  This is a technical note where we solve the additive eigenvalue problem
associated to a dynamics of a 2D-traffic system. The traffic modeling is not
explained here. It is available in \cite{Far08}. It consists of a microscopic
road traffic model of two circular roads crossing on one junction managed with
the priority-to-the-right rule. It is based on Petri nets and minplus algebra.
One of our objectives in \cite{Far08} was to derive the fundamental diagram of
2D-traffic, which is the relation between the density and the flow of vehicles.
The dynamics of this system, derived from a Petri net design, is non monotone
and additively homogeneous of degree 1. In this note, we solve the additive
eigenvalue problem associated to this dynamics.
