I don't have the link handy, but if you search back in some of my older posts, I relayed a link to a study that tried to assess the relative distractiveness of different activities undertaken while driving. That study put talking on a two-way radio as roughly half-as-distracting as talking on a handheld cell phone. But it put talking on a hands-free cell phone as very slightly less distracting as talking on a two-way radio. I don't think it's a simplex/duplex thing. I think it's a "holding something to your face" thing.