Percent of Person Miles Traveled on the Interstate and Non-Interstate NHS that are Reliable
These measures are calculated by determining the Level of Travel Time Reliability (LOTTR) for each reporting segment, rounded to the nearest hundredth. LOTTR is defined as the ratio of 80th percentile travel time to the 50th percentile travel time using travel time data in 15-minute intervals from the National Performance Monitoring Research Data Set (NPMRDS). The LOTTR metric is calculated for the following time periods:
- AM Peak – 6:00am – 10:00am on Monday – Friday
- Mid-day – 10:00am – 4:00pm on Monday – Friday
- PM Peak – 4:00pm – 8:00pm on Monday – Friday
- Weekends – 6:00am – 8:00pm on Saturday and Sunday
A segment is reliable if its LOTTR is less than 1.5 during all four time periods. If one or more time periods has a LOTTR of 1.5 or above, that segment is unreliable.
To obtain person miles traveled, vehicle miles traveled (VMT) for each segment are multiplied by the average vehicle occupancy for each type of vehicle on the roadway. FHWA published a national average vehicle occupancy of 1.7 for all roads and that is the number that was used for all the segments in Florida.
To calculate the percent of person miles traveled that are reliable on the Interstate or non-Interstate NHS, the sum of the number of reliable person miles traveled is divided by the sum of total person miles traveled.
`"Interstate and Non-Interstate Reliability" = ("∑"_(i=1)^(R)("SL"_(i) × "AV"_(i) × "OF"_(j)))/("∑"_(i=1)^(T)("SL"_(i) × "AV"_(i) × "OF"_(j)))`
Where,
Rtotal number of reporting segments that are exhibiting an LOTTR below 1.50 during all of the time periods;
I reporting segment ‘‘i’’;
SLᵢlength, to the nearest thousandth of a mile, of the reporting segment ‘‘i’’;
AVᵢtotal annual traffic volume to the nearest single vehicle, of the reporting segment ‘‘i’’;
Jgeographic area in which the reporting segment ‘‘i’’ is located where a unique occupancy factor has been determined;
OFᵢoccupancy factor for vehicles on the NHS within a specified geographic area within the State/Metropolitan planning area; and
Ttotal number of reporting segments.
Truck Travel Time Reliability Index
The Truck Travel Time Reliability (TTTR) Index is calculated using truck travel time data in 15-minute intervals from the NPMRDS. Average travel times are ranked for each segment of the Interstate for each of the following five time periods:
- AM Peak – 6:00am – 10:00am on Monday – Friday
- Mid-day – 10:00am – 4:00pm on Monday – Friday
- PM Peak – 4:00pm – 8:00pm on Monday – Friday
- Weekend – 6:00am – 8:00pm on Saturday - Sunday
- Overnight – 8:00pm – 6:00am on all days of the week
A TTTR ratio is calculated for each segment of the Interstate system by dividing the 95th percentile truck travel time by the 50th percentile truck travel time during each time period. The highest TTTR value for each segment among the five time periods is then weighted by segment length. The sum of the weighted values is divided by the total Interstate length to calculate the TTTR Index.
`"Freight Reliability" = ("∑"_(i=1)^(T)("SL"_(i) × "maxTTTR"_(i)))/("∑"_(i=1)^(T)("SL"_(i)))`
Where,
iAn Interstate System reporting segment;
maxTTTRiThe maximum TTTR of the five time periods of Interstate System reporting segment ‘‘i’’;
SLᵢSegment length of Interstate System reporting segment ‘‘i’’; and
TA total number of Interstate System reporting segments.
