Google Maps has a feature called "typical traffic," which lets you pick a day and time to see what the traffic has looked like historically — so if you're trying to get out of the city at 4pm on a Friday, you can avoid the most historically jammed-up highways. This feature has been available for highway routes for years, but Google is finally rolling it out to surface roads as well. It's built right into the desktop version of Google Maps: just enter a starting point and a destination, enable traffic, and then select the day and time to see what the traffic is typically like on your selected timeframe.
The feature is live now, and a quick look at Google Maps shows that this data is pretty widely available — pretty much any street we looked at that had traffic data available also had the historical traffic data as well. Unfortunately, this data only appears on the desktop version, but since the feature is more geared towards trip planning than on-the-go traffic checks, we don't think mobile users will miss it too much.