# Steady-Speed Traffic Capacity Analysis for Autonomous and Human-Driven Vehicles
## 5 Discussion  
This study investigated the impact of car-following behavior on traffic characteristics using both traditional human-driven vehicles and different ACC-equipped vehicles. Controlled empirical tests have been conducted to collect data through driving at multiple test steady speeds. The diverse selection of ACC sensors provides insights into the anticipated behavior of future promised AVs and their impact on traffic flow and capacity. During the experiment, the shortest setting of the ACC controller was used, which maintains the  
Figure 16: Summary of average clearance, space headway, and density at all tested speeds. Figure 17: Density and Capacity Relationship. shortest following distance or following time as specified by the vehicle manufacturer. This setting yielded the maximum flow under controlled conditions, representing the capacity of the studied roads. The results indicate that human drivers exhibit the shortest following distances, resulting in higher capacities compared to the tested ACC systems across all driving speeds. However, variations in behavior among different ACC systems are evident. The study demonstrates the disadvantages of ACC technology, as evidenced by the percentage decrease in capacity compared to conventional driving. The results also show that although there was uncertainty at 30 km/h (especially for Volvo), this might have resulted from the very low speed applied to examine the systems. The Subaru ACC system, represented by the camera-based sensor 'Eyesight,' outperformed other tested ACC systems, exhibiting a percentage decrease ranging from 3.2% to 12.9% at speeds between 40 km/h and 80 km/h. Following closely is the combined camera- and radar-based ACC in Volvo. Despite ranking last in terms of capacity increase linearity, as illustrated in Figure 17, it demonstrated a moderate decrease in capacity ranging from 9.5% to 13.5% at speeds from 40 km/h to 80 km/h. Conversely, VW showcased a uniform increase in capacity values corresponding to the rise in driving speed. This could be attributed to the efficiency of electric engines, renowned for their precise management of acceleration and deceleration owing to the instantaneous torque delivery of electric motors. This capability enables the radar-based ACC system to execute precise adjustments, ensuring the maintenance of a steady following distance. However, in comparison to human drivers, the VW ACC system results in a higher percentage decrease in capacity, ranging from 14.0% to 24.1% at driving speeds between 40 km/h and 80 km/h. At higher speeds, while human-driven vehicles continued to provide higher capacities, there was an oscillation in capacity decrement percentages for ACC vehicles. However, there was an observed oscillation in the capacity decrement percentages for ACC-equipped vehicles. Specifically, the Subaru and VW systems exhibited notable decreases, particularly at 100 km/h and 110 km/h, with a range of 2.6% to 0.8% for Subaru and 7.2% to 4.4% for VW. In contrast, the performance of the Volvo ACC system was less satisfactory, displaying higher percentages of capacity decrement, reaching 14.3% at 110 km/h. This disparity could be attributed to the larger frontal area of the Volvo sport utility vehicle, which leads to increased aerodynamic resistance, as the vehicle encounters greater air resistance or drag. Overall, when comparing our study's results to those of other empirical studies in the literature, the average human-driven time headway in our investigation was found to be 1.175 s. This figure is lower than the measurements reported by [PERSON] et al. [31], where the ACCs' minimum and maximum time headway ranged between 1.36 and 2.5 s in their empirical study. Additionally, the average time gap observed in our study for conventional vehicles was 0.903 s, which is less than the average time gap for Eco short drive (reported as 0.966 s for driving speeds of 50, 70, and 90 km/h) in ref. [30].