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Domenic D'Andrea
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Congestion Management Process
Performance Measures

What are performance measures?

Performance measures are objective ways to determine the degree of success a project, program, or initiative has had in achieving its stated goals and objectives.  In other words, they are ways to track progress.  In this case, performance measures are what we use to track the region’s progress in reducing and managing congestion.

CMP Corridor In Murrysville, Allegheny County

What are the basic considerations for selecting performance measures?

Performance measures should be easily understood by the public, staff, and elected officials so they can be incorporated into the overall transportation decision-making process.  They should be sensitive to various modes of transportation such as freight and transit.  They should also be sensitive to peaking characteristics and the amount of time that congestion is experienced during the day.  Performance measures should be sensitive to congestion mitigation strategies so discernable changes can be detected.  Also, it is useful to have performance measures which can be modeled and forecasted in order to estimate future congestion levels.  However, the most significant constraint in selecting performance measures is the availability of data and the resources available to undertake data collection.  A comprehensive series of performance measures will not be useful without reliable data to back it up.

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What performance measures are used in Southwestern Pennsylvania?

SPC’s Congestion Management Process (CMP) uses Posted Speed Reliability Index, Expected Travel Time Reliability Index travel time, speed and delay as its fundamental performance measures.  These building blocks are then used to calculate additional performance measures such as Delay per Vehicle per Mile and Total Delay. 

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How is travel time and speed data collected?

For many years the “floating vehicle method” was standard for collecting this type of data.  Using this method, a traffic engineer or technician would drive up and down the corridor and collect data, initially with a stop watch and later by using a computer connected to a global positioning system (GPS) device.  The drawback to this methodology is that you gather a relatively small amount of data for the level of resources that must be expended to gather it.  In recent years, advancements in technology have provided new ways to gather greater volumes of data for lower cost and in less time.

Bluetooth is a wireless technology used for exchanging data over short distances and is found in many automobiles, cell phones, in-vehicle navigation systems, and hands-free devices.  Each Bluetooth device has a unique identification code known as a median access control (MAC) address.  For traffic data collection purposes, Bluetooth readers are installed at various locations along a corridor, and as a Bluetooth-enabled device travels past the readers, a time-stamp is created for that particular MAC address.  By matching MAC address time-stamps from different locations, travel times and speeds can be calculated for various segments along a corridor.  Note: Data is aggregated and individual MAC addresses are not tracked.

Another emerging source of travel time and speed information is vehicle probe data gathered and compiled by private companies. These companies amass huge amounts of real-time traffic data from public and private sources such as roadway sensors, commercial fleets, in-car navigation systems, and cell phone apps.  They synthesize this data and sell it for a variety of uses. PennDOT and FHWA currently have contracts to purchase this data, which allows partners like SPC to access it for planning purposes.

These new methods for collecting travel time and speed data have the distinct advantage of providing 24/7 information, as opposed to the floating vehicle method which typically only focused on peak hour measurements.

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How is delay calculated?

Before travel time runs are performed, staff records the posted speed limit within each segment of the corridor so an ideal travel time (the amount of time it would take to traverse a given roadway segment at the posted speed limit if there were no interference) can be calculated.  The measured travel time data collected in the field is then compared to the ideal travel time to determine delay.

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How are other performance measures calculated?

Since segments and corridors vary in length, it is useful to factor distance into congestion calculations.  For example, a motorist who encounters 5 minutes of delay in a 5-mile corridor does not experience as much congestion as a motorist who encounters 5 minutes of delay in a 1-mile corridor. Therefore,  Delay per Vehicle per Mile is another useful performance measure. Delay is the amount of time it takes to traverse a given roadway segment minus the amount of time it would take to traverse that roadway segment at the posted speed limit if there were no interference. Delay per Vehicle per Mile is Delay divided by the length of the roadway segment. It is important to note that Delay refers only to peak times and not to the entire day. 

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What is Posted Speed Reliability Index?

This performance measure is the percentage of time that the measured average speed for a corridor is at or near (within 5 mph) the average posted speed for the corridor.  Furthermore, how often are you able to flow freely in the corridor without significant delay.  This performance measure is expressed in an percentage. 

What is Expected Travel Time Reliability Index?

This performance measure is the percentage of time the measured travel time for a corridor is within 10% (or below) of the median weekday travel time for that corridor.  Additionally,, how often are you able to travel the corridor in the amount of time you would expect with typical recurring levels of congestion.  This performance measure is expressed in an percentage.          

Note: Posted Speed Reliability Index and Expected Travel Time Reliability Index are used only for Tier 1 corridors (Interstates and other limited-access freeways and expressways.  Vehicle probe data is used to track congestion performance measures on these roadways.                 

For more information on data collection and proper data usage, please see the Technical Notes page.

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.

How do I interpret the information provided for individual corridors?

The following graphs are samples illustrating the type of information that is available for each corridor.

Tier 1-Vehicle Probe Data

The first graph and corresponding chart illustrate the Posted Speed Reliability Index and Expected Travel Time Reliability Index.  The graph is for Northbound traffic on I-79.  Each blue line represents a day (365 days) of traffic flows.  The chart represents the breakdown of the Posted Speed Reliability Index and Expected Travel Time Reliability Index by month.

Note: Posted Speed Reliability Index and Expected Travel Time Reliability Index are used only for Tier 1 corridors (Interstates and other limited-access freeways and expressways.

Example of Posted Speed Reliability Index and Expected Travel Time Reliability Index   Breakdown of the Posted Speed Reliability Index and Expected Travel Time Reliability Index by month

Tier 2-Bluetooth Data

The graph below illustrates the information collected from the Bluetooth counters.  The dotted green line represents travel time at the posted speed limit (i.e. how many seconds it should take you traveling at the posted speed limit from Hardies Road to S.R. 910).  The blue line represents the average travel time in seconds gathered from the Bluetooth traffic counters.  The dotted purple line represents the posted speed limit in miles per hour.  The red line represents the average speed gathered from the Bluetooth traffic counters.

From the information gathered in the graphs, we are able to calculate the performance measure (Delay, Delay per Vehicle per Mile, Speed) associated with the Tier 2 corridors.

Graph illustrating the information collected from the Bluetooth counters

These graphs illustrate Delay per Vehicle per Mile during the morning peak hour.  The first graph is for southbound traffic; the second graph is for northbound traffic.

As shown in these graphs, during the AM peak the Delay per Vehicle per Mile in both the northbound and southbound directions is greatest between Saxonburg Boulevard and Kittanning Street.  The fact that Delay per Vehicle per Mile, which is a measure that does not explicitly factor in traffic volumes, is significantly greater between Saxonburg Boulevard and Kittanning Street than in other segments indicates that targeted operational improvements may be an appropriate congestion mitigation strategy for this segment.

Note: Graphs with blue bars indicate Northbound or Eastbound travel and graphs with cranberry colored bars indicate Southbound or Westbound travel.

AM Peak Example

AM Peak Example

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For more information about our Congestion Management program, contact SPC at (412) 391-5590 or e-mail comments@spcregion.org.

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