Why Field Measurements Differ from Model Estimates: Analysis Framework for Capacity and Level-of-Service Analysis of Unsignalized Intersections

Several questions are considered relating to the variability between field measurements and model forecasts, with a focus on the need to moderate user expectations about this variability. Considered first are the degree of variability observed in field measurements of delay and the stochastic effects in delay estimates produced by microscopic simulation. Examined next are the structure of the models of two-way stop-controlled (TWSC) intersection capacity and delay and how this structure might cause differences between field measurements and model estimates. How much the end user can moderate these differences using backcalculations, observation, and calibration and a “correct” perspective for the end user regarding these differences and variability are also discussed. These questions are considered using data collected as part of the NCHRP project used to develop the TWSC intersection capacity and level-of-service procedures contained in the Highway Capacity Manual models.