A comprehensive evaluation of roll and yaw stability of tractor/semitrailers with single and dual trailers in city and highway conditions is conducted. Commercial vehicles fundamentally behave differently in city driving conditions than at high speeds during highway driving conditions. In order to closely examine each, this study offers two distinct evaluations of commercial vehicles: 1) low-speed driving in tight turns, representative of city driving; and 2) high-speed lane change and evasive maneuvers, typical of highway driving. Specifically, for city driving, the geometric parameters of the roadway in places where tight turns occur—such as in roundabouts—are closely examined in a simulation study in order to evaluate the elements that could cause large vehicle body lean (or high rollover index), besides the truck elements that have most often been studied. Two roundabout geometries, 140-ft single-lane and a 180-ft double-lane, are examined for various truck load conditions and configurations. The vehicle configurations that are considered are a straight 4x2 truck, a tractor with a 53-ft semi-trailer (commonly known as WB-67), and two trucks in double-trailer configurations. Five potential factors are identified and thoroughly studied: circulatory roadway cross-section, roundabout tilt, truck configurations, truck apron geometry, and truck load condition. The results of the study indicate that when the rear axles of the trailer encounter the truck apron in the roundabout, the climbing and disembarking action can cause wheel unloading on the opposite side, therefore significantly increasing the risk of rollover. Interestingly, in contrast to most high-speed rollovers that happen with fully-loaded trailers, at low speeds, the highest risks are associated with lightly loaded or unloaded trucks. For high-speed driving conditions, typical of highway driving, a semi-truck with a double 28-ft trailer configuration is considered, mainly due to its increasing use on U.S. roads. The effect of active safety systems for commercial vehicles, namely Roll Stability Control (RSC) for trailers and Electronic Stability Control (ESC) for the tractor, is closely examined in a test study. Various trailer loading possibilities are evaluated for different combinations of ESC/RSC on the tractor and trailer, respectively. The results of the study indicate that 1) RSC systems reduce the risk of high-speed rollovers in both front and rear trailers, 2) the combination of ESC (on tractor) and RSC (on trailer) reduce the risk of rollover and jackknifing, and 3) RSC systems perform less effectively when the rear trailer is empty.