## 6 Results for the Tokyo Region  
Figure 15a shows the survival probability of all of the bridge panels. The survival probability fell to 50% in 45 years. Figure 15b-d show the survival probability of three category explanation variables: edge, design code, and girder type. For each variable, each group shows a different declination trend, indicating that these groups affect the deterioration rate. In particular, a large gap between design codes is shown in Figure 15c, which reveals that safety was greatly improved by the design code overhaul after 1964. [PERSON] et al. (2016) also stated that according to their analysis, slabs designed before 1964 showed shorter fatigue life [22]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Standard** & **Minimum Distributing Bar** & **Minimum Slab Thickness** \\ \hline
1956 & 25\% greater than main rebar & \\
1964 & 14 cm \\
1967 & 70\% greater than main rebar & \\
1973 & 16 cm \\
1980 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of minimum design distributing bar and slab thickness between different design standards. The multivariate Cox regression analysis result is shown in Table 7. Unlike the East Japan results, risk increased more than 30% when traffic volume increased by one standard unit. From the engineering point of view, a slab undergoing frequent wheel load from traffic reaches its fatigue limit faster. However, increasing the slab thickness decreased the risk, because a thick slab has enhanced loading capacity. According to our data, the average slab thickness in the Tokyo region was only 19.8 cm. Thus, unlike in East Japan, repeat traffic loading was a major factor in deterioration. These results reveal that slab thickness should be increased to resist traffic load. As in East Japan, the risk decreased as the slope increased because of faster water runoff. Moreover, winter precipitation showed high risks. Since winter precipitation was highly correlated with precipitation in other seasons, this result indicates that precipitation increased the deterioration rate. As in East Japan, slabs usually have a longer lifespan under relatively low ambient humidity. [PERSON] et al. (1982) pointed out that the degradation rate of reinforced concrete slabs was faster if water was present. If water has sufficiently penetrated the cracks of a concrete slab, there is a possibility of fatigue fracture even under the design load, which is about one fifth of the static stress ([PERSON], 1983). Water penetration through cracks probably promotes scratch polishing and high water pumping pressure on the cracked surface, and rapidly increases crack width. Thus, in both East Japan and the Tokyo region, slab deterioration is significantly influenced by water. Figure 15: Survival probability of some category variables in Tokyo region; (**a**) all panels; (**b**) edges; (**c**) design code; (**d**) girder type. Panels at the edge of the deck tended to have a higher likelihood of deterioration, because these panels were more frequently in contact with water such as rainfall. The report from the Chugoku Regional Development Bureau (2016) also revealed that deterioration of the edge of the slab was more serious than in the center [24]. The results also show that continuous girders had a lower deterioration rate. From the engineering perspective, continuous girders are safer because they have fewer expansion joints. Thus, water has less opportunity to penetrate and contact the unprotected part of the slab. However, as Figure 15d shows, the survival probability declination of continuous girders was faster. In the specifications for highway bridges [25], the minimum slab thickness designation for continuous girders is smaller than for simple girders, as Table 8 shows. In terms of the design code, the slab panels constructed according to the Specifications for Steel Highway Bridges issued in 1956 and 1964 showed high risk, indicating that a small amount of distributing bar and thin slab thickness greatly decreases the safety of slabs.