## 2 Methodology  
### Scheme Design  
In the conventional design for an eight-lane expressway under the management strategy in China, four lanes are equally allocated to passenger vehicles and mixed traffic in each direction, but without a guardrail separating them. This lane restriction (LR) scheme is frequently taken on mainlines with a design speed of 120 km/h, and restrictions are removed around interchanges to provide conditions for diverging traffic to change lanes. However, passenger vehicles, especially those in the fast lane, must make consecutive lane changes within diverging times. In addition, trucks in the curb lanes disturb the interweaving of traffic. Consequently, congestion frequently occurs around interchanges during periods of heavy traffic. Figure 1 shows the conventional design for an interchange exit ramp, which features two-lane ramps in each direction of the 8-lane mainline.  
Figure 2 presents a solution that employs PVTS along the mainline. This approach allows passenger vehicles and trucks to travel and diverge continuously on their dedicated lanes. For one thing, that eliminates the barrier effect produced by trucks that LR failed to fully address, promoting a considerably smoother traffic flow. For another, the separated section facilitates diverging traffic, allowing them to change lanes only once at most to exit into the deceleration lane. However, the implementation of PVTS requires significant extra construction, including an auxiliary lane and the addition of segregated subgrade or barrier facilities.  
Research has elucidated the mechanisms underlying capacity bottlenecks resulting from consecutive lane changes occurring within seconds at the diverging areas, which provide a clear path for optimizing the design by reducing the lateral range of lane changes. While PVTS provides an advantageous solution, an optimization approach can be attempted to avoid the drawbacks of significant consumption. Specifically, as shown in Figure 3, optimized scheme A proposes partitioning the four-lane main-line into dual two-lane  
Figure 1: Conventional interchange exit ramp (CI) for an eight-lane expressway. The design features four lanes in each direction connecting to a two-lane exit ramp, typically with an auxiliary lane included to maintain lane balance.  
Figure 2: Interchange exit ramp under the PVTS strategy. The upper two lanes in the figure denote the dedicated lane for passenger vehicles, while the bottom two lanes denote the dedicated lane for trucks.  
sections only around interchanges, enabling vehicles to exit with only one lane change, while the basic section still applies the LR strategy. This design obviates substantial segregated subgrade and barrier facilities and also omits auxiliary lanes, thereby reducing the required mainline pavement area. However, the barrier effect produced by trucks can still hardly be prevented in the mixed trac section.  
Along this line, the authors also propose an alternative design and organization strategy to eliminate the necessity of section separation for each direction. Scheme B, as depicted in Figure 4, features a single-lane exit ramp on each side of the mainline. In this way, merely a short, separated central divider is needed around diverging areas, which obviates section separation in the same direction. Inevitably, the left-side exit ramp would have a lower capacity and a higher crash risk since the speed differences between vehicles in the diverging areas of the left-side ramp can always be higher under the right-hand drive rule in China [37].