It was on TV. I should have said "tests" instead of "studies". It was an episode of Alan Alda's Scientific Frontiers on PBS... maybe a year ago?

As I recall, they tested drivers in simulators sober, with some alcohol (but legal level), legally impaired, and sober but engaged in cell phone use. Those who were sober but using the cell phone scored about the same as those who had some alcohol, but not enough to be legally impaired.

In order to justify banning cell phone use while driving, to be just, we'd have to ban driving with something as low as .04 blood alcohol level (which we do for 16-17 year olds in CA). By the way, I'm all for banning cell phone use while driving for those under the age of 18. They are much more prone to making mistakes than those who are older and more experienced, and much more prone to being seriously distracted by cell phone use.