The traffic information is integrated with Google Maps and is available in more than 30 American cities, including San Francisco, Dallas, Chicago and New York.
The data is provided for major highways and is color-coded to signify traffic conditions: green means no congestion; yellow is for minor holdups; and red means significant slowdowns.
According to Google product manager Carl Sjogreen, the data is aggregated from several sources, including road sensors, as well as car and taxi fleets.
As a result, Sjogreen said, there is sometimes not enough data to report road conditions. In that case, the roads appear gray.
For the time being, he added, Google will provide traffic data only for major highways, but it may expand the offering eventually.
"We're always looking to improve the comprehensiveness of our traffic information and our coverage areas over time," Sjogreen said.
Google is not the only company providing real-time online traffic data. Yahoo Maps also offers the service and provides symbols designating specific traffic incidents. But its color-coding for traffic flow is hard to understand.
Google's maps show only traffic flow, however, and not specific incidents.