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section will focus on travel time reliability and the measures cal conditions. By its very nature, roadway performance is
that quantify it. highly variable and unpredictable, in that on any given day,
Travel time reliability is the consistency or dependability unusual circumstances such as vehicle accidents can dra-
in travel times, as measured from day to day and/or across matically change the performance of the roadway, affecting
different times of the day. It better represents a commuter's both travel speeds and throughput volumes. Because travel
experience than a simple average travel time measurement. conditions are so unreliable on congested highways, travel-
It is important because most travelers are less tolerant of ers must plan for these problems by leaving early to avoid
un expected delays (nonrecurring congestion) than of every- being late [2.5.1].
day congestion, and they also tend to remember bad traffic In terms of an agency/operator using this as an internal
days over an average daily travel time throughout the year. measure for an APM system, it may not be meaningful because
The recommended measures used to quantify travel time airport APM systems tend to have a high level of availability
reliability are 90th or 95th percentile travel time, buffer index, and do not have wide variances in the travel times found on
and planning time index [2.5.4]. the roadways (i.e., whereas the 90th or 95th percentile travel
times for a highway will provide a measurable level of travel
time and unavailability over a month's time, an airport APM
2.5.1.1 90th or 95th Percentile Travel Time
system may not).
The 90th or 95th percentile travel time measure is sim-
ply an estimate in minutes of how bad delays will be on
2.5.1.2 Buffer and Planning Time Indices
certain routes during the heaviest traffic days. The 90th or
95th percentile represents those days in the month that are Another effective travel time reliability measure in the
the heaviest traffic days causing the greatest congestion and highway industry is the buffer index. The buffer index, pre-
longest travel times; it is the near-worst-case travel time. This sented as a percentage, represents the extra time travelers
method requires continuous tracking of travel times in order must add to their average travel time when planning trips
to provide an accurate estimate. State departments of trans- in order to ensure an on-time arrival most of the time. It
portation employ this method for use by the public online. is expressed as a percentage, and its value increases as reli-
A traveler can, for example, determine on a website that the ability gets worse. For example, if the average travel time is
95% reliable travel time for a particular route is 59 min, 30 min and the buffer index is 40%, a traveler should allow
which means that if the traveler allows 59 min for the trip on an additional 12 min to rely on an on-time arrival 95% of the
that route, he or she would be on time 19 out of 20 weekdays time. The extra 12 min is called the buffer time. The buffer
in the month [2.5.4]. index is computed as the difference between the 95th per-
This measure is ideally suited for traveler information centile travel time and the average travel time, divided by the
in that it provides a gauge of how many times in a month, average travel time.
for example, the travel time can be relied upon. It does not The planning time index measure represents the total
obviously predict the day(s) when the 90th or 95th travel travel time that should be planned for a trip and includes
time may occur, but used in conjunction with other mea- an adequate buffer time to rely on an on-time arrival of
sures to be described in the following subsections, a reason- 95%. For the previous example, the planning index would
able probability of arriving on time can be computed. From be 1.40. The planning time index differs from the buffer
the agency's view, this measure may be useful in that it index in that it includes typical delay as well as unexpected
can track the creep in the average travel time over a period delay. In addition, where the buffer index is used in deter-
of time (i.e., the 59-min travel time may creep to 63 min mining the additional time necessary to make the trip, the
over time). This would be useful in planning for the mitiga- planning index is used in determining the total travel time
tion of congestion, whether in the form of providing additional necessary for the trip. The planning time index is computed
infrastructure or employing other techniques such as HOV as the 95th percentile travel time divided by the average
lanes, directional lanes by time of day, or other operational travel time.
strategies. In the context of the airport APM industry, it may not be
As applied to the airport APM industry, providing this expected that passengers would be traveling daily on an air-
measure to the traveling public may not be as useful as in the port APM, nor that the travel time on an airport APM would
highway industry since APM travel times tend to be relatively vary to a degree great enough or frequent enough to make
short, thereby making the measure less meaningful. In addi- these measures meaningful for use by the public or an APM
tion, the high availability of airport APM systems (99%+) owner/operator. The additional time that must be allowed to
provides more dependable transportation than a highway make a trip within an established time, or the total time to
system, where there is a great variation from average or typi- be allowed for planning a trip within such a time, are mea-
