Comparison of uncongested speed-flow relationships using data from German autobahns and North American freeways

The use of speed-flow data from German Autobahns provides a greater range of free-flow speeds than that available from North American freeways. On the basis of these additional data, three conclusions address current questions about the speed-flow relationship. First, the speed at capacity is not independent of free-flow speed but is higher on facilities on which the free-flow speed is higher; speed approaching capacity is as high as 90 to 100 km/hr on one Autobahn. Second, the range of flows over which average speeds remain constant (at the free-flow speed) decreases with increasing free-flow speed. For example, with a free-flow speed of 135 km/hr, speeds remain constant out to flows of 1,000 passenger car units per hour per lane. Third, for the range of flows in which speeds decrease with increasing flow, the relationship between speed and flow appears to be linear. This means that there is no need to postulate a curved (e.g., quadratic) relationship near capacity, since there is not an increasing rate of decline in speeds at those flows.