NOTE: The structurally deficient rating is an early warning sign for engineers to use to prioritize funding and to initiate rehabilitation 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 2015. This submission reports that there are 2565 bridges on the Maryland State Highway System, of which 81 are classified as structurally deficient, or about 3%. The number of structurally deficient bridges has decreased from 143 in 2006 to 69 in 2015.