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Travel Time Reliability and Freight Reliability Performance Measures
National performance measures are a central part
of Transportation Performance Management, or TPM.
Of these measures, three national travel time reliability performance measures
assess performance of the National Highway System
and freight movement on the Interstate System.
To help explain why these travel time reliability performance measures
were established, this video covers:
What the travel time reliability measures are;
What the travel time reliability performance measures assess; and
Insight into how the measures were established.
What Are the Travel Time Reliability Performance Measures?
There are three national performance measures for travel time reliability.
They are:
Interstate Travel Time Reliability;
Non-Interstate National Highway System (NHS) Travel Time Reliability; and
Truck Travel Time Reliability Index for the Interstate System,
or TTTR Index.
What Performance Measures Assess
A formal definition for travel time reliability is
the consistency or predictability in travel times,
as measured from day to day, and/or across different times of the day.
Travel time reliability is significant to many transportation system users,
whether they are vehicle drivers, transit riders,
freight shippers, or even air travelers.
Personal and business travelers value reliability
because it allows them to accurately
plan their trip times and departures and arrivals.
For these reasons, the first measure, Interstate Travel Time Reliability,
and the second measure, Non-Interstate NHS Travel Time Reliability,
assess the percent of the person-miles traveled that are reliable.
Shippers and freight carriers also require predictable travel times.
Manufacturers rely on “just-in-time”
and “lean manufacturing” practices to maximize efficiency.
This requires time-certain delivery targets
to provide the right material, at the right time in the production cycle.
If a truck can't make a delivery to a manufacturer or supplier
due to unexpected traffic delays,
this can have a costly ripple effect on production.
Other common shipments that require a high degree
of on-time performance include expedited/high-value shipments,
perishable products, and cargo that needs to be transferred to another mode.
For that reason, the third measure, the Truck Travel Time Reliability Index
for the Interstate System, assesses freight reliability.
Let's look at the Interstate and Non-Interstate
NHS Travel Time Reliability measures.
They assess person-miles traveled instead of vehicle-miles traveled
because that better accounts for the movement of people
by factoring in the number of people in a vehicle using occupancy factors.
The occupancy factors are based on national survey results
and traffic volume data.
Occupancy considerations also make the measures more sensitive to congestion
in areas where more people travel.
The FHWA believes this is an appropriate way to measure
the reliable movement of people to make investment decisions.
The performance measure related to freight movement on the Interstate
uses truck speed and travel time reliability data
to calculate the Truck Travel Time Reliability Index
for the Interstate System.
This measure can be used to identify and quantify
major freight truck bottlenecks along Interstate highways.
Freight bottleneck locations routinely experience
recurring congestion and backups
because traffic volumes exceed highway capacity.
Bottlenecks are estimated to account
for about 40 percent of vehicle hours of delay
and therefore are important in assessing factors
that influence reliability.
Insight into why these measures were selected
In developing the measures,
FHWA determined the Interstate and Non-Interstate
NHS Travel Time Reliability Measures
should include both urban and non-urban areas.
In developing the measures, FHWA learned that outside of urban areas,
these measures capture rural road reliability problems
that spike as a result of weather events, special events,
tourist attractions, and so forth, especially on weekends.
In fact, FHWA learned that on both urban and rural segments
of the Non-Interstate NHS roadway,
weekend travel times resulted in reliability measures
that were increased by 5 percent to 7 percent
over measures derived solely from weekday travel times.
In finalizing the freight reliability performance measure,
a few ideas did not advance.
For example, based on feedback during the rule-making process,
FHWA evaluated a freight performance measure
for “Percent of the Interstate System Mileage Uncongested”
and ultimately determined that changing
that proposed measure to consider the factors
expressed through comments would be complicated
and overly burdensome to implement.
Therefore, FHWA made the Truck Travel Time Reliability (or TTTR) Index
the only freight-specific national performance measure.
FHWA also evaluated the impact of weighting
the measure by truck volumes on segments of roadway,
but ultimately did not do so.
This measure only applies to the Interstate System,
which plays a key role in interstate commerce.
If the measure were applied to a range of roadway functional classifications,
instead of just the Interstate System,
then weighting the measure for truck volume would be more important
in determining which roadways serve as major freight routes.
FHWA concluded that the measure would only be applied to the Interstate System
and that it's important to provide for reliable travel times
across the Interstate System,
regardless of the level of use by trucks
on a single segment of the Interstate System.
For additional information on how to implement
the three national travel time reliability performance measures,
contact your local FHWA Division Office and visit
For a more detailed discussion on how
these performance measures were established and evaluated
with stakeholder input, please review the summary
and discussion of comments in the final rule.
Links to all the TPM related rule-makings are at:
Download the Companion Resource
with helpful citations and other information at:
Working together, we can provide strategies to improve our nation
and people's lives through great transportation planning.
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