I've definitely noticed a lot of change in my thought processes from really having to think hard & check the map book a lot to begin with to visualizing the route in head to now I just ride in the right direction - don't often have to plan the route out other than deciding order of stops, consciously anyway.
I think one advantage of being on a bike not in a car is that the shortest route is usually the quickest whereas one ways and traffic etc means that cabbies would have to take more circuitous routes, needs a bit more planning.
"The hippocampus was only active when the taxi drivers initially planned their route, or if they had to completely change their destination during the course of the journey. "
Couriers have to change the course of our journey a lot more than cabbies so maybe we get even bigger hippocampuses?