A trailer skirt includes a plurality of panel modules attached under and along sides of a trailer. Each panel module comprises an outwardly angled flat panel with upper edge attached to a trailer undercarriage frame and a forward edge tabbed to slots in an inward-stepped rear edge of an adjacent identical panel module. The stair-stepped surface and leak-free connection provides excellent low air drag, and also provides good orientation of reflectors for lights when passing another vehicle. A modularity of the panel modules and their low weight allows them to be molded in an efficient and low cost manufacturing operation, shipped in a very compact and dense arrangement, quickly installed, and allows individual panel modules to be replaced without total disassembly of the skirt.