The motorcycle is the main transport mode for commuters in Viet Nam, owing to its lower costand higher mobility than the car when running on congested city roads. A better understanding ofcharacteristics of motorcycle traffic flow can be very helpful in formulating proposals for trafficmanagement policies. Under congested conditions, motorcycles are observed to change theirdirections and speeds very often. This study focuses on the zigzag movements of motorcycles inmotorcycle-only traffic, which are termed non-lane-based movements. The paper researches onthe mechanisms of non-lane-based movements considering how two behaviors of a subjectmotorcycle—acceleration and deceleration—are affected by the velocity changes of leadmotorcycles. A concept of safety space is introduced to explain these behaviors. Calibration datafor the proposed model were extracted from video clips taken at road segments in Ho Chi MinhCity, Viet Nam. By calculating the root-mean-square error of the estimated value and the fieldvalue, the proposed model reproduced the speed and direction of motorcycles with highreliability. A computer simulation was used to reproduce two basic types of non-lane-basedmovements—oblique following and swerving—and to verify the difference in the speed-flowrelationship between lane-based and non-lane-based movements.
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