Drivers who wheel and deal on their car phones are more likely to reach out and bash someone than those who just chitchat, a study has found.

The study, for the American Automobile Association Foundation for Traffic Safety, found that dialing a car phone or holding a simple conversation on one was no more distracting for a driver than tuning a car radio. Drivers doing either are 20 percent more likely not to notice a potential traffic hazard than motorists concentrating only on their driving, the study found.But drivers holding a complex conversation run a 30 percent greater risk of not noticing that a tree has stepped into their path. "Intense business conversation could well divert a driver's attention to the point that cues of potential danger may be overlooked," said a summary of the study by the National Public Services Research Institute of Landover, Md.

The study also found that drivers over age 50 are twice as likely as younger motorists to be less attentive to traffic when talking on the phone.

Car phones have grown rapidly in popularity since the early 1980s, and the Cellular Communications Industry Association, a trade group, says that about 5 million are in use now.

The study's conclusions are based on tests in which 151 drivers, about half of them regular users of car phones, operated driving simulators while talking on cellular phones. Like most units now sold, the phones were the "hands-free" type, which need not be removed from their cradles to be used. But they lacked the capability to dial preprogramed numbers with the touch of one button.

A spokesman for the cellular-phone association, Norman Black, did not dispute the findings and said that the industry is trying to educate consumers to be judicious in their use of car phones.

"You've got to use common sense," he said. "You do not try to punch in numbers when you're in a driving rainstorm. You do not try to close the most important deal of your life when you're trying to drive quickly home or to pick up your child from the day-care center."