change how one intersection worked so that cars would go slower and it would be safer to cross the street. We were told, basically, that the change couldn’t be done because it would make the cars go slower.