Errors and variability in volume data for project-level traffic forecasts
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NCHRP Report 765 reviewed literature about variability in traffic volume (Average Daily Traffic, or ADT) data. Although there were disagreements between studies as to methodology and results, the following equation, expressing the consensus of these studies, should be used for highway projects.
where CV is the coefficient of variation (or the ratio of standard error to the mean) and V is the ADT.
Figure: example with ADT=1,000 to 50,000