Talking on a cell phone while driving is not only dangerous, but a new study
shows it also be to blame for traffic jams.
Researchers at the University of Utah suggest motorists who talk on cell phones
drive more slowly on the freeway, pass sluggish vehicles less often and take
longer to complete their trips.
"At the end of the day, the average persons commute is longer because of that
person who is on the cell phone right in front of them," said University of Utah
psychology Professor Dave Strayer, leader of the research team. "That SOB on the
cell phone is slowing you down and making you late."
"If you talk on the phone while youre driving, its going to take you longer to
get from point A to point B, and its going to slow down everybody else on the
road," said Joel Cooper, a doctoral student in psychology.
Cooper is scheduled to present the study in Washington during the Transportation
Research Boards annual meeting later this month. The board is part of the
National Academies, parent organization of the National Academy of Sciences,
National Academy of Engineering and Institute of Medicine.
Cooper and Strayer conducted the study with Ivana Vladisavljevic, a doctoral
student in civil and environmental engineering, and Peter Martin, an associate
professor of civil and environmental engineering and director of the University
of Utah Traffic Lab.
Martin says that, combined with Strayers previous research, the new study shows
"cell phones not only make driving dangerous, they cause delay too."
In recent years, Strayers research group has published studies showing that:
• Handsfree cell phones are no less dangerous while driving than handheld cell
phones because the conversation itself is the major distraction.
• When young adults talk on cell phones while driving, their reaction times
become as slow as reaction times for senior citizens.
• Drivers talking on cell phones are as impaired as drivers with the 0.08
percent blood alcohol level that defines drunken driving in most states.
Highway statistics suggest drivers on cell phones are four times more likely to
be in an accident, and Strayers earlier research suggests the risk is 5.36
The Cellular Telecommunications and Internet Association claims 240 million U.S.
subscribers in a nation of 303 million people. An insurance company survey
estimated 73 percent of wireless users talk while driving.
Another survey found that during any given daytime moment, 10 percent of U.S.
drivers are using cellular phones.
The researchers note that 50 countries have adopted laws banning handheld phones
while driving. But they say handsfree phone conversations are distracting and,
"Thus, the majority of current regulation appears to be misguided."
The earlier studies found that cell phone users follow at greater distances, are
slower to hit the brakes and are slower to regain speed after braking. But such
research didnt examine how traffic efficiency is influenced by individual cell
That led to Strayer and Martin discussing the possibility of using computers to
simulate numerous individual cell phone users driving behavior and thus overall
traffic. So their doctoral students – Cooper and Vladisavljevic – conducted the
new study as a step toward an eventual computer "microsimulation" of numerous
drivers and vehicles.
The new study used a PatrolSim driving simulator. A person sits in a front seat
equipped with gas pedal, brakes, steering and displays from a Ford Crown
Victoria patrol car. Realistic traffic scenes are projected on three screens
around the driver.
The new study involved 36 University of Utah psychology undergraduates. Each
student drove through six, 9.2milelong freeway scenarios, two each in low,
medium and high density traffic, corresponding to freeway speeds of 70 mph to 40
mph. Each 9.2mile drive included 3.9 miles with two lanes in each direction and
5.3 miles with three lanes each way. Traffic speed and flow mimicked Interstate
15 in Salt Lake City.
Each student spoke on a handsfree cell phone during one drive at each level of
traffic density, and did not use a cell phone during the other three drives. A
volunteer on the other end of the phone was told to maintain a constant exchange
The drivers were told to obey the 65mph speed limit, and use turn signals. That
let participants decide their own speeds, following distances and lane changes.
"We designed the study so that traffic would periodically slow in one lane and
the other lane would periodically free up," Cooper said. "It created a situation
where progress down the road was clearly impeded by slower moving vehicles, and
a driver would benefit by moving to the faster lane, whether it was right or
"Results indicated that, when drivers conversed on a cell phone, they made fewer
lane changes, had a lower overall mean speed and a significant increase in
travel time in the medium and high density driving conditions," the researchers
In medium and high density traffic, drivers talking on cell phones were 21
percent and 19 percent, respectively, less likely to change lanes (roughly six
lane changes per 9.2mile drive versus seven or eight lane changes by drivers
not on cell phones).
That may seem minor, "but if you have a lot of people who are not changing lanes
and driving slower, this could substantially reduce traffic flow," Cooper said.
When considered with the earlier studies, "its going to increase traffic
congestion," says Strayer. "You have motorists on cells phones who tend to drive
slower, their reaction times are slower, if they do hit the brakes it takes them
longer to come back up to highway speed, and they are less likely to change
lanes. Overall, they are more likely to gum up the highways."
In low, medium and high traffic density, cell phone users spent 31 percent, 16
percent and 12 percent, respectively, more time following within 200 feet of a
slow lead vehicle than undistracted drivers. That meant they spent 25 to 50 more
seconds following another vehicle during the 9.2mile drive.
"If you were not distracted by talking on a cell phone, you would overtake and
pass the slower vehicle and come to your destination faster," Vladisavljevic
"If you get two or three people gumming up the system, it starts to cascade and
slows everybodys commute," Strayer said.
Slower not always safer
He acknowledges that, "in itself, staying in a lane and not passing might be
construed as being safer, just as driving slightly slower or having a greater
following distance also could be considered safer. But if you are doing that so
you can take your mind off the road and talk on the phone, that isnt safer."
Compared with undistracted motorists, drivers on cell phones drove an average of
2 mph slower and took 15 to 19 seconds longer to complete the 9.2 miles. That
may not seem like much, but is likely to be compounded if 10 percent of all
drivers are talking on wireless phones at the same time, Cooper says.
Vladisavljevic already has begun computer "microsimulations" of multiple
vehicles. She tried the simulation repeatedly with the proportion of drivers on
cell phones ranging from none to 25 percent.
"We saw an increase in delays for all cars in a system, and the delays increased
as the percentage of drivers on cell phones increased," she says.
Strayer says it is important to show how cell phone use affects traffic because
"when people have tried to do costbenefit analyses to decide whether we should
regulate cell phones, they often dont factor in the cost to society associated
with increased commute times, excess fuel used by stopandgo traffic and
increased air pollution, as well as hazards associated with drivers distracted
by cell phone conversations."
Martin says transportation analysts include two components – accidents and delay
– when they calculate the "user costs" associated with road travel.
"A fatal accident could cost as much as $5 million when we take into account
medical, property and lossofincome costs," says Martin. "Delay is measured by
a composite number representing a measure of the value of a typical American
travelers time. Today, this number is about $13 per hour.
"While the costs associated with accidents seem high, there are so very few of
them, comparatively, they actually are dwarfed by the user costs associated with
delay. If we compile the millions of drivers distracted by cell phones and their
small delays, and convert them to dollars, the costs are likely to be dramatic.
Cell phones cost us dearly."
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