## 4 Effect of Moving Bottlenecks on the Contraflow Lane
### Effect of Truck Moving Bottlenecks
#### 4.2.2 Vissim Simulation Results and Discussions  
The determined average speeds of the passenger car and truck platoons in each scenario are summarized in Figure 5. The truck ratio significantly impacted the effectiveness of the traffic flow, particularly for the passenger car platoons. With an increasing truck ratio and traffic volume, the average speed of the passenger car platoons reduced sharply to the average speed of the truck platoons. In the NR scenario, each 10% increase in the truck ratio decreased the average speed of the passenger car and truck platoons by 20-30 and 10-20 km/h, respectively. In other words, in the event of an emergency, a high truck ratio would substantially reduce the benefit of the contraflow strategy, posing a high risk to evacuees. Another possible reason for this was that the trucks needed to slow down to much slower speeds of 10-20 km/h to cross the highway median at the entry and exit from the contraflow lane. Moreover, faster vehicles in the normal lanes had opportunities to overtake slower vehicle platoons. However, when the road was saturated and the truck ratio was high, faster vehicles could not overtake slower ones, and this prevented their improvement to the traffic flow. The NT, NC, and TL schemes were also found to improve the traffic under contraflow conditions, including a significant improvement of the passenger car platoon flow. However, they had little effect on the performance of the truck platoons, and even sometimes decreased their speed. Among the NT, NC, and TL schemes, the first maintained the fast movement of passenger cars. For example, for a traffic flow of 300 veh/h and truck ratio of 40%, the passenger car platoons could still maintain at least 60 km/h under the NT scheme. This also highlights the benefit of a contraflow lane for improving outbound road capacity when coaches and trucks are forbidden from it. The bottlenecks created by coaches and trucks are the reason why the benefit of using a highway contraflow strategy is often much less than expected in practice. Independent sample \(t\)-tests of the speeds of the passenger car platoons under different traffic volumes and truck ratios with contraflow revealed a significant difference between the passenger car platoon speeds for the NR and NT schemes for (1) a truck ratio of 20-40% and traffic volume of \(>\)2500 veh/h, and (2) a truck ratio of \(>\)40% (\(p<0.05\)). Thus, when there is more than one evacuation route, a combination of contraflow with the NT scheme and the redirection of trucks to alternative routes is recommended for substantially higher traffic flow. Furthermore, when the traffic flow and truck ratio were small, the NC scheme was found to have a better effect on the passenger car platoons when compared with the TL scheme. Otherwise, the TL scheme was better. The main difference between the NC and TL schemes was in the possibility of passenger cars to overtake slower vehicles on the normal lanes. Under the former, passenger cars and trucks could both drive in normal lanes, whereas the latter restricted them to the left and right lanes, respectively. For small traffic flows and truck ratios in the NC scheme, passenger cars could both use the contraflow lane and overtake slow vehicles on the normal lanes, whereas the TL scheme prevented overtaking and constrained passenger cars to car following, while also preventing full utilization of the road. With an increasing traffic volume and truck ratio, passenger cars in the NC scheme found it more difficult to overtake, resulting in reduced mixed-traffic flow. This is representative of when the TL scheme is more beneficial, with the lane separation of passenger cars and trucks allowing each to maintain relatively high speeds. In addition, the truck lane is more utilized by the increased number of trucks. Because the speed limit of the present test highway is 80 km/h, the selected traffic scheme should allow passenger cars to reach at least 60 km/h as much as possible.