# Do Heavy Vehicles Always Have a Negative Effect on Traffic Flow?
## 3 Characteristics of Traffic Flow  
### Data Collection and Description  
A total of nine detectors were installed along a 61.4 km section of the Seoul-Chuncheon Expressway. Two types of detectors were installed: a vehicle detector equipped with a double-loop detector that collects traffic flow data such as traffic volume, speed, and occupancy; and an AVC detector equipped with piezosensors that are capable of classifying 12 types of vehicles (see Figure 2 for example). The characteristics of the piezosensor, other than the loop detectors that are commonly found on the road, are as follows. According to the Korea Ministry of Land, Infrastructure, and Transport (MOLIT) report [13], this piezosensor measures the electrical signal generated as a result of the mechanical stress applied to the sensor by the axle of a car when it passes over the sensor embedded on the road. If two piezosensors are installed on the road at a certain distance from each other, they can be used to accurately measure a vehicle's speed. They can also be used as weighing-in-motion apparatus, which is the measurement of a vehicle's weight when the effect of temperature is compensated. These methods for measuring the number and weight of the axles are accurate and reliable; thus, these sensors are frequently used as traffic flow survey equipment. The reliability of these piezosensors is shown in Table 5.  
The detectors along the Seoul-Chuncheon Expressway maintained accuracy above the level stipulated for the evaluation; performance evaluation inspections and calibrations are conducted at least once every two years by the Korea Highway Corporation, which is responsible for the management of the detectors. Advanced level is a grade that can be obtained when the accuracy of collected traffic information such as traffic volume and speed is \(\geq\)90%. In this study, the vehicle detectors satisfied the required accuracy for the advanced level, thereby ensuring reliable results. Examination of road geometry of the sections using the vehicle detectors showed that the maximum longitudinal slope and the cross slope were 2%. This indicates that the roads are flat, in agreement with the KHCM [3] standards. Therefore, it was assumed that the geometry of the data acquisition points would have a negligible impact on data analysis. Figure 2: Examples of piezosensors. \begin{table}
\begin{tabular}{c c} \hline \hline
**Measurement** & **Reliability** \\ \hline Speed measurement error & Less than \(\pm\)1.0\% \\ Wheelbase measurement error & Less than \(\pm\)5 cm \\ (for classification of vehicle type) & Vehicle-length measurement error \\ (for classification of vehicle type) & Less than \(\pm\)2.5\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Reliability of piezosensors [13]. For an accurate analysis, raw data of individual vehicles must be continuously acquired, but actual raw data was collected and aggregated every 30 s for efficient storage and utilization of highway vehicle detection system data. Given that it is not feasible to obtain raw data for individual vehicles, raw data already collected along the Seoul-Chuncheon Expressway was used for the analysis in this study. The collected data are traffic volume and speed data for 12 types of vehicles. The data were organized as shown in Table 6. As shown in Table 7, 5 min of traffic volume and 5 min of average speed data weighted by traffic volume were calculated and used for the analysis. As shown in Table 8, for the traffic data collected on the roads managed by the Korea Expressway Corporation, vehicles are classified into 12 types [13]. This is a regulation defined in the traffic survey vehicle classification guide published by MOLIT [14]. For road capacity and service level analysis, heavy vehicles are classified into three types: small, medium, and large-sized heavy vehicles. Small-sized heavy vehicles are small passenger cars and vans with a capacity of <16 passengers (Class 1) and trucks weighing <2.5 tons (Class 3).