Random walk on the randomly-oriented Manhattan lattice

In the randomly-oriented Manhattan lattice, every line in ${\mathbb{Z}}^d$ is assigned a uniform random direction. We consider the directed graph whose vertex set is ${\mathbb{Z}}^d$ and whose edges connect nearest neighbours, but only in the direction fixed by the line orientations. Random walk on this directed graph chooses uniformly from the $d$ legal neighbours at each step. We prove that this walk is superdiffusive in two and three dimensions. The model is diffusive in four and more dimensions.