# Do Heavy Vehicles Always Have a Negative Effect on Traffic Flow?
## 4 Speed Changes According to Traffic Volume and Heavy Vehicles Ratios  
### Changes According to Traffic Volume  
The following plot shows the mean and median speeds altered by the traffic flow in which heavy vehicles are included in the same ratio. Figure 4 shows the changes in the mean and median speeds according to the traffic volume in a traffic flow with only small-sized vehicles. Figures 5-7 show the speed changes in the traffic flow where the proportion of heavy vehicles is increased to 15%, 30%, and 50%, respectively. As shown in Figure 4, in the traffic flow that did not include heavy vehicles, the decrease in speed follows an almost constant slope. This is a downward slope similar to the one commonly seen in a typical traffic-speed graph. When the traffic flow rate in Figure 4 increases from  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Classification** & **Small** & **Middle** & **Large** \\ \hline Average speed (km/h) & 105.4 & 97.2 & 95.1 \\ Standard deviation (km/h) & 14.5 & 9.9 & 12.5 \\ Variance & 211.7 & 98.9 & 157.3 \\ Count (vehicles) & 639.521 & 113,811 & 55,286 \\ (Proportion; \%) & (79.1\%) & (14.1\%) & (6.8\%) \\ \hline \hline \end{tabular}
\end{table}
Table 10: Characteristics of speed data.  
This trend changed as the proportion of heavy vehicles increased. When the proportion of heavy vehicles increased, the decrease in speed was relatively small despite the increase in traffic volume. Table 11 shows the results when the first regression equation was applied to find the speed changes according to the changes in traffic volume in cases where the ratios of heavy vehicles were the same. For each case, a regression equation was calculated using two dependent variables, one set to speed \(v\) and the other set to traffic flow rate \(q\). Here, the slope of the straight line indicates the relationship between traffic flow rate and speed. Because all slopes are negative, it can be concluded that the speed decreases as the traffic volume (or traffic flow rate) increases. As the proportion of the heavy vehicles increases, the absolute value of the slope gradually decreases. Thus, it indicates that even if the traffic volume (or traffic flow rate) increases, the degree to which speed decrease is relatively small. In other words, when the proportion of heavy vehicles increases, the impact of traffic volume on speed decreases. In addition, it is the basis on which the traffic flow becomes more stable as the degree to which speed increases or decreases becomes smaller, regardless of the changes in traffic volume. This study targeted the Seoul-Chuncheon Expressway, which is characterized by a continuous flow and high-speed movement. In this expressway, the traffic flow with a high proportion of heavy vehicles occurs during dawn and night time, when the total traffic volume is low. Therefore, it is difficult to interpret the results for cases in which the statistical valid sample number (n = 30) is satisfied (transport rate of 1100 veh/h/ln or less), as shown in Figures 6 and 7.  
In reality, it is extremely rare that the proportion of heavy vehicles (% of HV) is more than 50%. This is the reason why the model reliability in the case of % of HV = 50 in Table 11 is low. Efforts were made to acquire data corresponding to the corresponding part, but only the data set for the case where the condition was satisfied momentarily was formed, and the case of continuous acquisition could not be observed. Since % of HV = 50 is satisfied when the traffic volume is less than 10, it may be meaningless to form such a model. Nevertheless, the model formula was presented together with the aspect of informing the characteristics of the data.