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Congestion Management Process
Performance Measures: Technical Notes On Data Collection & Usage

What performance measures are used in Southwestern Pennsylvania?

SPC’s Congestion Management Process (CMP) uses 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, Total Delay, and Total Delay per Mile.

How is travel time and speed data collected?

Travel times and speed data are collected by the “floating vehicle” method, in which a vehicle moves with the flow of traffic during morning and evening peak hours.  An on-board computer captures travel times, average speeds, and distance measurements, which are then downloaded for analysis.

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. Calculations are as follows:

SegmentIdeal Travel Time (min) = Segment Length (miles) * 60 (min/hr) / Speed Limit (miles/hr)

Corridor:  Ideal Travel Time (min) = S Ideal Travel Times for Segments

Delay / Vehicle (min) = Measured Travel Time (min) – Ideal Travel Time (min)

How is average speed calculated?

Average speed for each roadway segment is provided by the computer software used during data collection.  Once this data is downloaded, an average speed is calculated for the corridor as follows:

Corridor:  Avg. Speed (mph) = [S (Avg. Speed in Segment (mph) * Segment Length (miles))] / Corridor Length (miles)

How are crash rates calculated? [ disclaimer ]

Reportable crash data for the most recently available 3-year period is compiled for each corridor using PennDOT crash databases.  Once this data is compiled, a crash rate is calculated for the corridor (within the defined CMP limits) as follows:

Crash rate (crashes per million veh-miles) = (# of Crashes * 1,000,000) / (Corridor Length (miles) * AADT * 365 days/yr * 3 yrs)

How is severity index calculated? [ disclaimer ] [ see FHWA ]

Using the 3-year crash data referenced above, a severity index is calculated for the corridor (within the defined CMP limits).  This index is weighted to account for the severity of different types of injuries (Major, Moderate, Minimal, and Unknown) and is calculated as follows:

Severity index = [((# of Fatalities + # of Major Injuries) * 12) + (# of Moderate Injuries * 3) + (# of Minor Injuries + # of Unknown Injuries * 2) + (# of Property Damage Only Crashes)] / Total # of Crashes

How is typical park-n-ride utilization calculated?

Lot capacity and utilization levels are monitored for all park-n-ride lots in the region on a periodic basis.  This data is collected on typical days, similar to CMP travel time data.  Each park-n-ride lot is assigned to various corridors where it is anticipated to help relieve congestion, and an overall utilization rate is calculated for each corridor as follows:

Typical PnR Utilization (%) = ∑PnR Spaces Utilized / ∑PnR Capacity

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, calculating Delay per Vehicle per Mile is another useful performance measure. Delay per Vehicle 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 per Vehicle divided by the length of the roadway segment. It is calculated as follows for individual segments and corridors:

                Delay / Vehicle / Mile (min/mile) = Delay / Vehicle (min) / Length (miles)

Because different roadways carry different amounts of traffic, it is also useful to consider Total Delay as a performance measureTotal Delay considers the number of vehicles that are impacted by delay.  For example, a roadway carrying 500 vehicles in the peak hour with five minutes of Delay per Vehicle per Mile is generally not as critical a congestion problem as a roadway with 5 minutes of Delay per Vehicle per Mile that carries 5,000 vehicles in the peak hour. Total Delay is calculated as Delay per Vehicle multiplied by the peak hour volume in that roadway segment. It is important to note that SPC’s CMP considers AM and PM peak periods, so Total Delay refers only to peak times and not to the entire day.  Total Delay is calculated as follows:

Segment:  Total Delay (veh-hr) = Delay / Vehicle (min) * Hourly Volume (veh) / 60 (min/hr)

Corridor:  Total Delay (veh-hr) = S Total Delay in Segments (veh-hr)

As indicated previously, since segments and corridors vary in length, it is useful to have additional performance measures that factor in distance.  Therefore, SPC’s CMP also considers Total Delay per Mile, which is calculated as Total Delay divided by the length of the roadway segment. It is calculated as follows for each corridor:

                Total Delay / Mile (veh-hr/mile) = Total Delay (veh-hr) / Length (miles)

Each of the above mentioned performance measures are calculated for both AM and PM peak periods.  Two additional measures are utilized to look at the overall congestion levels in each corridor: Total Peak Hour Delay and Total Peak Hour Delay per Mile.  As is shown below, these measures are a total of the congestion in the AM and PM peaks.

