# Evaluation of Mixed Traffic Flow Efficiency and Safety on Hard-Shoulder-Running Freeways
## 4 Discussion and Conclusions  
The majority of the current literature on the HSR strategy for freeways primarily concentrates on traffic flow involving HDVs, neglecting the potential impact of CAVs with varying penetration rates on macroscopic traffic dynamics. In this study, we adopted a microscopic simulation with a factorial experimental design to investigate whether the effects of mixed traffic flow under the freeway HSR strategy differ from those of the traditional traffic flow or not. Our findings indicate that when the HSR policy is combined with the characteristics of CAVs, it will have a relatively complex impact on traffic flow efficiency and safety. Initially, in the absence of CAVs, the execution of the HSR strategy has the potential to markedly enhance traffic efficiency across a range of varying traffic demands. Furthermore, upon incorporating CAVs into the existing traffic flow, it has been observed that the increase in their penetration rate does not result in an immediate enhancement in traffic flow efficiency if the HSR is not utilized. In fact, a notable improvement in overall traffic flow efficiency is only achieved when the penetration rate surpasses a certain threshold. This observation resonates with findings from prior studies [21, 22], affirming the efficacy of the VISSIM simulation technique employed in this study. Once the HSR is implemented, the overall traffic efficiency within the designated area demonstrates a pronounced elevation as the penetration rate of CAVs increases. This positive trend is observed across all prevailing traffic flow conditions, encompassing free-flowing, saturated, and congested scenarios. Nevertheless, different speed limits for the HSR can lead to varying degrees of efficiency improvement under different traffic demands. Within a CAV penetration rate bracket of 0.4 to 0.7, the implementation of a 120 km/h speed limit does not consistently yield superior traffic efficiency compared to a speed limit of 80 km/h. Finally, concerning the traffic safety of HSR when integrated with CAVs, the finding indicates a complex interplay between traffic flow conditions and the impact of HSR on _TTC_ values and overall traffic safety. We present a slightly different conclusion from [PERSON] et al. [23], showcasing that the traffic safety of HSR improves linearly as the penetration rate increases, without exhibiting any abrupt surge, and, in the scenarios of free and saturated traffic flow, the opening of hard shoulders consistently results in lower _TTC_ values compared to when they are closed, indicating that HSR under these conditions contributes to reduced traffic safety risks. Conversely, during congested traffic flow, the opening of hard shoulders leads to higher _TTC_ values, suggesting that their utilization during congestion poses a heightened traffic safety risk. This observation can be logically explained by the fact that, during times of congestion, opening the hard shoulder as an extra lane for traffic appears to be a reasonable approach to alleviate the flow. However, the effectiveness of this measure is complicated by the fact that vehicles using the hard shoulder must ultimately merge back into the already congested main carriageway. This merging process, where multiple vehicles attempt to join a single lane with limited space and slow-moving traffic, introduces additional complexity and the potential for collisions. As vehicles on the hard shoulder attempt to merge, they may need to brake suddenly or change lanes abruptly to avoid other vehicles, leading to a decrease in the Time-To-Collision (_TTC_) value. A lower _TTC_ value indicates that vehicles are getting closer to one another at a faster rate, increasing the risk of a collision. Therefore, even though the hard shoulder might provide some initial relief from congestion, it ultimately contributes to a higher level of safety risk by creating more opportunities for conflicts and accidents during the merging process.