Considerable research has been conducted over the past decade on the response of both vehicles and occupants to low speed rear impacts. This research has employed various conditions of target vehicle braking and target occupant awareness. Relatively little effort has been devoted to quantitatively comparing vehicle and occupant responses under different braking and awareness. Given the variety of potential braking and awareness conditions in actual rear impacts, it is desirable to better understand the influence of these reactions on both vehicle and occupant dynamics.Low speed vehicle-to-vehicle rear end collisions were conducted with instrumented vehicles and an instrumented human subject. Six conditions were evaluated: 1) unaware occupant without braking, 2) aware occupant without braking 3) unaware occupant braking “normally”, 4) aware occupant full-braking, 5) unaware occupant with brakes mechanically fully applied, and 6) aware occupant with brakes mechanically fully applied. Three closing speeds were investigated (nominal 4, 8 and 13 km/h). The same occupant and vehicles were used for all impacts.Vehicle chassis velocity and acceleration were measured using 5th wheels and accelerometers, respectively. Occupant response was measured by accelerometers attached to the occupant's head and lumbar spine. Accelerations at the head static center of gravity were obtained via a 9-accelerometer headgear array.Little difference was found between vehicle kinematics for conditions in which the unaware occupant was not braking and the unaware occupant was braking “normally”. Decreased target vehicle velocity changes and increased bullet vehicle velocity changes were observed with fully or mechanically applied braking. Full or mechanical braking decreased the collision duration for the target vehicle, while bullet vehicle collision durations were not affected. Awareness of the impending impact decreased occupant head kinematic parameters and the target vehicle occupant described those impacts for which he was prepared as feeling less severe than those for which he was unaware. No differences were found in target vehicle occupant head kinematics for the unbraked and fully braked tests.