There's so many issues with trying to RP with people in cars in the tubes that I've pretty much abandoned all hope of it. Unless they're in the back, where the screen isn't getting constantly spammed. So cabbies I guess would be fine.
I'd like to see the proposed changes I think you, or possibly someone else brought up a year or two ago, about making the tubes only a handful of rooms and greatly extending the driving times through them, or allowing automatic traffic control when entering/exiting tubes. I really don't think we'd be losing much if we couldn't get out of our cars and off our bikes in the middle of the tubes, personally.