Variability of Speed-Flow Relationships on German Autobahns

On German autobahns the average speed is strongly dependent on traffic volume. In addition to traffic volume, there is a comprehensive set of parameters that also cause a time dependency of speeds on autobahns. Speed-flow relationships play a key role in the determination of the type of highway for a given design flow and a given desired traffic flow quality. The question, however, is whether the speed-flow relationships from the guidelines are reliable over longer periods and to what extent the short-term speed variations are larger than the flow dependencies. Therefore, the objective of this research project was to determine typical fluctuations of average speeds on autobahns that are not the result of different volumes. Moreover, the consequences of differing speed-flow relationships on the maximum flow (i.e., capacity) should be pointed out. Studies of influences on the speed-flow relationships for German autobahns are presented. In addition to the fundamental influencing factors such as traffic volume and traffic mix, time dependency is also considered. As a data base, the results from 15 sites equipped with automatic traffic counting machines were used, each over a period of 3 years. This study was based both on conventional analysis, that is, separating different sets of data from the whole sample and on analysis of variance methods. The investigations show that two types of time-dependent influences exist. At first, there are changing environmental factors such as daylight or darkness or weather conditions. Darkness causes an average reduction of velocities by about 5 km/hr and wet roadway conditions reduce velocities by about 10 km/hr. Consequently, a reduction in capacity must be expected. The second type of time dependency is induced by varying driver behavior and traffic mix. Here seasonal variations can be found over the year and also over the days of the week. During predominantly leisure traffic, such as on Sundays or during the summer vacation season, lower average speeds can be detected.