Congestion Management: Introduction

What is congestion?

Virtually everyone who has ever traveled America’s roadways, whether it be in their own personal automobile, on a bus, or behind the wheel of a big rig, has had the experience of sitting in traffic.  Congestion is the level at which transportation system performance is no longer acceptable due to traffic interference. The level of acceptable performance can vary by the type of transportation facility, by location within the region, and by time of day. For instance, commuters typically expect and are generally willing to accept a certain amount of traffic during morning and evening “rush hours”. However, they may not be willing to accept that same level of performance in the middle of the day.

In general, highway congestion results when traffic demand approaches or exceeds the available capacity of the highway system. The level of traffic demand can vary significantly depending on the season, the day of the week, and the time of day. Also, the capacity of the highway system, which is usually thought of as constant, can change because of weather, work zones, traffic incidents, or other non-recurring events.

What is a Congestion Management Process (CMP)?

Federal transportation legislation (SAFETEA-LU) requires that each metropolitan planning area in the United States have what is called a Congestion Management Process, or CMP.  The CMP is a regional program to address and manage congestion within the 10-county Southwestern Pennsylvania region in order to facilitate the movement of people and goods.

The CMP is a broad, regional level planning tool designed to help manage congestion by identifying congested corridors and recommending multimodal strategies for congestion mitigation. The goal of the CMP is to provide information that helps transportation planners, professionals and others to understand the overall congestion climate in individual corridors and the region. Data on the congestion climate helps SPC, in partnership with other agencies, to formulate congestion management strategies. Data and information from the CMP benefits the transportation planning process by helping the region focus limited federal transportation dollars where they can have the greatest impact.

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How does the CMP work?

One of the main requirements of a CMP is the establishment of a coordinated program for data collection and system performance to define the extent and duration of congestion. SPC’s CMP includes a monitoring program that measures travel time and delay on approximately 100 roadway corridors in Southwestern Pennsylvania that currently experience congestion or serve growth areas that are likely to experience future congestion.  Data is collected by a vehicle traveling along with traffic during morning and evening peak hours, which are typically times with the highest amounts of congestion.  This data is analyzed in order to get a quantitative measurement of congestion in each corridor.  Once this data is analyzed, it is reviewed in consultation with other agencies and members of the community to compare and validate the patterns of congestion and identify sources of congestion.

The final data is then used to help assess various strategies for managing congestion.  The region’s transportation agencies and municipal governments then look for ways to implement appropriate strategies into on-going or new projects in those corridors.  Once strategies are implemented, a follow-up assessment is often conducted to determine the effectiveness of the improvements.

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What is being measured?

The CMP measures travel times between defined points in each corridor called nodes.  These travel times are compared to the amount of time it would take to traverse a given roadway segment at the posted speed limit if there were no interference. The difference between the ideal travel time and the actual measured travel time is referred to as delay.  The two basic measures of travel time and delay can be used to calculate a variety of performance measures including Delay per Vehicle, Delay per Vehicle per Mile, Total Delay, and Total Delay per Mile.  ThePerformance Measures section of this website provides more details about how these measures are calculated.  This information can also be found in the Glossary.

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How often is this data collected?

Because SPC’s CMP network is so extensive, it takes approximately 3 years to gather data on all the corridors in the network, so each corridor is measured about once every 3 years.  As the CMP network evolves, this data collection schedule may be adjusted in order to gather data on certain corridors more frequently and others less frequently.

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What is not measured?

The CMP is designed to measure what is called recurring congestion; that is, normal, everyday congestion that you would experience on a “typical day”. The CMP does not measure non-recurring congestion, which can be caused by special event traffic, traffic accidents, construction or other short-term or temporary occurrences. The CMP does not yet measure non-recurring congestion. SPC hopes to integrate non-recurring congestion measures at a later date. The charts below illustrate the type of recurring congestion that is being measured in the CMP and the type of non-recurring congestion that is not measured in the CMP.

As shown in Figures 1 and 2, traffic volumes typically follow recurring patterns.  In the SPC region, these patterns generally include morning peak hours between 7:00 and 9:00 a.m. and afternoon peak hours between 4:00 and 6:00 p.m.  SPC follows standard traffic engineering practice by conducting all of its CMP travel time runs during peak hours on Tuesdays, Wednesdays, and Thursdays to avoid potential anomalies that can occur on Mondays and Fridays.  Figure 3 illustrates an example of traffic patterns in a non-typical week, in this case Memorial Day week.  SPC avoids conducting travel time runs in weeks with holidays and other events like this that could skew traffic patterns.

Figure 1 - Typical monthly traffic volume patterns.

Figure 2 - Typical weekly traffic volume patterns.

Figure 3 - Non-typical weekly traffic volume patterns.

Figures 4 and 5 illustrate an example of non-recurring congestion, which SPC’s CMP does not measure.  Figure 4 shows typical spot speeds on the southbound side of the Veteran’s Bridge in Pittsburgh.  As Figure 5 shows, two incidents the week of August 22 – 28, 2005 caused significant congestion.  One of these incidents was a scheduled event at Mellon Arena which impacted the AM peak rush hour.  The second incident occurred during the Friday PM peak.  This could have been the result of very heavy traffic volumes, a traffic accident, or another incident.

Figure 4 - Typical weekly spot speeds.

Figure 5 - Non-typical weekly spot speeds.

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What can be done about non-recurring congestion?

Non-recurring congestion is caused by temporary disruptions, usually caused by traffic incidents (ranging from disabled vehicles to major crashes), work zones, weather, and special events. As they are frequently unexpected, and in many cases, unpredictable, non-recurring events can dramatically reduce available capacity and reliability of the entire transportation system. Therefore, measuring and dealing with non-recurring congestion can, by its nature, be very difficult since often you can not predict when and where it will occur.   The management of non-recurring congestion has received increased attention in recent years, primarily due to the recognition that it often causes more severe problems beyond recurring congestion.

While SPC’s CMP does not currently measure non-recurring congestion, there are regional strategies that help address non-recurring congestion. These strategies include incident management, special event planning, and road weather and work zone management. You can find out more about these in the Congestion Management Strategies and Monitoring Strategy Effectiveness portions of this website. SPC & PennDOT have also developed a Regional Operations Plan (ROP) (PDF, 1.65 MB) to help address non-recurring congestion.

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Where can I find out which corridors are included in SPC’s Congestion Management Process?

Check out the System Definition section of this website.

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Where can I find the detailed corridor data collected through SPC’s Congestion Management Process?

Check out the Performance Monitoring & Data section of this website.

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For more information about our Congestion Management Process, contact Doug Smith at (412) 391-5590 x327 or e-mail dsmith@spcregion.org.

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