Even when they used "hands-free" devices, young drivers who normally have the quickest reflexes drove like 70-year-olds, with slower reaction and a tendency to miss what was right in front of them, the team at the University of Utah found.
"If you want to act old really fast, then talk on a cellphone while driving," Frank Drews, an assistant professor of psychology who worked on the study, said on Wednesday.
"If you put a 20-year-old driver behind the wheel with a cellphone, their reaction times are the same as a 70-year-old driver who is not using a cellphone," added David Strayer, a psychology professor who led the study and who has been studying the effects of cellphone use on driving for years.
Writing in the journal Human Factors, Strayer's team said they tested people aged 65 to 74 against drivers aged 18 to 25.
Preliminary tests showed older people were slower in processing information, which is normal and expected.
Then the volunteers used a driving simulator with dashboard instruments, a steering wheel and brake and accelerator pedals, surrounded by three screens showing freeway scenes and traffic.
Cellphone use in cars significantly
slows down reflexes
An image showed a car in front repeatedly hitting its brakes.
Each volunteer drove four simulated 16km freeway trips lasting about 10 minutes each, talking on a cellphone with a research assistant during half the trips and driving without talking the other times.
Only hands-free devices were used.
The older drivers hit the brakes more slowly to avoid the car in front, tended to hit the brakes twice, took longer to regain speed and had a greater following distance.
Cellphone use made older people drive even worse and younger drivers act like elderly drivers.
"Once drivers on cellphones 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."
"Once drivers on cellphones hit the brakes, it takes them longer to get back into the normal traffic flow"
Psychology professor David Strayer, University of Utah
Braking time slowed 18% when young or elderly drivers used a cellphone, the researchers found.
Chatting on the telephone caused a 12% greater following distance, apparently an effort to compensate for paying less attention to the road.
But that tactic did not always work.
"There was also a twofold increase in the number of [simulated] rear-end collisions when drivers were conversing on cellphones," the researchers wrote.