Google Maps has now been updated to give you more accurate estimates of travel duration by using real-time traffic datas. Now when you use Google Maps to get directions, it will provide you with two estimated durations, one without traffic and one based on the current traffic conditions.
Previously, Google had introduced a similar feature that could estimate travel time, but that was based on historic rather than real-time traffic data. It also provided two scenarios, but the traffic data was based on average conditions for an area instead of the actual conditions at the time of your travel.
The new feature utilizes data gathered from Android users that enable the My Location feature through Google Maps on their devices. The more Android users that participate the more traffic data Google Maps is able to collect in order to provide the real-time estimates. So it's possible that you may not see current traffic information for a certain route because no data has been collected from any Android users on that route.
[via Google Blog]