My understanding is that the drivers are often doing their best in maintaining schedules. If nature calls, or they know there is an opportunity, it is only fair for them to take it, then to risk the safety of passengers or the health of the operator him/ herself. It is selfish of you to suggest, or at least come across that your schedules or activities are of greater importance. Bus drivers are also people. You should take the opportunity to thank them, rather than frown at them for simply relieving themselves when nature calls. Just keep in mind, bus drivers aren't always within reach of a washroom when they need it. The likelihood of them holding out until the next opportunity, I'd be willing to bet, is larger than a good portion of us, who (I'd say) are generally within fair proximity to the facilities.