CHICAGO, USA: If you're late for work, a driver using a cell phone may be to blame.

U.S. researchers said on Wednesday that people who use cell phones while behind the wheel impede the flow of traffic, clog highways and extend commute times.

"It's a bit like breaking wind in the elevator. Everyone suffers," Peter Martin of the University of Utah's Traffic Lab said in a telephone interview.

Prior studies have equated the risk of driving while talking on a cell phone with driving while drunk. Some 50 countries have banned use of hand-held phones while driving.

The latest study shows the impact of cell phone use on traffic patterns. "It has to do with the reaction to changes in speed," said Martin, who teaches civil and environmental engineering.

"When a driver who is not distracted is in a traffic stream and the vehicle in front slows down, the driver will brake in response. When a vehicle speeds up in front, the driver will respond and speed up," he said.

Martin and a team of researchers devised a study involving 36 university students, each of whom drove through six 9.2 mile-long (15 km) freeway scenarios in low- to high-density traffic at speeds that resembled driving on an interstate highway.

The drivers used a hands-free phone during half their trips and no phone in the other half. They were told to obey posted speed limits and use turn signals but the rest of the driving decisions were up to them.

What they found is that when the drivers were distracted by a phone conversation, they made fewer lane changes, drove slower and took longer to get where they are going.

In medium- and high-density traffic, drivers were about 20 percent less likely to change lanes. They also spent about 25 to 50 seconds longer following slow-moving vehicles before changing to an open lane. And they drove about 2 mph (3.2 kmh) slower than the undistracted drivers and took 15 to 19 seconds longer to complete the 9.2 mile (15 km) trip.

For an undistracted driver, these accommodations might make driving safer. "But if you are doing that so you can take your mind off the road and talk on the phone, that isn't safer," said University of Utah psychology professor Dave Strayer, who led the team.

Those delays can add up, especially in light of studies that suggest as many as 10 percent of U.S. drivers are using a cell phone at any one time.

"Delays in traffic streams of very small amounts grow into massive numbers when you project it across a highway and across a nation," Martin said.

The next step is to use computer models to determine just how much those delays are costing drivers in time and extra fuel costs that result from traffic delays.

"What we've done here indicates already that those numbers are likely to be significant," Martin said.