Relative rollover conditions of a heavy vehicle are analyzed to establish an array of potential dynamic rollover indicators towards development of an early warning device. A relative roll instability indicator defined as Roll Safety Factor (RSF) is proposed and shown to be a highly reliable indicator regardless of vehicle configurations and operating conditions. The correlation of various potential rollover indicators with the roll safety factor are then investigated for a 5-axle tractor semi-trailer combination using a comprehensive directional dynamic analysis model to determine the reliability of the proposed indicators over a range of operating conditions. The indicators are further examined in terms of measurability, lead time, and potential for application in an early warning system. The study shows that the trailer lateral acceleration and axle roll angles are closely correlated with the RSF. The steering factor and the tractor lateral acceleration indicators provide considerably large lead time for the driver's actions when compared to that of the RSF. These indicators thus offer considerable potential for use in generating an early warning of impending dynamic rollover. Different designs of a rollover early warning device to manipulate the dynamic response signals are finally discussed.