This section contains documentation of nuanced discussion regarding the tools, functionality, data, and utility. It is intended to highlight partner use-cases, understand challenges faced when working with NPMRDS data, archive solutions or workarounds to those challenges, and share dialogues that provide insightful paths forward.
The Travel Time index (TTI) is the ratio of a measured travel time during congestion to the time required to make the same trip at free-flow speeds. For example, a TTI of 1.3 indicates a 20-minute free-flow trip required 26 minutes.
LOTTR is a MAP-21 PM3 measure that looks at peak hour speed distributions for a given segment. LOTTR is a ratio calculation for each segment of road that takes the annual 80th percentile travel time for peak hours over the annual 50th percentile travel time for peak hours. Higher LOTTR values indicate that there is a wider gap in distribution of travel times between the 80th and the 50th percentiles. A wider gap (higher LOTTR score) means that delay during peak travel times is inconsistent and highly variable.
TTTR is a MAP-21 PM3 measure that looks at peak hour speed distributions for a given segment. TTTR is a ratio calculation for each segment of road that takes the annual 95th percentile travel time for peak hours over the annual 50th percentile travel time for peak hours. Higher TTTR values indicate that there is a wider gap in distribution of travel times between the 95th and the 50th percentiles. A wider gap (higher TTTR score) means that delay during peak travel times is inconsistent and highly variable. TTTR is a more stringent reliability score than LOTTR to highlight the most highly variable segments that could impact freight delivery times.
Excessive delay measures the extra amount of time spent in congested conditions defined by speed thresholds that are lower than a normal delay threshold. For the purposes of this measure, the speed threshold is 20 miles per hour or 60 percent of the posted speed limit, whichever is greater. This measure multiples the total hours of delay by traffic estimates for each segment to determine vehicle hours. The measure is then multiplied by the federal vehicle occupancy estimate which is calculated by the Texas Transportation Institute (TTI) to determine person hours.
This measure uses only truck speed data to calculate Total Excessive Delay. TED measures the extra amount of time spent in congested conditions defined by speed thresholds that are lower than a normal delay threshold. For the purposes of this measure, the speed threshold is 20 miles per hour or 60 percent of the posted speed limit, whichever is greater. This measure multiples the total hours of delay by traffic estimates for each segment to determine vehicle hours.
Speed limit
Freeflow Speed