## 4 Design Scheme and VISSIM Simulation  
### Design Scheme  
For two interchanges spaced evenly apart, the upstream acceleration lanes emerge into the mainline at the taper, the downstream deceleration lanes diverge from the mainline at the taper, and the normal roadway is in between. For small spacing interchange, the traditional connection method will produce weaving in the basic section of the mainline. In particular, for two-lane entrance and exit ramps, the distance between the entrance and exit taper sections is shorter when designed individually, making the use of auxiliary lane connections the most appropriate option. The Baqiao-Tianwang compound interchange uses two auxiliary lanes to connect the upstream acceleration lane and the downstream deceleration lane, called the conventional auxiliary lane connection. Details of the four auxiliary lane design options are as follows.  
* Scheme 1: Current Baqiao-Tianwang Interchange Design Scheme  
The design is the conventional compound interchange connection method, using auxiliary lanes with the same number of lanes as the ramps to connect the two interchanges, as in Figure 4a. The currently widely adopted design is the conventional auxiliary lane, which is relatively simple. Due to the lack of relevant regulatory guidance, this form of auxiliary lane is typically used for dual-lane entrance and exit ramps. However, when traffic volume is low, the outer auxiliary lane may have a lower utilization rate. Schemes 2-4 differ from Scheme 1 regarding the type of weaving and the utilization of auxiliary lanes. However, Schemes 1-3 are essentially two-lane auxiliary lanes, and Scheme 4 is a single-lane auxiliary lane because it is a B-type weaving.  
* Scheme 2: Tapered Auxiliary Lanes  
The tapered auxiliary lane is a new design form, as shown in Figure 4b. Based on the conventional auxiliary lane form, the two-lane auxiliary lane tapers to a single lane for a certain distance at the entrance, and then, near the exit, it tapers to two lanes. In this scheme, the auxiliary lane in the basic road section of the mainline is a single lane, which can ensure that the weaving behavior is on the auxiliary lane. However, the auxiliary lane expands to a double-lane configuration near the entrance and exit so as to ensure the efficiency of vehicles diverging and emerging. The use of this design saves land and improves the auxiliary lane utilization ratio.  
* Scheme 3: Extend Auxiliary Lanes  
According to the \"Guidelines for Design of Highway Grade-Separated Intersections\" in China, when the traffic volume of the entrance ramp approaches the design capacity, it  
Figure 4: Design form scheme of the auxiliary lane. (**a**) Conventional auxiliary lane design; (**b**) tapered auxiliary lane design; (**c**) extended auxiliary lane design; (**d**) B-type weaving auxiliary lane design.  
is specified that the auxiliary lane should be extended at the diverging point, as shown in Figure 4c. This scheme extends the auxiliary lane at the diverging nose for a certain distance and then gradually emerges into the mainline for vehicles that mistakenly enter the auxiliary lane or cannot change lanes in time to emerge into the mainline after entering the auxiliary lane from the acceleration lane. Extending the auxiliary lane beyond the diverging nose ensures sufficient lane length for vehicles to emerge into the mainline and also prevents vehicle parking at the diverging point, which would affect the efficiency and safety of the traffic. However, the \"Guidelines\" do not specify in detail its application in terms of traffic volume, emerging rate, etc., and there is a lack of guidance in actual design.  
* Scheme 4: B-type Weaving Auxiliary Lanes  
As shown in Figure 4d, the scheme is to connect the inner lane of the two-lane ramp to the outermost lane of the mainline at the emerging point. Only one auxiliary lane is provided to connect the outer lane of the two-lane ramp. Vehicles located on the inner lane of the ramp can emerge and diverge the mainline without changing lanes.