From FHWA: Census population data are used in many transportation planning applications including population forecasting, travel demand modeling and microsimulation, Title VI analysis and land use analysis. The US Census definition of "urbanized areas" (UZAs) is used in the Federal determination of MPOs.
Answers to frequently asked questions about census urbanized areas and MPO designation are available on the Federal Highway Administration (FHWA) website, here. For a schedule of the census UZA definition process, click here. Additional resources are also available, here.