Abstract: A modular bridge deck system supported on a plurality of cooperating girders and the deck system that comprises a plurality of deck panels secured together to form a modular bridge deck. Each deck panel is preferably formed by longitudinally shop friction-stir welding a plurality of elongated, multi-void, extruded aluminum structural elements. A top surface of each respective deck panel and the longitudinal shop-welding form a substantially continuous top surface of the modular bridge deck. In addition, the modular bridge deck has a depth and weight that is substantially equal to a weight of a steel open-grid deck of a moveable bridge or fixed span bridge to be replaced by the modular bridge deck system.