An instrumented vehicle study was performed on a motorway in the United Kingdom to examine the behavior of drivers faced with the cut-in of a vehicle lane-changing into the space between themselves and the preceding vehicle. Data concerning this activity are in short supply and may be used not only in formulating models of human response in driving but also in designing and optimizing driver assistance aids such as adaptive cruise control (ACC). The cut-in vehicle used was equipped with a rear-facing radar unit enabling it to monitor the degree and speed with which drivers attempted to restore their original headway. Cut-ins from both directions were examined--moving in from a slower lane (94 events) and from a faster lane (72 events). The criticality experienced by the follower vehicle ranged from moderately severe [time to collision (TTC) around 10 s and time gap around 0.35 s] to noncritical (lead car's speed at cut-in greater than follower speed and time headway beyond steady-state values). Findings indicate that the "pullback" behavior, at least over the initial 5 to 10 s, can be described by a constant pullback speed (the rate of decrease of the initial speed), and causative models for this response have been derived using "instantaneous" variables (those that may be calculated on cut-in, such as relative speed and TTC) and longer-term "target" variables, such as desired headway, the former of which have been most effective in describing behavior. Finally, empirical responses have been compared with those that would be produced by ACC systems; it was found that a comparatively close match is produced for low values of relative speed.