Traffic Monitoring Guide

Appendix K. LENGTH BASED CLASS MEMO

INFORMATION:

Reporting of Length Based                                                                                                     February 24, 2000

Vehicle Classification Data to the Highway

Monitoring System (HPMS)

Director, Office of Highway Policy Information HPPI-30

Division Administrators

The HPMS calls for the annual reporting of various types of vehicle classification data. This reporting ranges from the percent of single-unit and combination trucks on HPMS sample sections to highway functional class level summary reporting of 13 vehicle classes. Because collecting such data on multi-lane, or high volume facilities is difficult, some States have proposed collecting vehicle classification data using a limited number of vehicle length categories. To date, we have not seen information that objectively compares the data collected through length based methods with that collected through the more traditional methods based on the 13 categories described in the Travel Data by Vehicle Type section of Chapter III of the HPMS field Manual. Without the review and approval of such information by my office, vehicle length based classification data are not to be reported to the HPMS.

The States proposing to report vehicle length based classification data to the HPMS must provide the following information.

• A description of the length categories to be used and how they relate to the 13 categories. For example, if four length categories are used, the description should explain how each of the 13 categories relates to a particular length category;
• A description of the method used to test how well each of the length categories captures the vehicles classes identified in point 1 and the results of those tests;
• If a State intends to disaggregate length based data into the 13 categories the imputation method must also be described; and
• Documentation on the situations in which length classification will be used. For example, a State might propose to such techniques only on high volume urban streets or a State may want to use length based classification for collecting information on percent trucks for reporting on HPMS sample sections, but will use other methods to report the Travel Data by Vehicle Type.