# Do Heavy Vehicles Always Have a Negative Effect on Traffic Flow?
## 6 Discussion and Conclusions  
In this study, traffic data were collected for the Seoul-Chuncheon Expressway, which is a one-way, two-lane road, and the effect of heavy vehicles on traffic flow was analyzed. Although individual vehicle data should ideally be acquired and used for the analysis, there were some limitations in data collection, i.e., data were collected in 30-s cycles and used as raw data for an efficient storage and utilization of data. However, it was found that heavy vehicles had a positive effect on stabilizing traffic flows. In particular, these results had a positive effect on reducing the speed deviation of the vehicle group, which suggests that the probability of a conflict between vehicles can be reduced, so that the accident risk can be reduced when compared with traffic flows comprising only passenger cars. The conclusion that traffic safety is helpful when the speed deviation is small is also shown in the previous studies [20; 21; 22; 23; 24]. Furthermore, according to [PERSON] et al. [24], a reduction in speed variance and deviation is interpreted as stabilization of traffic flow. In addition, for stabilizing the traffic flow, when speed was decreased by 5.6 mph at Nottingham Road, England, there was a reduction of 40% in serious injuries and 30% in minor injuries. Studies conducted in Korea showed that by inducing speed reduction using a section speeding crackdown, there was a 3.2% decrease in the number of accidents per day on the West Coast Expressway and a 12.2% decrease on the Yeongdong Expressway [24]. This was achieved by speed reduction using the section speeding crackdown, as well as the decrease in speed variance and standard deviation by heavy vehicles, which is the result of this study. Thus, with effects such as stabilizing traffic flow and reducing traffic accidents, this can reasonably be interpreted as a positive intervention. Such modeling is expected to be able to follow the methodology of existing studies of heuristic verification and extended modeling studies using it [25] and traffic flow modeling [26]. This study used speed among traffic data to investigate the effect of heavy vehicles. If density measurement is possible in the future, the effect of heavy vehicles on traffic flow can be derived more accurately. Moreover, unlike in two-lane roads, the probability that the leading heavy vehicle of a vehicle group can be overtaken increases on roads with three or more lanes; thus, it may be possible to obtain results different from those obtained in this study. In particular, as there may be more conflicts between vehicles due to an increase in the possibility of overtaking, further analysis is required for traffic flows including various lanes, points, and crackdown flows. It may not be enough to say that heavy vehicles have a positive effect on traffic flow alone. Nevertheless, as a result of analyzing the phenomena shown in the data, it was confirmed that heavy vehicles can have an effect on reducing the speed deviation of surrounding vehicles, thereby contributing to the homogenization of the traffic flow. This part was suggested as a positive effect of heavy vehicles on traffic flow. If the characteristics of low-speed and large-sized vehicles shown in this study are used, it is expected that they can be used to establish various operational strategies for traffic flow management. In addition, the contents of quantitative evaluation of the threat of heavy vehicles on surrounding vehicles or the impact on road structures should also be considered. We intend to present these contents as future research. Furthermore, it is possible to compare the research results of the effect of platoons by heavy vehicles and platoon by connected and automated vehicles (CAV) on traffic flow after CAV is introduced. CAV causes lane changes of human-driven vehicles and forms platoons [27] that are similar to those of heavy vehicles [19].