Random Count Site Selection Process for Statistically Valid Estimations of Local Street Vehicle Miles Traveled

The Texas Department of Transportation has identified the need for a more accurate estimation of vehicle miles traveled (VMT) on streets functionally classified as local. The Texas Transportation Institute developed a procedure to select traffic count sites randomly on local streets that results in a statistically valid estimation of local street VMT. The primary goal was to yield a statistically valid median traffic volume to apply to road segments on which no count is taken. This procedure involves grids overlaid on maps depicting the functional classifications of streets in a given area. All grid cells are assigned sequential numbers, and then a relative range of random numbers is generated. The actor then locates the grid cells that correspond to the randomly selected numbers in the order in which they were generated. Each time a cell that contains a local street is selected, the actor highlights it and marks a count location on the map. This process is repeated until an appropriate number of sites have been selected. Statistical analyses have determined how many count locations are necessary to provide a representative sample of traffic counts in each area, depending on the population. By an oversampling in a variety of areas, median values of observed volumes identify the number of counts at which the diminishing rate of return on the investment in the counts occurs. This procedure has resulted in median traffic count volumes on local streets that more realistically represent the variety of local streets that exist. These median volumes derived from randomly selected sites are considerably lower than the median volumes derived from historical count locations that are not randomly selected. In addition, there are now randomly selected traffic count locations in area types of some counties in which no counts were previously performed. FHWA has approved this random traffic count site selection process for use and has acknowledged it as a “most noteworthy state activity” related to estimating local street VMT (www.fhwa.dot.gov/ohim/statepractices.pdf).