Earlier quoted context omitted.
Sounds plausible that it's more distracting, but I'm not convinced that the difference is more varied than found among different types of passengers (back-seat driver/kids/dumb friends/audiobook(?) etc) If you have a few studies that show more than that, I would be glad to check it out.
I don't know of any that have looked at different types of passengers. They have usually just compared an adult passenger talking with the driver to an adult on a phone talking with the driver. Here was a recent study that was pretty interesting: https://news.illinois.edu/blog/view/6367/204502 Here's an older one: https://www.sciencedaily.com/releases/2008/12/081201081917.h...
thanks, I appreciate the links :)