Comparison of Freeway Capacity Estimation Methods

The objective of this research was to investigate various methods that can be used to arrive at an estimate of freeway capacity values, and to recommend one of these methods to the FDOT for use in developing their own estimates of capacity for Florida freeways. Three methods were investigated: one that fits a mathematical function to plotted speed-flow data points, from which the apex of the function is taken as capacity; one that estimates a breakdown probability distribution based on flow rates preceding breakdown events, from which capacity can be taken to correspond to a certain percentile value of the breakdown probability distribution; and one that uses a simple average of a specified percentage of the highest flow rates observed at a site. Based on the various advantages and disadvantages of each of the methods, the following was concluded. The method based on identifying breakdown events is most suitable for the determination of capacity at a site where a detailed operational analysis is desired.