Vehicle Classification Data Expansion

Nationwide surveys of departments of transportation, metropolitan planning organizations, and classification vendors/producers were conducted to determine the state of practice on equipment and methodologies used to determine truck vehicle miles traveled (VMT). The current Illinois Department of Transportation (IDOT) methodology was evaluated and it was found that it overestimated truck VMT for multi-unit trucks on all eight functional classes except on the minor urban arterials. The average overestimation was 11.5% and it varied from -10% to +44%. The current method overestimated truck VMT for single-unit trucks in five and underestimated in three functional classes. The under/over estimation ranged from -6% to +35%, but the average value was close to zero. To calculate truck VMT more accurately, this study proposed two different methods based on average truck percentage (ATP) and average section length (ASL). In the ATP method, truck VMT is calculated by multiplying the ATP for a group of roadway sections by the total VMT of that group. The ATP method should be used when the ATP and the total VMT by volume groups are available. In the ASL method, the total truck volume for the sampled sections is multiplied by the ASL. The ASL method should be used when the information required for ATP is not available or not reliable. Sample size influences the accuracy of truck VMT estimation and the decision on sample size must consider the error level that is acceptable. This study looked at the likely error for different sample sizes and recommended using 8% to 16% of the number of roadway sections. The sections should be distributed among the volume groups. Recently, IDOT collects vehicle classification data for three categories at about 10,000 sections, biennially. It is recommended to evaluate the truck VMT calculation using recent data.