Federal Way residents who commute to Seattle on Interstate 5 saw a 12-minute decrease in their morning commute time between 2007 and 2009, according to a traffic study released Nov. 26 by the state Department of Transportation.

But the study results are paradoxical: The commute time decreased while the number of commuters increased.

The state published the “2010 Congestion Report,” an “analysis of system performance on state highways,” with a section specifically devoted to the commute from Federal Way to Seattle.

Sreenath Gangula, who helped analyze data that went into the report, said the 22-mile Federal Way to Seattle commute was highlighted because it showed a decreased commute time, but more commuters. It’s also a corridor that has seen no major upgrades in recent years, unlike Interstate 405 in Bellevue, for example.

In 2007, the average morning peak travel time was 47 minutes, but dropped to 35 minutes by 2009. At the same time, the number of peak morning commuters increased by 3 percent in 2009 over 2007, according to the report.

Gangula gave several explanations for the change:

• Accidents have decreased. Gangula said that collisions on the stretch between Federal Way and Seattle decreased by 30 percent between 2007 and 2009, while other highways in the Puget Sound region saw a decrease of only 8 percent. He doesn’t know if Federal Way drivers are safer compared to other drivers, but the decrease in major collisions leads to a decrease in minor collisions, both of which contribute to slower traffic. If there’s a big accident on the highway, it’s more likely that fender benders will occur in the traffic surrounding that accident, further slowing traffic.

• The economy. Though the number of commuters has increased, the report indicates that the recession and subsequent job losses may have taken some commuters off of the highway.

• New transit options. In July 2009, Sound Transit’s light rail service began operating between SeaTac International Airport and downtown Seattle. It’s possible that different commuting options had an effect on the overall commute, but Gangula said ridership data from local transit agencies was not immediately available to factor into the department’s report.

Yet another factor might be people changing their driving behavior. If commuters discover that their commute is getting shorter, they may change their schedule, leaving for work at different times.

“If you’re driving to work at 5:30 a.m. in 2007, and you feel like you’re reaching work 20 minutes early, the next day maybe you’ll start at 6:05 a.m.,” he said. “That’s self-metering.”

Commuters are also able to drive closer to the speed limit with less congestion on the road. The lowest average speed in 2007 was around 30 mph, but increased to 40 mph in 2009.

The state uses both human and computer technology to monitor commute times. A series of sensors are embedded every half mile in the roadway between Seattle and Federal Way to track the number of cars on the road and their speeds. Human monitors are used to track vehicle occupancy in high occupancy vehicle lanes.

The evening commute between Seattle and Federal Way also dropped between 2007 to 2009 from 36 minutes to 32 minutes.