At the preliminary design stage for a highway bridge 81 m long, a three-span continuous girder, including a span of about 30 m, is preferable from the point of construction cost. Generally, the concern is that girder bridges with spans of 30 m, which have a fundamental natural frequency of about 3 Hz, have large traffic-induced vibrations due to the coupling dynamic response with heavy vehicles having a bouncing natural frequency of about 3 Hz. Furthermore, large traffic-induced vibration in girder bridges causes low-frequency sound (LFS) as one of the environmental vibration problems. Two kinds of girder bridge with a length of 81 m - a concrete hollow slab bridge and an eight-girder steel bridge with the same spans - are proposed. For assessing the low-frequency sound radiated from bridges, firstly, the traffic-induced vibrations of two kinds of girder bridge are compared with each other by analysing the coupling dynamic response of bridges due to moving heavy vehicles in a dynamic system.