Finally, empirical proof you can blame chatty 20-somethings for stop-and-go traffic on the way to work.

A new study confirms that the reaction time of cell phone users slows dramatically, increasing the risk of accidents and tying up traffic in general, and when young adults use cell phones while driving, they're as bad as sleepy septuagenarians.

"If you put a 20-year-old driver behind the wheel with a cell phone, their reaction times are the same as a 70-year-old driver who is not using a cell phone," said University of Utah psychology professor David Strayer. "It's like instantly aging a large number of drivers."

The study was announced today and is detailed in winter issue of the quarterly journal Human Factors.

Traffic jams and death

Cell phone distraction causes 2,600 deaths and 330,000 injuries in the United States every year, according to the journal's publisher, the Human Factors and Ergonomics Society.

The reason is now obvious:

Drivers talking on cell phones were 18 percent slower to react to brake lights, the new study found. In a minor bright note, they also kept a 12 percent greater following distance. But they also took 17 percent longer to regain the speed they lost when they braked. That frustrates everyone.

"Once drivers on cell phones hit the brakes, it takes them longer to get back into the normal flow of traffic," Strayer said. "The net result is they are impeding the overall flow of traffic."

Strayer and his colleagues have been down this road before. In 2001, they found that even hands-free cell phone use distracted drivers. In 2003 they revealed a reason: Drivers look but don't see, because they're distracted by the conversation. The scientists also found previously that chatty motorists are less adept than drunken drivers with blood alcohol levels exceeding 0.08.

Separate research last year at University of Illinois at Urbana-Champaign supported the conclusion that hands-free cell phone use causes driver distraction.

"With younger adults, everything got worse," said Arthur Kramer, who led the Illinois study. "Both young adults and older adults tended to show deficits in performance. They made more errors in detecting important changes and they took longer to react to the changes."

The impaired reactions involved seconds, not just fractions of a second, so stopping distances increased by car-lengths.

Older drivers more cautious

The latest study used high-tech simulators. It included people aged 18 to 25 and another group aged 65 to 74. Elderly drivers were slower to react when talking on the phone, too.

The simulations uncovered a twofold increase in the number of rear-end collisions by drivers using cell phones.

Older drivers seem to be more cautious overall, however.

"Older drivers were slightly less likely to get into accidents than younger drivers," Strayer said. "They tend to have a greater following distance. Their reactions are impaired, but they are driving so cautiously they were less likely to smash into somebody." But in real life, he added, older drivers are significantly more likely to be rear-ended because of their slow speed.

Other studies in the journal found:

Telephone numbers presented by automated voice systems compete for drivers' attention to a far greater extent than when the driver sees the same information presented on a display.Interruptions to driving, such as answering a call, are likely to be more dangerous if they occur during maneuvers like merging to exit a freeway.Things could get worse. Wireless Internet, speech recognition systems and e-mail could all be even more distracting.Are Cell Phones Really So Dangerous? Posted Feb. 2, 2005 at 10:15 a.m. ET

Several readers wrote to LiveScience questioning whether cell phones were really so bad for drivers. Here is some additional information that helps illuminate the death statistic.

The estimates of annual deaths reported in this week's article (2,600) may well be low. The number, for U.S. deaths related to drivers using cell phones, comes from a 2002 study by the Harvard Center for Risk Analysis (HCRA). Researchers then estimated that the use of cell phones by drivers caused approximately 2,600 deaths.

Because data on cell phone use by motorists are limited, the range of uncertainty is wide, those researchers said. The estimate of fatalities in that HCRA report ranged between 800 and 8,000.

Importantly, the researchers noted (in 2002) that increasing cell phone use could be expected to cause the annual death estimate to rise. The 2002 estimate, for example, was up from an estimate of 1,000 deaths in the year 2000. Logic suggests the number -- though just an estimate -- could be much higher in 2005.

The estimates are based largely on mathematical models, but they are not without basis. In 2001 in California, for example, "at least 4,699 reported accidents were blamed on drivers using cell phones, and those crashes killed 31 people and injured 2,786," according to an analysis by The Los Angeles Times. That number can expected to be low, because of the lack of formal procedures for noting cell phone use as a cause of a traffic accident.

The Times also noted a 1997 study of Canadian drivers "who agreed to have their cell phone records scrutinized found that the risk of an accident was four times greater while a driver was using the phone."

Each year, about 42,000 people die in U.S. auto accidents.

Here is how the new University of Utah simulations were conducted:

Participants in the simulator used dashboard instruments, steering wheel and brake and gas pedals from a Ford Crown Victoria sedan, surrounded by three screens showing freeway scenes and traffic, including a "pace car" that intermittently hit its brakes 32 times as it appeared to drive in front of study participants.

If a participant failed to hit their own brakes, they eventually would rear-end the pace car. Each participant drove four simulated 10-mile freeway trips lasting about 10 minutes each, talking on a cell phone with a research assistant during half the trips and driving without talking the other half. Only hands-free phones were used to eliminate any possible distraction from manipulating a hand-held cell phone.

Thirty times each second, the simulator measured the participants' driving speed, following distance and - if applicable - how long it took them to hit the brakes and how long it took them to regain speed.

-- RRB

Behind the Statistics

Are Cell Phones Really So Dangerous?