Researchers at the University of Utah have found that motorists talking on cell phones drive more hesitantly than undistracted drivers and, as a result, are increasing everyone's average drive time by 5 percent to 10 percent.
For someone with a two-hour round-trip commute, that means as much as 12 extra minutes behind the wheel each day. Over the course of a year, the excess time in traffic easily could top 50 hours.
"On your commute home tonight, your commute will be slower because of people who are using their cell phones," said psychologist Dave Strayer, whose findings will be presented Jan. 16 at the annual meeting of the Transportation Research Board, which is part of the National Academies.
Strayer and his colleagues put 36 college students behind the wheel of a converted Ford Crown Victoria outfitted with screens to simulate driving conditions on a 9.2-mile stretch of Interstate 15 near Salt Lake City. Each student drove in low-, medium- and high-density traffic while talking into a hands-free cell phone, then again with the phones shut off.
Previous studies of phone use while driving have focused on the hazards of mixing the two. Epidemiologists examining records of accidents have found that the risk of crashing is more than four times higher for drivers talking on cell phones. That is the equivalent of driving with a blood alcohol level of 0.08 percent, high enough to qualify as drunk in most states, including Maryland.
In the latest study, Strayer found that students drove 2 mph slower when talking on the phone and took 15 to 19 seconds longer to reach their virtual destinations.
While engaged in conversation, the students were 19 percent to 21 percent less likely to change lanes in medium- to high-density traffic, the study found. The total number of lane changes was six, compared with seven or eight when the phones weren't in use.
In preliminary computer models, the researchers found that those small changes would accumulate and clog up the roads for everyone.
The National Highway Transportation Safety Administration says that 10 percent of drivers talk on their phones during daylight hours. The overall effect is to boost average drive times by 5 percent to 10 percent, Strayer said.
Freeways reach a tipping point when traffic slows to about 35 mph and cars are separated by a few dozen feet instead of several hundred, said Hamid Bahadori, a transportation engineer for the Automobile Club of Southern California. Under those conditions, a pokey driver forces others behind him to brake rather than reduce their speed gradually. The more they slow down, the longer it takes to get back up to speed.
"Any slowing down sends a kind of shock wave through the traffic," said Steve Bloch, senior traffic safety research for the auto club.
Some traffic experts are skeptical that the patterns seen in the driving simulator would hold true on the road. They also noted that although some lane changes improve traffic flow, they aren't always beneficial.
Sometimes drivers think an adjacent lane is moving faster, only to find after merging that they want to move back. That slows traffic even more.
"If by using the cell phone you don't do that, then you're helping," said Petros Ioannou, director of the Center for Advanced Transportation Technology at the University of Southern California.
Strayer said more than wasted time is at stake. Chatty drivers also mean more gasoline consumption, more pollution and worse health, he said.
"When people are stuck in traffic, their blood pressure -- especially for males -- starts to really go up," he said.
Karen Kaplan writes for the Los Angeles Times.