Here is San Francisco:
And Boulder, Colorado:
"These maps show how long it takes to get everywhere else via walking and public transit," Hardin writes in an email. "This allows you to make some important comparisons, such as 'if I move here, I can reach half the city in 50 minutes if I start at 8 a.m.'" His paperexplains more of the technical details.
Hardin says the maps change dramatically according to the time of day, as you can see from this animation for Boulder.
He sees several real-life uses, including assisting apartment-searchers who want to compare places to live based on the "average minimum travel time to the rest of the city," aiding transit planners who want to examine the impact of "adding or removing transit stops or routes," and helping "suburban commuter[s] decide if public transit and walking provide a viable alternative to a daily car commute."
Hardin's maps, which are based on Open Street Map data, aren't completely accurate. For example, they assume people cross rivers directly, when normally people take bridges or tunnels. "Frankly, better tool exists for planning your commute," he admits. The point is to offer approximations, so you know immediately how far two destinations are from each other.