Improved fluid-dynamic model for vehicular traffic

The fluid-dynamic traffic model of Kerner and Konh\"auser [Phys. Rev. E 48, 2335 (1993); 50, 54 (1994)] is extended by an equation for the vehicles' velocity variance. It is able to describe the observed increase of velocity variance immediately before a traffic jam develops. Another modification takes into account the finite amount of space that each vehicle needs. As a consequence, the improved traffic model does not produce densities that exceed the maximum vehicle density or negative velocities, like former models did.