Want to know where exactly you can and can't go in the next hour? Trulia's new map service can show you a "heat-map" of the places you can travel within a certain time limit, and by either public transit or by automobile.
As Mark Wilson of Fast Company reports, "...in a single search, you can spot the congestion of an entire city and plan your day accordingly...Maybe you're a tourist in a new town. You'd like to hit up some sites, but you have no idea how much time to plan. Or maybe you're juggling errands on the weekend, and you want to hit up whichever stores are lightest on traffic first."
After some playing around with the maps, and reading the comments of other users, the maps do seem to be off in some ways. Arrival times for suburban and rural areas tend to be drastically misjudged, usually off by twice the normal time it would take to get there by automobile. But, for larger and traffic-clogged metros at peak rush hours, the times seem to be, for the most part, correct.
As one commenter put it, "[r]oadway classification seems to be downplayed too much, as in local streets moving almost as fast as major arterials. Are signalized or signed intersections assigned impedance values?...what time of day is this? Is this a weekday or a weekend? I hope under/overpasses are uncoupled. The data is out there to support such an analysis, but it will cost you dear." Still, it's worth a try.