## 1 Introduction
Traditionally, researchers collect traffic data at peak points of non-emergency implementations of the contraflow strategy, such as during special events, based on which they attempt to assess the effects of using the contraflow strategy for emergency evacuation [9; 10]. The results of such studies have shown that the adoption of an empirical observation enhanced the benefits of this approach. However, this was overly dependent on the prevalence of the appropriate situations, with respect to factors such as traffic volume, traffic composition, road structure, and the length of the contraflow lanes. Different algorithms and simulation methods have been used to evaluate the effects of different contraflow schemes to attempt to develop an optimal scheme. Software packages like DynusT [5], DYNASMART (Dynamic Network Assignment-Simulation Model for Advanced Roadway Telematics) [11], and CORSIM [4] were developed for the estimation of traffic volume and travelling time under different contraflow schemes. Heuristic [12], genetic [13], greedy [14], and Tabu search [15] algorithms have also been used to search for the best contraflow schemes. Through the modelling and simulations performed with the aid of such algorithms, major breakthroughs have been achieved in the use of traffic contraflow for mass evacuation. Nevertheless, this research method requires a large amount of real data for parameter adjustment and to prove the reliability and feasibility of the employed models. Despite the effectiveness of the contraflow strategy for managing a substantial increase in traffic volume during emergency evacuation, several problems still arise from its implementation [16]. The most serious of the problems is travel delay, which is mainly caused by \"fixed bottlenecks\" at the entries and exits of the contraflow lanes. The bottlenecks are usually too narrow for vehicles to quickly pass through, resulting in decreases in the traffic flow speed, typically of 15-25 km/h [5]. The diversion flow at the entries and merge flow at the exits also impact the evacuation efficiency. There is also the problem of \"moving bottlenecks,\" which are traffic buildups caused by slow-moving large or heavily loaded vehicles such as trucks and coaches. Many researchers have investigated the reduction of \"fixed bottlenecks\" through the optimization of the design of the entries and exits of contraflow lanes. However, to the best of our knowledge, few studies have considered the travel delay due to \"moving bottlenecks\" in contraflow lanes. Because the effects of moving bottlenecks are closely related to the ratio of heavy vehicles, previous researchers have eliminated or neglected their impact by forbidding trucks and coaches or setting a constant ratio of trucks and coaches [17]. For example, some studies assumed that the percentage of heavy vehicles was 15%, which is twice the 7-8% indicated in the 2001 Federal Highway Administration (FHWA) Highway Statistics Annual Report [3; 5; 16]. Some researchers also assumed that the evacuation traffic was composed of 98% passenger vehicles and 2% heavy vehicles, based on the fact that most evacuees prefer using their own vehicles to escape from hazardous areas [18]. In reality, during mass evacuation, the ratio of trucks and coaches may be relatively high and may also vary among different regions. Further research is thus required to explore the effect of \"moving bottlenecks\" on traffic flow when using the contraflow strategy, with respect to the heavy vehicle ratio. In the present study, traffic data were collected during peak traffic hours on the Xihan Highway under contraflow and regular traffic conditions. The data were used for a microscopic investigation of the traffic characteristics of the contraflow and normal traffic lanes. Trucks were not allowed to enter the test section during the observation periods, and thus only traffic data for passenger cars and coaches was obtained. The effects of the moving bottlenecks caused by the coaches under contraflow strategy were examined.