This study investigated the mechanism of drivers' hazard judgement for lane change decision. Subjects were tested to determine a critical inter-vehicle gap by which the subject judges between safe and unsafe lane changes. Then, a regression equation to correlate the critical gap and relevant vehicle state variables was obtained. It is shown that the equation's structure paralleled the subjects' hazard perception mechanism and its parameters corresponded with their judgement criteria. Comparing these subjective hazard criteria with an objective hazard criterion, safety margins in the subjects' judgements were analyzed and the need for lane change support systems was discussed.