Planning Time Index (PTI): A common method to operationalize and measure travel time reliability. The PTI is the total travel time that should be planned to guarantee an on-time arrival 95% of the time (95th Percentile Travel Time / Free-flow Travel Time). The PTI includes reoccurring delay as well as unexpected delay because it includes the 95th percentile travel time. Thus, the PTI compares near-worst case travel time to a travel time in light or free-flow traffic. For example, a planning time index of 1.60 means that, for a 20-minute trip in light traffic, the total time that should be planned for the trip is 32 minutes (20 minutes x 1.60 = 32 minutes) to ensure an on-time arrival. In other words, 95% of the time, this 20-minute trip will take 32 minutes or less. The larger the value, the less reliability can be inferred.

Travel Time Index (TTI): A common method to operationalize and measure congestion. The TTI is the measured travel time represented as a percentage of the “ideal” travel time (Travel Time / Free-flow Travel Time). A value of 1.6 indicates the completion of a trip took 60 percent longer than it would have at free-flow speeds. For example, a TTI of 1.60 means that a 20-minute trip in light traffic took 32 minutes at the measured timepoint (20 minutes x 1.60 = 32 minutes). The larger the value, the more congestion can be inferred.

Annual Hours of Peak Hour Excessive Delay Per Capita (PHED): PHED represents the amount of time a roadway experiences significant congestion per year divided by the population of that region. The metric is designed to track progress towards the goals of the Congestion Mitigation and Air Quality Improvement (CMAQ). The threshold to trigger excessive delay is based on the travel time at 20 miles per hour or 60% of the posted speed limit travel time, whichever is greater. PHED is summarized by Equity and Non Equity Focus areas and represents the congestion experienced on all non-interstate roadway segments in the National Highway System. This data is not available at the corridor level and therefore the graphs are static.

Level of Travel Time Reliability (LOTTR): LOTTR was developed to assess the Performance of the National Highway System's reliability. LOTTR is defined as the ratio of the 80th percentile travel time to a “normal” travel time (50th percentile). A roadway is deemed reliable if the LOTTR ratio is less than 1.5 for the defined time period. For example if an average trip along a roadway takes 5 minutes during the AM Peak period, the roadway is deemed reliable if the impact of non-reoccurring congestion does not cause the 80th percentile travel time to exceed 7.5 minutes (5 X 1.5). The total length of segments with a LOTTR of less than 1.5 is divided by the length of all segments to arrive at the final LOTTR percentage. This data is not available at the corridor level and therefore the graphs are static.