Congested traffic states in empirical observations and microscopic simulations

We present data from several German freeways showing different kinds of con gested traffic forming near road inhomogeneities, specifically lane closing s, intersections, or uphill gradients. The states are localized or extended , homogeneous or oscillating. Combined states are observed as well, like th e coexistence of moving localized clusters and clusters pinned at road inho mogeneities, or regions of oscillating congested traffic upstream of nearly homogeneous congested traffic. The experimental findings are consistent wi th a recently proposed theoretical phase diagram for traffic near on-ramps [D. Helbing, A. Hennecke, and M. Treiber, Phys. Rev. Lett. 82, 4360 (1999)] . We simulate these situations with a continuous microscopic single-lane mo del, the "intelligent driver model," using empirical boundary conditions. A ll observations, including the coexistence of states, are qualitatively rep roduced by describing inhomogeneities with local variations of one model pa rameter. We show that the results of the microscopic model can be understoo d by formulating the theoretical phase diagram for bottlenecks in a more ge neral way. In particular, a local drop of the road capacity induced by para meter variations has essentially the same effect as an on-ramp.