What is the bridge made of?
The bridge is made of concrete.
    src        edge_attr  dst
0     1               in    2
1     4         close to   12
2     4   on the side of    0
3     4               on    0
4     4  to the right of    5
5     5  to the right of   13
6     5   to the left of    4
7     5               on    0
8     7  to the right of   10
9     8             near   12
10    9               on   10
11   10           behind   12
12   10   to the left of    7
13   12      in front of   10
14   12             near    8
15   13   to the left of    5
16   13               on    0