There's the bias between users willing to share that data and those not willing. All sorts of correlations could be hiding there.
I can't find out what the rate of refusals are, but I'd guess them to be rather low, maybe 10% at most. The survey is incredibly easy to perform and very transparent about colleted data.
The important thing to realise is that the survey is very consistent, so while the baseline may have some bias, the trends are very representative.
Like what? Are there special roads that busses can drive down but a sedan gets stuck on? Some kind of road made especially for the tires of the bus and no other vehicles? Like a tram system, or gondolas maybe?
Cars and busses are both road vehicles, and roads serve them both. We can put up signs and write rules about which vehicle can go where, but those are basically free to change.