<p>Increased downgrade safety is particularly important on mountainous roads since horizontal curves can be combined with a vertical alignment or steep slope. Urban and rural interchanges may also have a downgrade by combining the horizontal curve and vertical alignment, in addition to mountainous areas. Various studies have examined the safety effect of curves and alignments using the concept of vehicle dynamics modeling. However, few studies have been performed on how braking force, vehicle speed, and departure angle of run-off-the-road vehicles affect their performance over downgrades with a foreslope. In this study, the effect of negative longitudinal slope on the safety of downgrade roads was investigated using lateral acceleration, yaw moment and roll moment. The results showed that changing the foreslopes’ slope has a negligible impact on the heavy vehicle’s lateral acceleration, which is more effective than altering its speed and then its departure angle from the road. Reducing the longitudinal slope of the downgrade, softening the foreslope, and lowering the vehicle’s departure angle result in lower yaw moment values. This, in turn, lowers the skidding and sliding potentials of the rear axle of the vehicle around the front axle. The greatest change in the roll moment of Sedans occurs at 100&#xa0;km/h and 7.5° departure angle, while for SUVs and trucks, it occurs at 120&#xa0;km/h with the same departure angle. In addition, the braking force has a significant impact on the safety of vehicles, so that by increasing the braking force, the safety of the vehicle increases by reducing the roll moment of the vehicle.</p>