Traffic congestion

Traffic congestion is a specific condition in transportation systems. It is defined by slower speeds and longer trip times. It also involves increased vehicular queuing, where cars form long lines.

This phenomenon occurs when the demand for road space exceeds the available capacity. When vehicles are fully stopped for long periods, it is called a traffic jam.

This state can lead to driver frustration and even road rage. Road rage involves aggressive or angry behavior by a driver. This can include verbal insults or unsafe driving maneuvers.

Engineers and scientists study traffic using complex mathematical models. One common method is to model traffic as a flow through a fixed point. This approach is very similar to the study of fluid dynamics.

In this view, traffic moves much like a fluid through a pipe. When the volume of vehicles reaches 75% to 100% of a road's capacity, the flow shifts. It moves from a free-flowing state to a congested state. In this state, both the volume of cars and their speeds are reduced.

Congestion can be classified into different types based on its cause. About half of all traffic congestion in the United States is recurring. This recurring congestion is caused simply by the sheer volume of traffic. The other half is non-recurring, meaning it happens due to specific incidents. These incidents include vehicle crashes, road work, or weather events.

For example, rainfall can reduce the capacity and speed of a road. Even a single car braking heavily can trigger a cascading failure. These ripple effects are known as traffic waves. They can create a sustained traffic jam even if the original flow was normal.

Recent scientific discoveries have explained how these jams form. MIT mathematicians developed a model to describe "phantom jams." These are jams that appear without a clear obstacle like a crash. They are caused by small disturbances, such as a driver braking too hard. These small movements can become amplified into a self-sustaining jam.

The researchers call these mathematical structures "jamitons." Interestingly, the math describing jamitons is very similar to detonation waves from explosions. This discovery helped scientists solve traffic equations that were first theorized in the 1950s.

Economic theories provide another way to understand why congestion happens. Some economists view congested roads as a "tragedy of the commons." Because most roads are free to use, there is little incentive to limit driving. This leads to over-use until the traffic system collapses into a jam. Economist Anthony Downs argued that rush-hour congestion is almost inevitable. He noted that standard workdays cause many people to travel at once.

In a capitalist economy, goods are often allocated by queueing. This means the first person to arrive gets the space. This is different from pricing, where the person who can pay gets the space.

There are several ways to try to manage or reduce congestion. A common response is to increase road capacity by adding lanes. However, this often leads to "induced demand." This means that more people start driving because the road is wider. A 2011 study found a "fundamental law of road congestion." It showed that vehicle-kilometers traveled increase in proportion to available lane-kilometers. This implies that building new roads often just brings in more traffic.

Instead of adding supply, some advocate for demand-side solutions. This includes road pricing or congestion pricing to ration demand.

Cities also use specific infrastructure to organize traffic flow. Some use HOV lanes, which are for vehicles with multiple riders.

Other systems use reversible lanes that change direction during peak times. Junction improvements, such as grade separation using bridges, can also help.

Cities may also use bus lanes or express toll lanes to manage movement. These tools aim to increase the number of people moved through a space. By managing how space is used, planners try to reduce the economic loss caused by delays.