## 7 Conclusions  
In this study, visual inspection data of bridge panels in East Japan and metropolitan Tokyo highway bridge panels were used for survival analysis in order to facilitate optimal maintenance decision making. First, unlike in previous research, a data cleaning process was used; then, by considering the effect of repair work, the precise survival time of each bridge panel was obtained. After correlation checking and variable selection, a KM curve estimator was used to clarify the basic information of important variables. Cox regression was then applied to identify the hazard ratio of each variable. The results can be summarized as follows. First, the different deterioration characteristics of different regions were clarified. Reduplicative dynamic traffic load was a major reason for the deterioration of bridge deck slabs in the Tokyo region. However, it was not a high risk in East Japan, because the traffic volume there was not as high as  
Figure 16: Risk score distribution in the Tokyo region. Figure 17: Risk score map of the Tokyo region. in the Tokyo region. Correspondingly, thicker slabs had much greater loading capacity; therefore, a decreased hazard ratio was evident in the Tokyo region. In East Japan, because of lower traffic volume and thicker slabs, slab thickness was not a major deterioration factor. Second, water including winter precipitation increased the deterioration rate in both regions. Especially in East Japan, during the extended snow melting process, salt water penetrates into cracks and accelerates the deterioration process. Under this condition, rebar is corroded and slab is seriously damaged unless there is appropriate protection and repair work. Correspondingly, water flow is faster for slabs with greater slopes; thus, the hazard ratio tends to be smaller. Moreover, waterproofing in East Japan reduced risk, indicating the importance of preventing water from penetrating the slab. Third, panels in different positions had different deterioration rates. Usually, the panel on the edge of the slab or near the expansion joint had high risks because of the high possibility of contact with water. In Tokyo, even panels in the center of the slab that directly carried heavy traffic loads had a smaller hazard ratio than the edge area. In addition, for both areas, because the water flow was more rapid with a greater slope, the hazard ratio was small. These results statistically prove that water plays an important role in deterioration. Therefore, waterproofing on the edge should be enhanced against severe environments. Fourth, using geographical coordinate information, risk scores were mapped; distinctions between high-risk and low-risk areas were shown. In East Japan, due to its special geographical environment, the Japanese seaside is snowy in winter, resulting in the use of large amount of de-icing salt. Therefore, the deterioration condition and rate in this area is relative high. In the south of the Tokyo region, since traffic volume in those areas is relatively high and most slabs in this area were relatively thin according to design codes prior to 1964, the total risk score is high. As a result, through this quantitative multiple deterioration factors assessment, the priority of maintenance in the case of an insufficient budget will be determined by more accurate statistical analysis rather than personal subjective judgment. Briefly speaking, it can be applied to optimize decision-making. Therefore, under the premise of ensuring more efficient and accurate countermeasures, financial burdens can be lessened. In this study, the risk factors for bridge deck slab components were quantitative evaluated. The results for these two typical regions can be extrapolated to the entirety of Japan and used for optimal decision making. By considering the content and quantity of risk factors, deterioration patterns can be clarified. Therefore, design, maintenance and rehabilitation can be rationalized.