Describing the Congestion Problem
Congestion has been increasing in cities of all sizes. While the congestion problems faced by travelers and freight shippers in metropolitan regions of more than 1 million people consume more travel time and waste more fuel than congestion faced in cities less than 500,000, the burden and frustration has increased across all population groups (see Exhibit P-1).
Exhibit P-1. Congestion Growth Trend
The process of congestion relief begins by understanding the problem. Exhibit P-2 depicts the results of a national study on the sources of congestion. All these sources also interact, meaning that the strategies in the Congestion Reduction Toolbox typically address more than one problem.
Exhibit P-2. Sources of traffic congestion. (FHWA)
- Bottlenecks—points where the roadway narrows or regular traffic demands cause traffic to backup–are the largest source of congestion.
- Traffic incidents—crashes, stalled vehicles, debris on the road–cause about 1/4 of congestion problems.
- Work zones—for new road building and maintenance activities like filling potholes–are caused by necessary activities, but the amount of congestion caused by these actions can be reduced by a variety of strategies.
- Bad weather cannot be controlled, but travelers can be notified of the potential for increased congestion.
- Poor traffic signal timing—the faulty operation of traffic signals or green/red lights where the time allocation for a road does not match the volume on that road–are a source of congestion on major and minor streets.
- Special events cause "spikes" in traffic volumes and changes in traffic patterns. These irregularities either cause delay on days, times or locations where there usually is none, or add to regular congestion problems.
Is congestion the same everywhere?
Highway congestion, very simply, is caused when traffic demand approaches or exceeds the available capacity of the highway system. Though this concept is easy to understand, congestion can vary significantly from day to day because traffic demand and available highway capacity are constantly changing. Traffic demands vary significantly by time of day, day of the week, and season of the year, and are also subject to significant fluctuations due to recreational travel, special events, and emergencies (e.g. evacuations). Available highway capacity, which is often viewed as being fixed, also varies constantly, being frequently reduced by incidents (e.g. crashes and disabled vehicles), work zones, adverse weather, and other causes.
To add even more complexity, the definition of highway congestion also varies significantly from time to time and place to place based on user expectations. An intersection that may seem very congested in a rural community may not even register as an annoyance in a large metropolitan area. A level of congestion that users expect during peak commute periods may be unacceptable if experienced on Sunday morning. Because of this, congestion is difficult to define precisely in a mathematical sense – it actually represents the difference between the highway system performance that users expect and how the system actually performs.
Congestion can also be measured in a number of ways – level of service, speed, travel time, and delay are commonly used measures. However, travelers have indicated that more important than the severity, magnitude, or quantity of congestion is the reliability of the highway system. People in a large metropolitan area may accept that a 20 mile freeway trip takes 40 minutes during the peak period, so long as this predicted travel time is reliable and is not 25 minutes one day and 2 hours the next. This focus on reliability is particularly prevalent in the freight community, where the value of time under certain just-in-time delivery circumstances may exceed $5 per minute.