  This paper presents a new approach to congestion management at traffic-light
intersections. The approach is based on controlling the relative lengths of
red/green cycles in order to have the congestion level track a given reference.
It uses an integral control with adaptive gains, designed to provide fast
tracking and wide stability margins. The gains are inverse-proportional to the
derivative of the plant-function with respect to the control parameter, and are
computed by infinitesimal perturbation analysis. Convergence of this technique
is shown to be robust with respect to modeling uncertainties, computing errors,
and other random effects. The framework is presented in the setting of
stochastic hybrid systems, and applied to a particular traffic-light model.
This is but an initial study and hence the latter model is simple, but it
captures some of the salient features of traffic-light processes. The paper
concludes with comments on possible extensions of the proposed approach to
traffic-light grids with realistic flow models.
