From http://news.yahoo.com/s/nm/20080102/us_nm/usa_phones_traffic_dc:

Cell phone users tie up traffic: study

By Julie Steenhuysen Wed Jan 2, 3:26 PM ET

CHICAGO (Reuters) - 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 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 kph) slower than the undistracted drivers and took 15
to 19 seconds longer to complete the 9.2 mile 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.