Design Hourly Volume Estimation at Freeway Nodes From Short-Term Traffic Counts

This paper extends the concept of a design hourly volume (DHV) which is derived from the “nth hour” to a concept based on the nth highest saturated hour. To calculate this nth highest saturated hour at each ramp junction of a node, it is necessary to have permanent traffic counts (PTC) on all ramps and the main lanes. In practice, such counts are often not available. For such cases, the German Highway Capacity Manual proposes a method that enables the estimation of DHV through short-term traffic counts (STCs) and the extrapolation of the results using available PTC in the vicinity. This study examines how accurately the required nth highest saturated hour can be estimated with this method and similar concepts. Furthermore, we investigate to what extent the number and the location of the available PTC affect the accuracy of the estimation. Scenarios without PTC are also considered. The evaluation is based on a database with a total of 72 freeway nodes for which PTC data from three years (2017–2019) are processed. The results show that the estimation of the nth highest saturated hour with the method of the German Highway Capacity Manual works accurately, even if only one PTC is available on each inflowing approach. The results further indicate that STC are crucial to achieve accurate results when few PTCs are available. Acceptable results are also obtained by STC of one week, even without a projection at a PTC.