These animations of Portland, Oregon show the distance that can be traveled via public transit and walking. The dark gray border represents the maximum extent of travel given the time constraint indicated at the bottom of the frame. The shading within the boundary indicates the travel time. I assume a walking speed of 5 km/hr along roads and 3km/hr off-road.
The public transit schedule was obtained from the Trimet GTFS file and a custom path planner was used to determine the minimum arrival time at each stop. This was combined with data from the Census Bureau Tiger/LINE files to calculate the minimum arrival time at many points in the street system. Finally, the resulting data was rendered on top of a population density map.