# Variable Speed Limit and Ramp Metering for Mixed Traffic Flows: A Review and Open Questions
## 3 Research and Implementation
### Impact of AVs and CAVs on the Fundamental Diagram  
As mentioned, Table 1 summarizes obtained research results on the impact of AVs and CAVs on fundamental diagram without control algorithms. In [76], an evaluation of the impact of AVs on an existing motorway, national road network, and urban network (Dublin city center) was presented. AVs were formulated as Society of Automotive Engineers (SAE) level of automation 2, and CAVs were formulated as SAE level of automation 4 with \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Paper** & **Year** & **Control Strategy** & **Proposed Method** & **Penetration Rate** & **Compared with** & **Improvements** \\ \hline
[84] & 2020 & DVSL & DRL & 100\% CAVs & no control, QL, DQN VSL, & 8.1\% lower \(TT\), 3.7\% lower \(CO\) emissions \\ \hline
[85] & 2020 & VSL and LC & GA & 100\% CAVs & no control, VSL & 45\% lower \(TTS\) \\ \hline
[86] & 2020 & VSL & Multiclass CTM & 0\%–100\% AVs & no control & 33\% lower energy consumption \\ \hline
[87] & 2019 & Speed harmonization & Optimal control with Hamiltonian function & 100\% AVs & no control, VSL, SPD-HARM & 22\% lower fuel consumption, 30\% lower \(TT\) \\ \hline
[88] & 2019 & VSL & GA & 0\%–10\% CAVs & 7 scenarios with no control, VSL, CTM and I2V with V2I & 36\% lower \(TT\), 68\% lower delays, 66\% lower number of stops, 7.6\% lower emissions \\ \hline
[89] & 2019 & VSL & Rule-based VSL & 0\%–100\% AVs & no control & 26\% lower \(MTT\), 31\% lower fuel consumption \\ \hline
[90] & 2018 & Centralized VSL & Deep-RL (GRU) & 100\% AVs & no control, feedback RM & 25\% higher bottleneck throughput \\ \hline
[91] & 2017 & ACC and VSL & Rule-based VSL & 0\%–100\% AVs & ACC only, VSL only & 80\% lower \(TT\), 77\% lower \(TET\) \\ \hline
[92] & 2016 & C-VSL & Feedback control & 0\%–100\% AVs & P-VSL & 49.5\% lower delay time \\ \hline
[93] & 2015 & VSL & MPC & 50\% and 100\% CAVs & no control & 20\% lower \(TTT\), 11\% improved safety, 16\% lower fuel consumption \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The most representative frameworks for VSL in mixed traffic flows. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Paper** & **Year** & **Control Strategy** & **Proposed Method** & **Penetration Rate** & **Compared with** & **Improvements** \\ \hline
[94] & 2019 & LC, merging & rule-based CLCC, event-based CMC & 100\% CAVs & no control & 95\% lower total delay time, 52\% increased speed \\ \hline
[95] & 2019 & merging & trajectory planning & 100\% CAVs & no control & achieved desired merging speed \\ \hline
[96] & 2019 & LC & decentralized and centralized control & 0\%–100\% AVs & no control, ALINEA RM & 43\%–61\% lower \(TTT\) \\ \hline
[97] & 2019 & RM & look-ahead cruise control & 20\% AVs & - & queue length optimization \\ \hline
[98] & 2018 & merging & V2X communication & 100\% CAVs & no control & 5.3\% lower \(ATT\), 3.4\% higher average speed \\ \hline
[99] & 2017 & merging & CIDM safe time gaps & 0\%–25\% AVs & - & 72\% lower speed oscilations, 15.4\% lower \(ATT\) \\ \hline
[100] & 2017 & merging & Hamiltonian analysis & 100\% CAVs & no control & 48\% lower fuel consumption, 13.5\% lower \(TTT\) \\ \hline
[101] & 2017 & merging & nonlinear optimization & 100\% CAVs & no control, gradual speed limit reduction & reduced delay, increased average speed up to 95 km/h \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The most representative frameworks for merging control in mixed traffic flows. Cooperative Adaptive Cruise Control (CACC). Tested scenarios included penetration rates from 0% to 70% with mixed SAE levels of automation. CAVs showcased gradual improvement in safety and efficiency. Satisfactory and realistic results were observed at penetration rates of around 20% to 40%. The motorway scenario proved to be the one most affected by CAVs. At low CAV penetration rates, traffic congestion and conflict situations were increased in some regions of the motorway networks.