Evaluation of PeMS to Improve the Congestion Monitoring Program

The highway congestion monitoring program (HICOMP) report is based on data from tach vehicle runs or PeMS. Research under TO 5319 compares the accuracy and cost of these two alternatives. The detailed studies described in the report lead to the following conclusions and suggestions for follow-on work: 1. In freeway sections with good quality detectors spaced at most one-half mile apart, PeMS-based estimates of the magnitude, extent and duration of congestion are incomparably more accurate than those based on tach vehicle runs. 2. Because at most four days of tach run data are used to estimate the annual congestion over a freeway segment, the error in these estimates is on the order of 25 percent. (With only one day of tach runs, which is typical, the error is 50 percent.) The error in year to year comparisons is on the order of 35 percent, and so one can have no confidence in such comparisons published in the HICOMP report. 3. Because the location and number of detectors within a district varies significantly from year to year and the quality of the loop detector data is uneven, it is hazardous to use PeMS-based congestion estimates to make inter-district or year to year comparisons. However, careful analyses using PeMS data can yield valid comparisons. 4. PeMS data collection costs a fraction of tach-vehicle runs that yield estimates with similar accuracy. It would be more beneficial to divert resources from tach runs into improving the detection system. 5. PeMS estimates the components of recurrent and non-recurrent congestion; congestion ‘hotspots’; and travel time reliability. Inclusion of these estimates will make HICOMP reports much more useful. 6. An outline of a standardized congestion reporting protocol is offered. It takes into account the errors introduced from too few tach runs, loop detectors with large spacing or poor quality. It suggests a transition plan to move towards a PeMS-based report.