Overpass is an important hub for urban road network facility, its traffic capacity severely restricts that of the entire road network. Since overpass area is easy to gather water in urban road network, rain water under the overpass is an important incentive for traffic jams. In this paper, a reliable and easily maintainable method is discussed to detect the depth of the road surface water, which designs and implements a monitoring system of urban road network ponding depth. Based on this, technique of predicting travel time has been researched about overpass area under water-logging condition. Through a real example, the technique discussed in this paper has been proved to be highly effective and veracious, and can be used to provide basic data for traffic guidance to plan out sound routes.