Multiplied by the many people who might be talking while driving in any given rush hour, the findings indicate that mobile phones are slowing commutes.
"People need to realize that cell phones are not only unsafe [in traffic], but they actually generate extra delay," said Peter Martin, an engineering associate professor and director of the University of Utah Traffic Lab. "Whilst you can argue you should stop cell phone use while driving because it's dangerous, this just adds weight to the argument."
A mounting body of scientific literature suggests a 25 percent reduction in reaction times for motorists using cell phones. The U.'s study sought to determine whether those slow reflexes also make for slow traffic, and a simulator otherwise used to train police officers found strong evidence for that hypothesis.
National estimates currently peg the growing number of drivers talking on the phone at 10 percent of all motorists on the road at any given time. Pairing that with the slowdown that the Utah researchers observed among the study subjects negotiating simulated Interstate 15 conditions, they estimated the typical Salt Lake Valley freeway commute is 5 percent to 10 percent longer because of cell phones, according to lead researcher Dave Strayer, a psychology professor.
A reduced reaction time is universal among study subjects, Strayer said.
"Everybody seems to think that they're the exception and they can do it," he said.
In fact, those who in past U. safety studies claimed to have tested well while driving the simulator and talking on the phone missed the clues of impairment because their brains were busy forming words. Electrical monitoring of brain activity shows that speech formation affects other brain functions in ways that listening to conversations or music doesn't, Strayer said.
"The brain just doesn't process driving information when people are talking on the phone," he said.
The study found that chatty freeway drivers on average drove 2 mph slower than others and spent up to 31 percent more time following closely behind other slow drivers instead of passing in a faster-moving lane. They also were slower to regain freeway speeds after braking.
The talking drivers took 15 to 19 seconds longer to traverse a 9.2-mile segment of Salt Lake County-simulated freeway, according to the study. That sounds minimal, but the cumulative effects on all drivers are magnified because numerous drivers in real-world conditions likely would be on the phone slowing down others, said Joel Cooper, a doctoral student in psychology.
The motorist advocacy group AAA acknowledges that cell phones and driving are a bad mix, and even advocates nationally for state laws barring their use among drivers 19 and younger, AAA Utah spokeswoman Rolayne Fairclough said. The group has not gone so far as to endorse restrictions on adults, she said.
"We of course would like people not to use cell phones while they drive," she said.
Previous studies at the U. have determined a heightened danger of accidents when motorists talk on phones, with no improvement when using hands-free sets.
University of Utah students operating a driving simulator while talking on the phone on average:
* Drove 2 mph slower
* Followed slow-moving vehicles without changing lanes for up to 31 percent longer
* Took up to 19 seconds longer to drive a simulated 9.2-mile segment of Interstate 15
* Took longer to resume highway speed after braking in traffic