A route matrix is a matrix used to give the number of direct routes between
each pair of nodes in a network.
In the example shown, the arrows on some arcs of the network indicate one-way routes.
So, for example, there is a route from A to C but not from C to A. The loop from D to itself gives
an anti-clockwise route. If there had been no arrow then the loop could also have been
traversed clockwise, giving 2 direct routes from D to itself. Matrices of this kind enable details of
a network to be stored as numbers in a computer and manipulated as required.