In many underground mines, haulage vehicles (trucks or road trains) carry ore from underground loading stations to the surface. Vehicles travel in declines with occasional passing bays that allow descending empty vehicles to pull off the main path and wait for ascending laden vehicles to pass. The haulage productivity of a mine is affected by the number of passing bays and their locations. We have developed a simulator and a mixed integer programming model to study the optimal location of passing bays and the associated sizing and scheduling of the haulage fleet. This paper presents some practical implications of our study.