In this blog post, we’ll examine the causes of recurring highway traffic congestion during holidays and the shock wave theory, and explore the principles and operation of ramp metering, which is used to alleviate this congestion.
The two biggest holidays in South Korea are Seollal (Lunar New Year) and Chuseok (Harvest Festival). Whenever a holiday rolls around, we get excited about three days of uninterrupted rest and the joy of reuniting with relatives we haven’t seen in a long time. However, this excitement vanishes the moment we set out for our hometowns—all because of extreme traffic congestion. According to an article from some time ago, it took seven hours to travel from Seoul to Busan by express bus during Seollal. However, while expressways have bus-only lanes available for express buses, there are no such dedicated lanes for passenger cars, so families traveling by car may take even longer. Currently, South Korea is utilizing “ramp metering” as one of the methods to alleviate this traffic congestion. So, for what reasons is the government using ramp metering to resolve traffic congestion?
Before we explore ramp metering, let’s first examine the causes of traffic congestion. When you’re stuck in a traffic jam on a congested highway, moving slowly for two to three hours, you might suddenly wonder, “Surely the road ahead of the backed-up cars must be clear—so why aren’t they moving, making it so difficult for those behind them?” Ahead of the congested section, there is no sign of traffic, and you can see the congestion gradually building up. So, what causes this?
Among the causes of highway congestion are accidents resulting from collisions between vehicles, a reduction in the number of lanes, or congestion at on-ramps where other roads merge onto the highway. However, these reasons alone are insufficient to explain why the front section of a traffic jam is completely empty.
To explain this phenomenon, we can use shock wave theory. Simply put, shock wave theory views the cars traveling on the road not as individual units but as a single flow—much like flowing water—and applies the hydrodynamic principles of fluid flow to this phenomenon. According to shock wave theory, if a driver at the front or in the middle of a group of vehicles slows down due to inexperience or inattention, the vehicles following behind also slow down, and the speed gradually decreases further back in the line. Just as ripples spread out from the point where a stone is dropped into a calm lake, the low-speed section in a group of vehicles gradually spreads backward. This phenomenon has little impact on traffic congestion during times when there are few vehicles on the road, such as on weekday mornings; however, during periods of heavy traffic—such as national holidays—a single driver’s deceleration can have a significant overall impact. Therefore, on days with heavy traffic—such as during holidays—congestion can begin for minor reasons, such as changing lanes or reducing speed, even without an accident occurring. So, how exactly does ramp metering—a method designed to prevent such congestion—work?
The purpose of ramp metering is to maintain the number of vehicles on the highway at an appropriate level by controlling the flow of vehicles entering the highway, thereby preventing traffic congestion even if a traffic shockwave or an accident occurs.
The basic concept is simple. Assuming vehicles are moving from left to right, a downstream detector is installed at the front of the vehicle stream, and an upstream detector is installed at the rear. Using these upstream and downstream detectors, we can measure how many vehicles have passed through the section between them and how long it took them to do so. By measuring this data, we can determine whether the number of vehicles in the downstream or upstream section has exceeded the road’s capacity.
Once the number of vehicles in the upstream and downstream sections is determined via the detectors, a controller decides whether to allow vehicles on the on-ramp to enter the highway. To do this, the controller makes decisions based on the concept of “optimal capacity.” Optimal capacity refers to the maximum number of vehicles that can be present without causing congestion. For example, the downstream optimal capacity refers to the maximum number of vehicles that can be present in the downstream section without causing congestion. So, how does the controller use this information to manage vehicles entering the highway? There are three main scenarios in which the controller regulates traffic lights:
① Downstream optimal capacity > upstream vehicles + vehicles on the access road
→ No need to regulate entry.
② Downstream optimal capacity < upstream vehicles + vehicles on the access road
→ The controller regulates entry to ensure that the combined number of upstream and access road vehicles does not exceed the downstream optimal capacity.
③ Optimal capacity at the downstream section < vehicles from the upstream section
→ While it is impossible to completely prevent congestion, traffic lights are controlled or turned off to minimize congestion.
Countries such as the United States have implemented ramp metering to reduce congestion and lower traffic accident rates, thereby contributing to the reduction of social and economic costs. In South Korea as well, ramp metering is utilized as a traffic management tool to regulate highway traffic volume and alleviate congestion, and its adoption is included in recent plans to improve highway traffic congestion. However, the effectiveness of ramp metering can vary depending on how the appropriate metering rate is calculated and adjusted, taking into account road traffic conditions and incoming traffic volume. Therefore, ramp metering is not merely a means of restricting vehicles entering the highway but rather a method of traffic demand management designed to prevent the occurrence or worsening of congestion while maintaining a stable flow of traffic on the main roadway.
