Every one who has driven on a freeway has observed the phenomenon of lanechanging. This phenomenon is, of course, caused by the desire of most of the drivers not to be in a slow-moving lane. Therefore, the average driver who finds himself in such a lane moves into a neighboring faster lane, usually after a certain time-lag. This time-lag depends on the dynamic characteristics of the vehicle, the availability of acceptable gaps, and the driver risk, which is the value the driver places on the probability of collision during a maneuver, i.e. the higher the perceived probability of collision, the higher the time-lag. Modelling of the lane-changing phenomenon has been the objective of many investigators in the past. As will be shown later in this study, lane-changing is a very important component in highway traffic flow. In this study, a mathematical model to describe the lane-changing behaviour is suggested based on the lane-changing hypothesis that whenever there is a lane-changing maneuver, the average speed of the neighboring lane is faster than the average speed of the current lane. A set of data has been collected by a methodology which involves aerial photographic technique. The collected data are then used to test the validity of the lanechanging hypothesis, to calibrate and validate an existing lane-changing model, and to develop a gap acceptance function for freeway lane-changing maneuvers. / Master of Science