  At the intersection of two unidirectional traffic flows a stripe formation
instability is known to occur. In this paper we consider coupled time evolution
equations for the densities of the two flows in their intersection area. We
show analytically how the instability arises from the randomness of the traffic
entering the area. The Green function of the linearized equations is shown to
form a Gaussian wave packet whose oscillations correspond to the stripes.
Explicit formulas are obtained for various characteristic quantities in terms
of the traffic density and comparison is made with the much simpler calculation
on a torus and with numerical solution of the evolution equations.
