This visualization of daily bus and rail rides in London is based on research conducted by Jay Gordon (MIT). Below the fold is a summary of Gordon’s methodology:

This visualization merges all 16 million daily transactions made on London’s Oyster card with vehicle-location data from the city’s 8,500 buses to infer the travel histories of that day’s 3.1 million Oyster users. After inferring the times and locations of each bus boarding and rail alighting, bus and rail transactions are combined to reconstruct each cardholder’s daily travel history. Each pixel represents a 100-meter square section of Greater London, and the brightness of each of the three RGB color components indicates the number of riders in one of three categories:

1. Green indicates the number of passengers in the transit system, whether on a bus or in one of several rail modes.
2. Blue indicates the presence of riders prior to their first transaction of the day or after their last: it is assumed that the location of a rider’s first or last transaction approximates their place of residence.
3. Red indicates cardholders who are between transit trips, whether transferring, engaging in activities, or traveling outside the transit system.