I was really expecting this to be higher not lower due to factors like particulate inhalation from exhaust/brake dust/tire particles. Also there's a lot of sedentary-type problems you get while taxi driving like bad diet habits that are not conducive to brain health.

Dunno, did taxi driving for a few years. Mostly suburban for a small fleet, not gigging. I'm thinking newer drivers that rely on smartphones for navigation won't get the same benefit.

I seem to recall that at least some populations of taxi driver they have exams like The Knowledge (https://london-taxi.co.uk/the-knowledge/) where changes in structures of the brain can be measured after learning it.