100% a real thing. I haven't heard of anyone actually getting into an accident due to looking for mushrooms while driving, or at least admitting to it, probably in part because it's a hobby that people don't usually take up until they're well out of their most accident prone years. But I know a lot of people who have paid extra for lane assist and automatic emergency braking.

There is even a whole subset of mushrooms called the 55-mile-per-hour-mushrooms, so called because they're especially easy to identify while driving at highway speeds. Here is an old post referencing this: http://mpb.ou.edu/ben/ben416.html

Mushroom hunters are a bit notorious for being single minded about their hobby. One of the more popular books on the subject contains a reproduction of an actual event where the author ran off during the intermission of an orchestra that he was playing in to go hunt for mushrooms.

https://m.imgur.com/6Yv41