Ways to Estimate Speeds for the Purposes of Air Quality Conformity Analyses

About the report:

A speed post-processor refers to equations or lookup tables that can determine vehicle speeds on a particular roadway link using only the limited information available in a long-range planning model. An estimated link speed is usually based on volume, the percentage of heavy trucks, the free flow speed on the link, and the facility type (e.g., interstate, two-lane highway). These post-processed speeds are used to estimate motor vehicle emissions in conjunction with the U.S. Environmental Protection Agency's MOBILE model. At least two post-processors in the form of software packages are available to VDOT staff. One, developed by Michael Baker Jr., Inc., can be used immediately with minor modifications as an interim measure to perform conformity analyses for new nonattainment areas. The other is being updated by VDOT's Northern Virginia District; a new generation is expected within a few months. Another option is for VDOT staff to code speed-volume equations directly into a spreadsheet, as has been done in the Appendix. The authors recommend that over the next 12 months, either the Research Council, VDOT, or the two organizations work together to validate the post processors available by comparing their computed and actual speeds on a variety of facility types. In conjunction with this effort and to the extent that resources allow, the effect of different input data, such as vehicle age, vehicle type, and travel speeds, on mobile source emissions as predicted by the MOBILE model should be studied.

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