To Carnegie Mellon professor Marcel Just, the issue is clear: Driving while talking on a cell phone is dangerous.

After presenting a study that determined that listening to spoken sentences significantly reduces brain function in areas crucial to driving, Dr. Just told a state legislative committee yesterday that cell phone use while driving should be banned.

"Given that we know it impacts driving and brain function, there's no reason to allow people to take that risk," he told members of the House Transportation Committee at a hearing Downtown.

For lawmakers, the issue is more complex.They are considering several bills that would, among other things, prohibit drivers with learner's permits from using cell phones and having more than one passenger under 18; and ban text messaging and mandate hands-free phone devices for all drivers.

State Rep. Josh Shapiro, D-Montgomery, who authored the hands-free bill, sparred with Dr. Just, saying his bill would be a step in the right direction.

"It's just not enough," Dr. Just responded.

His research involved 29 volunteers who used a video game-like driving simulation while listening to sentences. Their brain functions during that time were captured by a functional magnetic resonance imaging machine, and the results showed a 37 percent drop in brain activity associated with driving.

Dr. Just said the distraction level would probably compare to that of listening to a book on tape in the car, but further studies would be needed to see if it applied to music as well.

Cell phones are more distractive because a driver can turn down the radio or cut off a conversation with a passenger if driving conditions require it, he said. Ending a phone call is more problematic.

The committee also heard from Sheila Klauer of Virginia Tech University's Transportation Institute, whose studies have focused more on the physical element than the cognitive.

Video cameras placed in 100 cars for a study showed the perils of distractions, especially among teens. Complex tasks that require multiple glances away from the road increase risk by two to three times, the study found, and 18- to 20-year-old drivers were involved in four times as many crashes and near-crashes than any other age group.

In one video that Dr. Klauer showed the committee, a young woman nearly ran over a child on a tricycle while adjusting her iPod.

"Had she not looked up at that critical moment, this could have been a devastating life event," said Dr. Klauer, who also recommended a full cell phone ban.

Matthew Sundeen, program principal for the National Conference of State Legislatures, and Brian Newbacher, director of public affairs for AAA East Central, testified about how states and localities are addressing the problem.

A total of 29 states and the District of Columbia have passed laws restricting cell phone use -- though no jurisdiction has an outright ban. New Jersey and Washington state have been the only ones to ban text messaging, though 21 states are considering such bills this year.

Committee members seemed to agree that action should be taken, but minority chairman Richard Geist, R-Altoona, said it's tough to legislate against cell phone use when so many other perilous driving behaviors are commonplace -- including putting on makeup, eating and even shaving.

He told the committee comprehensive action must be taken "rather than a rifle shot at one issue."

Rep. Paul Costa, D-Wilkins, admitted he's as guilty as anyone. While driving on the turnpike, he checks his BlackBerry. But he draws the line at the parkway, where the device goes untouched.

In combating distracted driving, Mr. Costa said, education can prove more important than legislation. Yesterday's event -- which was as much about getting word out as policy -- was the third such hearing on distracted driving, following meetings in Philadelphia and Harrisburg.

"If anything, our job is to continue to stress how dangerous this is until people start policing themselves," Mr. Costa said in an interview. "We're not there yet, but we're getting there."