You're driving along and your cell phone blurts out that familiar ding, chime or snippet of old school hip-hop. A text message has arrived.

Do u txt back?

Texting while driving is widely condemned as just about the most dangerous thing imaginable behind the wheel. And yet thousands of Americans -- particularly young ones -- just can't seem to help themselves.

Half of adolescents send text messages while driving, according to a study released this week from the National Center for Children in Poverty.

Other studies have pegged the percentage of all drivers who text at nearly 25 percent.

"It's so stupid," said Noah Caplan, 32, describing the practice that he does roughly "every single time I'm in my car."

Though he knows texting while driving is dangerous, it's also irresistably convenient, he said, explaining that friends and co-workers who text him want to hear back right away and that a text message is far more efficient than a phone call.

"Our generation is so hooked on instant gratification," he said. "Ninety-nine percent of the time, it's something that can wait. I shouldn't even pay attention to it."

He really shouldn't, said Carnegie Mellon University psychology professor Marcel Just, who has done brain imaging research showing that just listening to a cell phone conversation reduces the amount of brain activity dedicated to driving by 30 percent.

Writing a text message would be a "double whammy," said Dr. Just, because not only are drivers' brains occupied but their fingers and eyes are as well.

"It's even worse, much worse than using a cell phone," he said. "You don't really need research to show that texting is a very bad thing to do. It's a horrible risk to be taking. It has killed people and will continue to kill people until it's stopped."

In a Virginia Tech study released in July that monitored actual drivers with video cameras, researchers found drivers who were text messaging to be 23 times more likely to be involved in a crash or a near collision than non-distracted drivers.

That study found that drivers in the midst of writing a text message spent 4.6 seconds out of every six seconds with their eyes off the road -- the equivalent of a length of a football field while traveling at 55 mph.

Even in a nation that views the automobile as a moving entertainment center, portable conference room and a dinner-table-on-wheels, texting does give some people pause.

Holly Glymour, a 25-year-old law student at the University of Pittsburgh, admits to regularly talking on a cell phone while driving. But texting while driving?

"It scares me," she said. "I think it's a lot more likely that I'll get in an accident."

Still, she understands the temptation. Friends who text her expect an instant response, she said, with no allowance for other priorities. "If you don't answer people within two minutes, they send me more texts," she said. "If they don't hear back, they'll freak out and call me."

Web programmer Andy Cherep, 31, used to text with wild abandon. But nine months ago, after the birth of his son, he decided that risking his life to text was no longer worth it, particularly given that the stylus that he uses to text on his phone makes it a two-hand operation.

So now he only texts at red lights.

It's a practice that is also not foolproof, he learned recently when he had "more than a close call." While texting at a red light in Green Tree, he bumped the car in front of him -- resulting in an exchange of insurance information but not in any damage.

Whether any amount of texting should be allowed is an issue that has conflicted Pennsylvania legislators. Many legislators recognize the danger that hand-held cell phones and other distractions can pose for motorists, but there is a strong reluctance to impose "nanny government" on citizens.

Last spring House members narrowly rejected a bill by state Rep. Josh Shapiro, D-Montgomery, that would have banned texting while driving. It would have made it a "primary offense," meaning police could stop a car if they saw the driver distracted by cell-phone chatting or texting.

In April, the House approved a somewhat watered-down measure sponsored by Rep. Joe Markosek, D-Monroeville, that would prohibit "junior drivers" from using "interactive wireless communications devices," including cell phones, while driving.

The Senate, meanwhile, has taken a different approach, approving a measure by Sen. Robert Tomlinson, R-Bucks, that makes texting while driving a secondary offense for all drivers.

Sitting outside Pitt's law school with Ms. Glymour, law student Joe Hirschmann is an unapologetic texter firmly opposed to a state law.

Mr. Hirschmann, 24, insists that he can text safely because he knows his iPhone so well that he can send messages without looking down at his phone. And if he hits the wrong key on the touchpad, he said, his phone has a spellcheck function.

"People who want to pass a law are in the older generation, who aren't capable of driving and texting without being distracted," he said. "I'm safer texting than a woman driving putting on makeup."

And besides, "it's not illegal in Pennsylvania," said Mr. Hirschmann. "Yet."

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