Early identification of voids under a roadway structure is critical on preventing major failures from occurring. In this study, a 400MHz Ground Coupled Penetrating Radar (GCPR) was utilized to characterize the subsurface conditions of two roadway pavements (US290 and 34 th Street). A huge (1.8m*4.6m*3.7m=30.6m 3 ) void under US290's reinforced concrete pavement was successfully identified by GCPR. Video inspection indicated that a disjointed storm pipe had caused washout and void. Fortunately, the disjointed storm pipe was identified in time. Otherwise, the 30.6m 3 void would have continued to grow in size until the pavement collapsed, which may have caused human casualties and property loss. This study has successfully demonstrated that the GCPR is able to identify void locations. Despite leaking sand from a Mechanically Stabilized Earth (MSE) wall, no voids were detected by the GCPR surveys near the MSE wall on 34 th street. It is concluded that the erosion of the fill behind the MSE wall is in a very early stage, so that voids are not large enough to be detected by the GCPR. Therefore, it was recommended that regular inspections should be performed. © 2009 ASCE.