  Daytime population density reflects where people commute and spend their
waking hours. It carries significant weight as urban planners and engineers
site transportation infrastructure and utilities, plan for disaster recovery,
and assess urban vitality. Various methods with various drawbacks exist to
estimate daytime population density across a metropolitan area, such as using
census data, travel diaries, GPS traces, or publicly available payroll data.
This study estimates the San Francisco Bay Area's tract-level daytime
population density from US Census and LEHD LODES data. Estimated daytime
densities are substantially more concentrated than corresponding nighttime
population densities, reflecting regional land use patterns. We conclude with a
discussion of biases, limitations, and implications of this methodology.
