Google has added real-time traffic data for major highways in more than 30 American cities to Google Maps. The data is color-coded to signify traffic conditions: green means no congestion; yellow 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. "We"re always looking to improve the comprehensiveness of our traffic information and our coverage areas over time," Sjogreen said.