Studies show that the handset is not any less dangerous than the headset, and can be less so. The headset tends to get left on, and thus blocks sound from an area during driving. And, as well, the zoning mind tends to not care what the distraction is. The idea that one-handed driving is bad is still there, but many drivers are not "ten-and-two" people anyway, so there is no loss.

You're much worse on the road with a passenger that you want to address, or food you want to eat, or music you want to enjoy, than with any cell phone. Until they outlaw drive-thru cuisine and front seat riders, I don't expect any change on cellphone usage to be of any importance.