...at least when you get to the level of stardom that Willie is at... To the point of being soundproofed in several sections... separate sleeping areas for crew and separate one for headliner , living area, lavatory, and the driver's compartment so he/she can drive without disturbing the passengers, Look at the roof of these busses (or at the tops of the most expensive RV's on the road) and you will see 2 and 3 separate air conditioning units for zone by zone ventilation.
I'm not saying the door from the back to the driver's compartment wasn't open... not saying that the driver wasn't partaking himself (although in my experience with IATSE work - drivers are more likely to do speed than smoke pot, but more likely altogether, to be fired if caught under the influence) but a professional tour driver who does drugs while driving on tour is actually an extremely rare thing. You simply can't be stoned/drunk and keep up with the grueling tour schedule.
The likelihood of second hand buzz by the driver and a wall of smoke rolling out from the buss door when it opened (i know that is not described but I'm sure it is what many imagine) is improbable. Besides... I'm sure the driver was put through a field sobriety test immediately although the cops would have had plenty of time to do blood work/urine tests, and get results, considering how long the police would have taken tearing a tour bus apart along with luggage and equipment.
My opinion is the Toby Keith interview was a factor in the stop and search. In his 5 minute interview on Colbert... he spent more time talking about the prodigious pot smoking on Willies' bus than he did talking about himself.