The Manual on Uniform Traffic Control Devices (MUTCD) for Streets and Highways recommends hill signs be placed in advance of downgrade descent of mountain passes. Mountain passes increase the risk of a runaway, or out of control trucks and so the advance warning signs inform the driver to take special precautions such as reducing speed or using lower gears during the descent. The Wyoming Department of Transportation has installed steep grade advance warning systems on Wyoming mountain passes. However, concerns for out of control trucks on the mountain passes persist. The objective of this study is to evaluate the safety effectiveness of steep grade advance warning signs for trucks on Wyoming mountain passes. The safety evaluation was carried out by implementing a zero-inflated negative binomial modeling technique for predicting truck crashes on mountain passes. The outcome was two models that showed the risk of runaway truck accidents were high at locations where either the grades were steep and long or the grades were long with multiple vertical curves. The analysis showed the current advance warning systems were not significantly impacting truck crash risks at the high risk locations. The study, therefore, recommends some improvements to the current advanced warning signs or implementing an improved Federal Highway Administration Grade Severity Rating System based warning system that will significantly improve truck safety at the hazardous locations. The study informs policy makers on the safety issues on Wyoming Mountain passes with regards to runaway trucks and makes recommendations for reducing the risk of runaway truck crashes on mountain passes.