Emily Badger explains the above image:
[C]reated by Andy Woodruff of the great cartography blog Bostonography, [it] illustrates a single day’s worth of bus travel across Boston, by speed. Each little line on this map represents one bus (tracked as the crow flies between data points, not along its actual route), with red showing speeds of less than 10 miles an hour, yellow of 10-25 miles per hour, and green faster than that. …
The concept was inspired by an earlier Eric Fisher map of transit in San Francisco. … The project is primarily impressive for its design – “it’s at least half art,” Woodruff says – but these daily maps also have the potential to reveal patterns in how the system changes from day to day, between week days and weekends, between business as usual and system break-downs. If you live in Boston, they may also simply confirm your suspicions about your own commute.
A live version of the map, which shows bus speeds over the past three hours, is here.