Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Report
• Infrequent weather disruptions.
Carriers with greater exposure will exercise a stronger response to variability due to their increased
frequency of disruption. However, these are not characteristics of the carrier, rather characteristics
of the region in which they operate.
TRAVEL-TIME RELIABILITY PERFORMANCE MEASURES
It is important to identify travel-time performance measures that are relevant to passenger travelers
and freight movers with consideration of their respective needs. The following reliability
measures, found to be most relevant to the user categories, are most widely used by transportation
agencies:
• Planning Time (95 th Percentile Travel Time) - Average trip duration in minutes and
seconds for 95% or less of all trips. This measure estimates how bad the delay will be
during the heaviest traffic days.
• Buffer Index - The difference between the 95th percentile travel time and the average
travel time, divided by the average travel time. This represents the extra time (in
minutes or as a ratio) that travelers add to their average travel time when planning trips
to ensure on-time arrival. The buffer index increases as reliability gets worse. The
buffer index often produces counterintuitive results. When the average travel time
decreases (a positive outcome) and the 95th percentile travel time remains high, the
buffer index increases - indicating that reliability has become worse. Thus, the buffer
index should be used with caution.
• Planning Time Index - The 95 th percentile travel time divided by the free-flow travel
time index. The planning time index can also be understood as the ratio of travel time
on the worst day of the month over the time required to make the same trip at free-flow
speeds. Consequently, the planning time index represents the total travel time that
should be planned when an adequate buffer time is included.
While the buffer index shows the additional travel time that is necessary beyond the average travel
time, the planning time index shows the total travel time to complete a trip. The planning time
index is a useful measure during peak travel periods because it can directly be compared to the
travel time index on a similar numerical scale.
All these performance measures provide different perspectives and provide additional insight when
used with multiple time periods. For example, the 95th percentile travel time can be computed for
an entire peak period, or for each specific hour within that peak period. Comparing how these
measures change over the course of a day illustrates how reliability changes during the day.
Tracking changes in these measures by time of day describes whether peak spreading is occurring,
what benefits travel demand management programs that change when employees come and go to
work are likely to provide in terms of travel reliability improvements, and when incident response
resources are most needed.
In addition to the above mentioned performance measures, travel time index which is the ratio of
the travel time in the peak period to the free-flow, could also be used to compare measured travel
time conditions to free-flow conditions. However, since it can represent a ratio for one or more
trips it is not necessarily a travel-time reliability measure. A good practice for computing
reliability performance is to base them on measurements taken over an extended period of time.
The SHRP2-L03 (1) project recommends that six months of data be collected for urban freeways
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