Google has started rolling out traffic estimates for Maps users in selected areas. When you search for driving directions in places with the feature, you'll see an ideal or average time at the top, followed by a time marked "in current traffic." Google says it relies on "live and historic traffic data" to calculate that time. So far, we've tried it in several major US cities and saw it working in New York and Washington, DC.
Portland, Seattle, and suburban areas outside DC still don't seem to have the new feature, but Google says it will be expanding it over time. We imagine some of the information is pulled from public traffic authorities, but Google also looks like it's using data from people who have turned on the My Location feature in Android. Previously, Google included traffic data in a separate map overlay, but it wasn't incorporated into directions.