Derivation and application of congestion thresholds in the estimation of motorway link `capacity'.

This paper reviews the methodology used to calculate Congestion Reference Flows (CRF) to demonstrate the sensitivity of maximum link flow thresholds to the constituent formula parameters. It uses illustrative data from a dual 3 - lane commuter motorway and a dual 3 - lane strategic inter-urban motorway in the in the UK to demonstrate the responsiveness of flow thresholds to the percentage of HGVs and PSVs, peak to daily ratios and tidality. Having established CRF thresholds for both motorways, these are then used to determine the Years in which these thresholsds are exceeded. The paper concludes by identifying the limitations of the current method for estimating motorway reference flows. Recommendations are made for refining the formula.