  For even $n$ we prove that the genus of the complete tripartite graph
$K_{n,n,1}$ is $\lceil (n-1) (n-2)/4 \rceil$.
  This is the least number of bridges needed to build a complete $n$-way road
interchange where changing lanes is not allowed.
  Both the theoretical result, and the surprising link to modelling road
intersections are new.
