# Variable Speed Limit and Ramp Metering for Mixed Traffic Flows: A Review and Open Questions
## 3 Research and Implementation
### Merging Control Approaches in Mixed Traffic Flows
#### 3.5.4 Other Merging Control Approaches  
In [98], the motorway on-ramp merging control system was proposed to take advantage of V2X communication. CAVs share information in a vehicular ad-hoc network through dedicated short-range communication. It is assumed that all vehicles are CAVs. The vehicle sequencing protocol is designed to arrange vehicles in a predefined sequence to cooperate with each other before merging, thus avoiding high collision risks and excessive energy consumption and pollutant emissions when reaching the merging area [98]. The merging control approach was divided into three procedures. The first procedure calculated the maximum possible speed of on-ramp vehicles, the second procedure calculated the estimated arrival time, and the third procedure assigned vehicle sequence identification. The proposed protocol improved the \(ATT\) and average speed by 5.3% and 3.4%, respectively, compared to the no control scenario.  
A two-level LC control strategy utilizing capabilities of AVs to improve the traffic flow on motorways in the scenario of active bottlenecks at on-ramps and lane drops was analyzed in [96]. The first-level centralized control optimized traffic density across lanesto balance traffic flow among lanes and reduced the number of LCs near the merge area. Second-level decentralized control mitigated merging conflicts due to localized LC. This strategy was designed to eliminate merging conflicts between AVs on the mainline and vehicles on the on-ramp and generate safe gaps for targeted LC for AVs. The proposed strategy improved traffic flow compared to the baseline scenario and ALINEA RM and reduced \(TTT\) and \(TT\) variations among vehicles traveling on the mainline and on-ramps. The proposed control outperformed ALINEA for base demand level and minor increases in demand, thereby increasing the operational capacity of the motorway.  
In [94], an online merging control system for multi-lane motorway merging areas for a traffic flow with a 100% CAV penetration rate was presented, based on the optimization of the LCs and the trajectories of the following vehicles. A rule-based LC decision was used to balance the lane flow distribution before the merging area. A Cooperative LC Control (CLCC) optimization model was proposed to ensure safe and smooth LCs. Cooperative Merging Control (CMC) model was used for merging control near the merging area. A dynamic method with moving boundary point was proposed to coordinate the consecutive execution of the CLCC and CMC models. The CLCC model was based on a discrete-time linear system control model that optimized the longitudinal LC acceleration with the objective of maximizing the average speed. The CMC model assigned a merging sequence to each vehicle entering the cooperative merging area, and vehicle speeds were optimized. The simulations were conducted under four demand levels with three lane demand scenarios. The integrated CLCC and CMC results show a reduction in total delay time of 27% to 94.8% depending on the demand scenario. An increase in average speed was measured ranging from 2.7% to 52.2% depending on the demand scenario.