## 5 Conclusions  
This study proposed a calculation method for freeway passenger-dedicated lane width based on overtaking behavior. UAVs are used to record videos on the common two-way six-lane freeways in China. Parameters such as the position and outermost dimensions of the vehicle are extracted from the aerial video based on computer vision technology. To intuitively express the relative motion between vehicles, we introduced a vehicle coordinate system at the geometric center of the overtaken vehicle. Based on this, the lateral motion and lateral distance of vehicles during the overtaking process are analyzed. The following conclusions could be drawn. 1. Vehicles' lateral displacements are mainly related to behaviors such as lane changing, lateral distance maintenance and lane keeping. In the process of lane-keeping overtaking, the lateral approaching behavior and departing behavior exist in a certain proportion. The difference in lateral displacement behavior is related to the initial geometric center distance between vehicles and the lateral distance expectation of drivers. Taking the expectation of 4.24 m before overtaking and 3.75 m after overtaking as a baseline, when the initial geometric center distance is higher than this value, vehicles tend to approach each other; otherwise, they tend to depart from each other. 2. For lane-changing overtaking behaviors, it can be seen that the overtaking vehicle tends to prepare in advance before overtaking and completes overtaking over a longer distance with a lower lateral speed; on the contrary, the lane-merging process after overtaking is completed over a shorter distance with a higher lateral speed. The standard deviation of overtaking trajectory and lateral speed take the minimum when the vehicles are parallel, and the lateral distance at that moment is better as the control factor of lane width design to take care of the behavioral habits of most drivers. 3. The geometric center distance and wheel distance between parallel vehicles do not obey the normal distribution. Most of the geometric center distances are larger than the lane width of 3.75 m, which indicates that parallel vehicles cannot travel near the centerline of the lane at the same time. With the increasing the body width sum of vehicles, the geometric center distance does not change significantly, while the wheel distance has a significant decrease. In addition, speed does not significantly affect wheel distance in the freeway driving environment. 4. In this study, the geometric center distance between parallel vehicles is taken as one of the controlling factors in lane width design due to its lower sensitivity to the change in vehicle body width. Moreover, the vehicle body width and the lateral safety margin are also taken as the control factors for freeway lane width design. The general value of the passenger-dedicated lane width on freeways is recommended to be 3.5 m, and the limit value is 3.25 m.
5. In practical applications of freeway engineering, it is advisable to adopt a standard lane width of 3.5 m for passenger car lanes. Given a fixed construction budget, consideration may be given to utilizing the minimum permissible lane width of 3.25 m for inner lanes. The conserved lateral space can then be reallocated to widen the shoulder, thereby enhancing sight distance within curves to some extent. This study focuses on the relationship between vehicle width and lateral distance, which is crucial for addressing the challenges posed by vehicle diversity in lane width design. The proposed model enhances our ability to manage these challenges effectively. Nonetheless, this study has certain limitations that warrant discussion. Firstly, all scenarios examined in this study featured lane widths of 3.75 m. The impact of varying lane widths on the lateral distance between vehicles remains an open question for future research. A narrower lane width could alter driving behavior and consequently affect the distribution of lateral spacing between vehicles compared to the standard 3.75 m lanes.