NOTE: The structurally deficient rating is an early warning sign for engineers to use to prioritize funding and to initiate repairs or to begin the process to replace the bridge. The rating applies to three main elements of a bridge: 1) the deck (riding surface); 2) the superstructure (main supporting element of the deck, usually beams, girders, trusses, etc.); and 3) the substructure (supports to hold up the superstructure and deck, usually abutments and piers). These elements are rated on a scale from zero (closed to traffic) to nine (relatively new). If any of the three elements is rated as a four or less, the bridge is categorized as structurally deficient by federal standards. This does not mean that the bridge is unsafe. If a bridge becomes unsafe, it will be closed.
The summary below is taken from the annual submission (PDF, 29kb) to FHWA made in April 2009. This submission reports that there are 2,578 bridges on the Maryland State Highway System, of which 114 are classified as structurally deficient, or about 4%..