In this study, the safety margin allowed by air passengers departing from Hong Kong International Airport, and its relationship with the passengers' personal and trip characteristics and ground access mode choices is investigated. Safety margin, as used in this study is defined as the difference between the effective travel time and the actual travel time required, while the effective travel time is defined as the difference between air passengers' preferred arrival time at the airport terminal for check-in and their departure time from their trip origin. In the belief that the safety margin allowed by departing air passengers varies, it was felt that there is a need to further investigate departing air passengers' airport ground access mode choices decisions, and the implication, if any, on their allowed safety margin. Independent-sample t-tests were performed to distinguish similarities and differences in the size of safety margin allowed by (1) business and non-business travelers, (2) long-haul and short-haul travelers, and (3) Hong Kong residents and visitors. The analysis results revealed that, comparatively, business air passengers, long-haul travelers and those transported by franchised buses allowed larger safety margins than the air passengers with other agendas and airport access mode choices.