Accounting for composite travel time distributions within a traffic stream in determining Level-of-Service

This paper presents a procedure for assigning multiple Level-of-Service (LOS) ratings to travel time data sets containing multiple (composite) distributions. Different LOS modes within a mixed data set, the travel time data were fitted to multiple gamma-type distributions using an Expectation Maximization (EM) iterative process. The EM process was enhanced with a Monte Carlo style method, wherein the EM process was run 100 times with different random starting values and the best fit according to an R-squared value was taken. The number of underlying distributions was determined by fitting 1 to 5 distributions and using the Akaike Information Criterion to determine which number of fits maximized the information content of the fitted function. The resulting final posterior probabilities were then used to separate the data into their respective distributions. Reported here are the results of applying this procedure to travel time data collected in the Metro Atlanta area. It is believed that this method can provide enhanced LOS information, especially when the data contain multiple overlapping travel time distributions. This multiple LOS rating method is not intended to replace the current method, instead it is being developed as a supplementary tool to provide more detailed LOS information, for example providing a more detailed view that captures the underlying performance of each subgroup in addition to a single aggregate LOS measure.