Yet another study has just been released showing why California's new ban on driving with handheld cell phones won't work.

It's pretty clear to everybody by now that cell phones can and often do cause auto accidents. But opinions differ as to why: Is it because handheld phones take one of a driver's hands off the wheel, or because there's something about cell-phone conversations that is innately distracting? California's Legislature seems to think it's the former, while most studies, including one just completed by Carnegie Mellon University, show it's the latter.

Starting in July, it will be illegal for Californians 18 and older to drive while talking on a handheld cell phone, while drivers under 18 will be forbidden even from driving with hands-free devices. It's mystifying what this law's backers think it will accomplish. As the Carnegie Mellon study and others at the universities of Utah and Illinois have shown, it's the conversation itself that causes drivers to weave out of their lane or fail to see red taillights ahead of them. Researchers used brain imaging to show that simply listening to someone on the phone reduces the brain activity associated with driving by 37%. Many people find this counterintuitive — why should talking to someone on the phone be more distracting than talking to someone in the passenger seat? One answer is that passengers can sense when there's trouble on the road ahead and stop talking; also, the tendency of cell-phone signals to fade in and out requires extra concentration on the part of listeners.

California's new law will be a boon for cell phone companies and retailers, which will make a mint selling headsets and Bluetooth systems to drivers. But don't expect it to reduce the number of accidents.