Congestion Management Process Performance Metrics

  • Level of Travel Time Reliability (LOTTR) represents the consistency or dependability of travel times from day-to-day or across different times of day. LOTTR is calculated by dividing the 80th percentile travel time for a segment of road during peak travel times by the 50th percentile travel time.
  • Truck Travel Time Reliability (TTTR) represents the consistency or dependability of truck travel times from day to day or across different times of day. TTTR is calculated by dividing the 95th percentile truck travel time for a segment of road during peak travel times by the 50th percentile truck travel time.
  • Travel Time Index (TTI) is the ratio of peak period travel time to free-flow travel time. TTI represents the average additional time required during peak times compared to times of light traffic.
  • Planning Time Index (PTI) represents the total travel time that should be planned when an adequate buffer time is included. The calculation includes typical and unexpected delays. Thus, PTI compares near-worst case travel time to a travel time in light or free-flow traffic. PTI is calculated with the 95th percentile cross-peak travel time divided by the free-flow travel time.
  • Demand-to-Capacity (DC) ratio is a measure that reflects the level of mobility and the quality of travel of a roadway or a section of a roadway. The DC ratio compares the roadway capacity with the estimated trip demand generated directly from the travel demand models. The capacity of a roadway is defined as the theoretical maximum volume that can be processed by that roadway during a specified time period.
  • Vehicle Hours of Delay (VHD) is the sum total of delay experienced by all vehicles on the network. Delay is defined as the difference between estimated actual travel speed and free-flow travel speed
  • Person Hours of Delay (PHD) is calculated by multiplying VHD by the average vehicle occupancy rate. Because vehicle occupancy differs from place to place, the following rates are used: 1.48 for New York City counties, 1.75 for Nassau and Suffolk counties, and 1.44 for Westchester, Rockland, and Putnam counties.
  • Average Travel Speed (ATS) is the calculation for a weighted average of speed. The average speed for each element of the road system is multiplied by the amount of travel on the set of roads.
  • Lane Miles of Congestion (LMC) measures the road space that functions at less than free-flow speeds during the peak period and compares actual roadway volume with maximum acceptable volume for the roadway. It reflects the mobility of a roadway or section of a roadway, indicating the proportion that is congested. LMC can easily be aggregated from facility to corridors to sub-regional to region. For purposes of this performance measure, a roadway is defined as congested if the volume is greater than or equal to 85 percent of the maximum acceptable volume for that roadway (essentially the level of service E volume).
  • Vehicle Miles Traveled (VMT) is another performance measure developed by the post processor. VMT is the sum of distances traveled by all motor vehicles in a specified region. Travel demand forecasting is used to generate the average trip lengths for a region. The average trip length measure is then used to estimate VMT, which in turn is used in estimating gasoline usage or mobile source emissions of air pollutants.