From mySociety.org, time travel maps take into consideration the means of travel (car, rail, etc.) and the different paths available. Above is a map of London and shows time to travel from the center of town. The white contour lines represent half hour intervals, and the color coding has warm colors for the shortest times, and cool colors for the longest times.

The really interesting feature are the "islands". These small circles represent destinations that you can reach much faster than the surrounding area. Mainly stations for faster trains than have fewer stops.

Strangely similar in concept to an "event horizon".