This research investigates the actual effects of stringer flexibility and slenderness on the quality of Highway bridges and seeks to find improved criteria which can help the designer achieve a high quality, economical structure. The investigation considers effects of flexibility and slenderness on peak stresses, stress range and deflections due to live loads and their dynamic effects. These effects are directly related to gross yielding, deck cracking, and fatigue damage, and are correlated with possibilities of bridge deck deterioration and human discomfort from bridge motions.