Jamie calls it Shower Brain, for me it’s driving brain.

people’s tendency toward insightful or analytical thinking is evident during “resting-state” brain activity–while a person relaxes with no task to perform or expectation about what is to come.

When I had an hour long commute either end of the day, I would find breakthroughs in projects would come more readily. Now I no longer have a commute I find motorway or “big road” drives have the same effect. The task requires concentration but on a level that has become second nature to me, my mind can then work on things without my realising it.