A semi-trailer has a rear impact guard including an elongated horizontal member that attaches to a trailer by a pair of vertical guards. The horizontal member may have a top surface, a bottom surface, and a leading edge from which a surface of the guard member extends rearward to the top or bottom surface. The rear impact guard may include a pair of gussets that extend from the vertical guards. Each gusset may include a main generally planar portion, a first stiffening flange perpendicular to the main portion, and a second stiffening flange perpendicular to the first stiffening flange.