Describing Non–Lane-Based Motorcycle Movements in Motorcycle-Only Traffic Flow

The motorcycle is the main transport mode for commuters in Vietnam because it costs less than a car and has greater mobility on congested city roads. A better understanding of the characteristics of motorcycle traffic flow can be helpful in formulating proposals for traffic management policies. Under congested conditions, motorcycles are observed to change their directions and speeds quite often. This study focuses on the zigzag movements of motorcycles in motorcycle-only traffic, which are termed “non–lane-based movements.” Research was conducted on the mechanisms of non–lane-based movements with a consideration of how two behaviors of a subject motorcycle—acceleration and deceleration—were affected by the velocity changes of lead motorcycles. A concept of safety space was introduced to explain these behaviors. Calibration data for the proposed model were extracted from video clips taken at road segments in Ho Chi Minh City, Vietnam. By calculating the root mean square error of the estimated value and the field value, the proposed model reproduced the speed and direction of motorcycles with high reliability. A computer simulation was used to reproduce two basic types of non–lane-based movements—oblique following and swerving—and to verify the difference in the speed–flow relationship between lane-based and non–lane-based movements.