Measuring Improvements in the Movement of Highway and Intermodal Freight
Ratio of Variance to Average for Peak Period Trip Times at Freight-Significant Nodes
These two indicators measure congestion and reliability of travel in metropolitan areas of special importance to freight movement. The first of these is intended to measure the relative severity of peak-period congestion, the second to measure the predictability of travel times in peak periods. The term “freight-significant node” refers to cities or metropolitan areas that are designated as especially important nodes in the national freight network. These measures do not score well in terms of descriptive value. They are somewhat technical in character; the notion of a freight-significant node might be difficult to explain to a generalist audience. The terms “ratio of peak-period travel time to off-peak travel time” and “ratio of variance to average for peak period trip times” also are somewhat difficult to understand.
The notion of “freight-significant nodes” is parallel to that of freight-significant highways. Data on peak-period conditions in all large cities would have little focus on freight; hence the idea of identifying certain cities as of special importance for freight movement. Such an identification, however, appears more difficult for cities than for highways. Clearly, some cities originate or receive more freight than others, but that does not necessarily mean they are more critical in terms of movement of freight on the national network. There appear to be real conceptual difficulties in finding a simple, unambiguous standard for designating some metropolitan areas as freight-significant.
Data problems relating to identification of freight-significant nodes are closely entwined with the conceptual problems, i.e., it is not clear what data one would seek. Regarding the actual measures, ratio of peak to off-peak travel time should be derivable from the HPMS data on volumes and capacity based on assumptions about peaking patterns. It is known that, at one time, Cambridge Systematics developed a set of empirically derived relationships between AADT/capacity ratios and percentage of AADT in the peak period. Estimating the variance of peak-period travel times could not be done with data from the HPMS. Collection of data on the variance of travel time would require arrangements for on-site observations.
Scores: Ratio of peak-period travel time to off-peak travel time at freight-significant nodes
Medium to high
Scores: Ratio of variance to average for peak period trip times at freight-significant nodes