That's according to the American Community Survey, which calculates a statistic called the Commuter-Adjusted Daytime Population to estimate the number of people present in a particular city during normal business hours. Calculated by adding the number of non-working residents to the total working population, the figure underscores the idea that many cities dramatically expand and contract throughout the course of a day -- their true populations determined by much more than simply the number of people who actually live there. It also highlights the additional challenges faced by local governments responsible for planning and building infrastructure for both residents and all inbound travelers.
The San Francisco Municipal Transportation Agency inadvertently made it possible to visualize the rhythm of this daily migration. The agency's SF Park program, which raises and lowers the price of public parking over the course of the day, allows app developers to access real-time data on the number of cars parked in its municipal garages at any given time. Using figures from Monday, July 15, 2013, and combining that with traffic data from the California Department of Transportation's highway sensors, we created a visualization of a normal day in the life of San Francisco's central business district.
Mouse over each circle or line on the chart to see data for that specific garage. Use the time slider at the bottom to change the pace.
The circles represent public parking garages, their sizes changing according to "occupancy" -- the number of cars parked in each garage. Meanwhile, the thickness of the green and red highway lines shows the frequency of inbound and outbound traffic (respectively).
Of course, this sketch is just a microcosm of San Francisco's daily migration trends: it only includes cars in public garages and doesn't account for the hundreds of thousands of commuters who use public transportation to commute into the city every day. It does, however, illustrate the constant ebb and flow of the city's population and the unique rhythms generated by these transitions.
The chart below, using Census data, shows the population breakdown of the largest cities in the country as they appear during the middle of a standard business day. For San Francisco, 265,000 workers come into the city and 103,000 head out. The leader in overall daily migration change is Washington, D.C., where the daytime population is a whopping 79 percent higher than it is at night.