                Total Peak Hour Delay (veh-hr) = AM Total Delay (veh-hr) + PM Total Delay (veh-hr)

                Total Peak Hour Delay / Mile (veh-hr/mile) = AM Total Delay / Mile (veh-hr/mile) + PM Total Delay / Mile (veh-hr/mile)

Are these the same performance measures used by other regions?

Federal regulations do not dictate mandatory CMP performance measures for regions to use; rather Metropolitan Planning Organizations like SPC are encouraged to develop their CMPs to fit the region and the available program resources.  A statewide congestion management study done for PennDOT in 2005 indicated that planning organizations in Pennsylvania use a variety of performance measures to evaluate congestion in their areas. These include measures that are determined based on field data collection, like those used by SPC, and measures that are calculated based on computer modeling.  They include measures designed to address recurring as well as non-recurring congestion.  Each of these performance measures has its strengths and weaknesses; thus a combination of measures is preferable.

What are the pros and cons of various performance measures and data sets?

The following table illustrates some of the pluses & minuses of various performance measures used by SPC and other MPOs.

Performance measure

Pros

Cons

Accident Rates

  • a way of addressing non-recurring congestion
  • provides useful information for other safety-related initiatives
  • sensitive to mitigation strategies
  • difficult to get comparable data for roadways owned by different entities
  • confidentiality issues with some data
  • management and manipulation of data can be difficult
  • difficult to model and forecast

Average Speed

  • easy to understand
  • applicable to multiple modes
  • sensitive to mitigation strategies
  • can be modeled and forecasted
  • more appropriate than spot speeds
  • time consuming to collect data
  • need multiple runs for reliability

 

Delay / Vehicle

  • easy to understand
  • applicable to multiple modes
  • sensitive to mitigation strategies
  • can be modeled and forecasted
  • time consuming to collect data
  • need multiple runs for reliability
  • does not account for unequal segment and corridor lengths

Delay / Vehicle / Mile

  • easy to understand
  • applicable to multiple modes
  • sensitive to mitigation strategies
  • can be modeled and forecasted
  • accounts for unequal segment and corridor lengths
  • time consuming to collect data
  • need multiple runs for reliability
  • may over represent short segments and corridors

Level of Service

  • easy to understand
  • applicable to multiple modes
  • can be modeled and forecasted
  • LOS for segments is not very useful since congestion is usually at node points
  • LOS at node points requires extensive data collection
  • relatively macroscopic, so not as sensitive to mitigation strategies
  • assumptions are very important as is reliability of the data used

Qualitative Perception

  • no data collection required
  • very subjective
  • difficult to gauge modal impacts
  • difficult to accurately gauge changes resulting from mitigation strategies
  • can not be modeled and forecasted

Total Delay

  • easy to understand
  • applicable to multiple modes
  • sensitive to mitigation strategies
  • can be modeled and forecasted
  • time consuming to collect data
  • need multiple runs for reliability
  • does not account for unequal segment and corridor lengths
  • requires peak hour traffic volume data which may be difficult to obtain

Total Delay / Mile

  • easy to understand
  • applicable to multiple modes
  • sensitive to mitigation strategies
  • can be modeled and forecasted
  • accounts for unequal segment and corridor lengths
  • time consuming to collect data
  • need multiple runs for reliability
  • may over represent short corridors
  • requires peak hour traffic volume data which may be difficult to obtain

Transit Ridership

  • easy to understand
  • sensitive to mitigation strategies
  • can be modeled and forecasted
  • not particularly sensitive to other modes
  • management and manipulation of data can be difficult

V/C Ratio

  • applicable to multiple modes
  • can be modeled and forecasted
  • somewhat more difficult to understand calculations
  • mid-block v/c ratios do not include intersection delays
  • relatively macroscopic, so not as sensitive to mitigation strategies
  • assumptions are very important as is reliability of the data used

How is Posted Speed Reliability Index and Expected Travel Time Reliability Index Calculated?

This performance measure is used for Tier 1 corridors only and is represented in a percentage. 

1 - (# of hours where measured Avg speed dropped below 5 mph of Avg. Posted speed limit / # of hours in Month) = Posted Speed Reliability Index

1 - (# of hours where measured Avg. Travel Time > 10% Median Avg. Travel Time for that hour of the day / # of hours in Month) = Expected Travel Time Reliability Index

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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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