Drivers should put down their cell phones and keep their hands on the wheel. And a recent study shows they should turn their headsets off, too.

A study conducted by researchers at the University’s Beckman Institute found that using hands-free cell phones slows down talkers’ ability to react to changes in their environment, which could impair their driving ability.

The findings are at odds with legislation in several states, such as New York and New Jersey, as well as Washington, D.C., that prohibits the use of hand-held cell phones while driving but allows headsets. Similar laws have been discussed in the legislatures of all 50 states.

The study was completed by a group of University researchers and a professor from the University of Utah. The researchers tested the abilities of young and old people to react to changes in pictures of downtown Chicago as the images flickered across large virtual reality screens.

The team found that young people (mean age 20.6) performed worse on all facets of the test when talking with people out of sight.

“Young people were poorer, had slower times and made more errors across the board in all categories of the test when they were talking,” said Art Kramer, a psychology professor and researcher at Beckman.

Older participants (mean age 67.3) had a more difficult time differentiating important changes from unimportant ones, said Jason McCarley, an assistant professor of psychology and a member of the department of aviation that worked on the project.

“They wouldn’t catch the important stuff, like a child running across the street for a ball,” Kramer said.

Although the tests did not compare hands-free phones to hand-held ones, Kramer said other studies have suggested that accident rates are similar with both.

Carolyn Schamberger, a spokeswoman for Verizon Wireless, said it was important to consider the safety benefits of cell phones in addition to their dangers.

“A wireless phone is one of the best security tools since the inception of 9-1-1,” Schamberger said. “Cell phones make 150,000 calls per day to 9-1-1. A wireless phone in a car enhances safety when used responsibly.”

But using a phone while driving, even a hands-free phone, may not be very responsible. Kramer said the brain’s inability to multitask kept talkers from paying full attention to their environment.

“People think driving is visual, so as long as the hands are free, talking won’t make a difference,” Kramer said. “But generating speech causes a bottleneck, and you just can’t do several things at the same time.”

Silvia Manrique, a public relations manager for Cingular Wireless, commended the University on the study but said distracted driving goes beyond cell phones.

“It’s a bigger issue than that,” Manrique said. “It’s also people changing the radio while driving or eating while driving. Focusing on cell phones takes away from the bigger issue … We always encourage responsible cell phone use while driving.”

Hands-free cell phone use has become more prevalent in recent years as car companies have built voice recognition and cell phone technology into their cars.

Kramer pointed to the “On-Star” technology used by General Motor Corp., who helped fund the project, as part of the impetus for their work.

“I spent some time at GM over the years and was impressed with their engineers and all of their in-car stuff,” Kramer said. “But there’s nobody there that knows how speech generation works.”