Impacts between passenger vehicles and heavy vehicles are uniquely severe due to the aggressivity of the heavy vehicles; this is a function of the difference in their geometry and mass. Side crashes with heavy vehicles are a particularly severe crash type due to the mismatch in bumper/structure height that often results in underride and extensive intrusion of the passenger compartment. Underride occurs when a portion of one vehicle, usually the smaller vehicle, moves under another, rendering many of the passenger vehicle safety systems ineffective. Heavy vehicles in the US, including single-unit trucks, truck tractors, semi-trailers, and full trailers, are currently not required to have side underride protection devices. The NTSB, among other groups, has recommended that side underride performance standards be developed and that heavy vehicles be equipped with side underride protection systems that meet those standards. The work presented used virtual testing to evaluate the relative performance of example side underride devices compared with a baseline. We also evaluate the effects of different test conditions on underride guard performance. Crash test results were utilized for calibration purposes. A tractor-trailer, with and without side impact underride protection, was impacted by a passenger car and SUV under a range of impact conditions. Passenger vehicle intrusion metrics were calculated to provide an indication of relative risk for each impact condition. The results can support the development of side underride protection recommended practices.