Traffic Flow on Freeway Upgrades

Traffic flow characteristics on freeway upgrades in Germany were analyzed, and a combination of measurements, analyses of continuous counting results, and microscopic simulations was performed. From these results a macroscopic traffic flow model for freeway upgrades based on a two-stage linear speed-density relation was developed. All relevant external influences such as degree of gradient and length of upgrade together with traffic flow parameters such as volume or proportion of heavy vehicles may be included through specific parameters. On the basis of this analysis, it was found that capacity depended solely on the degree of the gradient. Capacity was not influenced by gradient length. Velocity, however, was determined not only by the degree of the gradient but also by the length of the upgrade (up to L ≤ 4,000 m). Results were formulated to be ready for application in practice. They form, for instance, the basis of the chapter on freeways in the new German version of the U.S. Highway Capacity Manual, Handbuch für Bemessung von Straßenverkehrsanlagen.