Guys, you must understand there's no sure way that everything will always work the same.
Vehicles don't always take the same time to arrive, because they don't always start from the same parking, and this depends how the airport is made so, you might see sometimes vehicles arriving at the same time, possibly clashing into each other, and sometime one might end up its service before the other one arrives.
So, we must plan for the worse, and assume they would always clash, and trying to prevent it.