## 2 Data Collection  
To obtain a better understanding of how the contraflow strategy affects evacuation traffic on highways and explore alternative methods tailored to the specific traffic conditions, both field experiments and traffic simulations were performed in this study. There were three reasons for performing the field highway contraflow experiments. First, the highway contraflow strategy has rarely been employed during emergency evacuations in the past, and the data from real contraflow traffic are presently unavailable. In the rare cases when a highway contraflow strategy was used, either the amount of traffic was small, the length of the contraflow segment was very limited, or the contraflow duration was short. This resulted in the failure of the experiments to reflect the true situation of highway emergency evacuation. Conversely, the conditions of the present field experiments on the Xihan Highway were similar to those during real emergency evacuations. Second, most previous studies on the contraflow strategy focused on the selection of the contraflow route and the estimation of the travel time on a macroscopic level, with only a few examining the traffic characteristics in the contraflow segments on a microscopic level. Third, although the concept of the contraflow strategy is easy to understand, its practical arrangement and management is challenging. Many of the associated problems can only be properly encountered in the field. The Xihan Highway connects Xi'an and Hanzhong, extending approximately 259 km and incorporating 43 junctions. It is a dual four-lane highway, with each lane measuring 3.5 m in width. To improve the road pavement condition of the highway, the Department of Transportation of ShaanxiProvince conducted pavement maintenance works along the entire stretch between 20 July and 31 August during 2017. The activities included pothole patching, minor reshaping, minor sealing, and crack filling. During the time of the project, trucks were forbidden from the highway and traffic contraflow was implemented along several segments of the highway. In Figure 1, the Xihan Highway is marked in red, with the contraflow sections highlighted in blue. The effect of the contraflow strategy was analyzed based on data collected at the section between K1244+700 and K1258+000 along the Xihan Highway (a section about 13.3 km long). Specifically, the contraflow section had its start and end points at K1422+700 and K1258+000, respectively (number 1 contraflow section in Figure 1). The geometric design of this road section was appropriate so that the effect of the road alignment on vehicle speed could be ignored. The speed limit within the road section during normal times is 80 km/h. During the experiment, a laser-based AxleLight vehicle classifier system (AVCS) was installed at 25 different observation points beside the road to collect traffic flow data from each contraflow and normal traffic lane, including the vehicle speed, travel direction, vehicle type, wheelbase, and headway. Additional observation points were set at the entry and exit of the contraflow section. The traffic flow data obtained by the AVCS were automatically and continuously stored and could not be lost, even when the system was switched off. The compactness of the AVCSs enabled their easy setup, simple operation, and concealment behind the highway side barriers. They generally facilitated the acquisition of large amounts of real traffic data under contraflow and regular conditions. As a major link between two famous tourist cities, Xi'an and Hanzhong, the Xihan Highway is always busy, especially so during the travel seasons. In this study, the contraflow conditions during peak periods of the highway were observed to examine the effectiveness of the contraflow strategy for mass evacuations during serious emergencies.