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reliability. It can be measured with a congestion index or more than a single data source, and more than one analysis
accessibility statistics. procedure. Mobility and reliability measures, when com-
· Reliability. The changing times, locations, and levels of bined in a process to uncover the goals and objectives the
mobility and reliability are important characteristics for public has for transportation systems, can provide a frame-
mobility and reliability measurement. This is particularly work to analyze how well the land use and transportation
important to freight movement operations that rely on the systems serve the needs of travelers and businesses and
transportation system as an element of their productivity provide the basis for improvement and financing decisions.
and to measuring the frustration level of travelers faced Exhibit 2.4 provides a quick reference to selected mobility
with an unexpected loss of mobility or reliability. and reliability measures discussed in more detail in this
chapter. It illustrates the measures, the input data required,
The total amount of mobility and reliability provided to and the general format of the equation required to calculate
travelers in an area is the volume of a box with axes of time, each measure.
location, and level. The reliability of the mobility provided to
travelers and residents is the change in the volume of the box
2.4 Individual Measures
from time period to time period or from day to day. Exhibit 2.3
illustrates the description of mobility and reliability with the Travel time, speed, and rate quantities are somewhat more
four aspects. These answer the key questions of travelers and difficult to collect and may require more effort than the traf-
residents: 1) When can I travel in a satisfactory amount of fic volume counts that currently provide the basis for most
time? 2) Where can I travel in a satisfactory amount of time? roadway analysis procedures. Travel speed-related measures
3) How much time will it take? 4) How much will my travel can, however, be estimated as part of many analysis processes
time vary from trip to trip? currently used. The ultimate implementation of a set of time-
Answering the key questions with measures of the four related mobility and reliability measures in most urban areas
components of mobility and reliability will encompass the will probably rely on some estimating procedures along with
needs of residents and travelers, as well as transportation and archived data. These measures may include current Highway
land use professionals. Capacity Manual-based analysis techniques (5), vehicle
density measures estimated from detectors in the pavement
or from aerial surveys or relationships that estimate travel
2.3 Performance Measure Summary
rate, or speed from generally available volume and roadway
The overriding conclusion from any investigation of mo- characteristics. The use of estimating procedures will be
bility and reliability measures is there is a range of uses and particularly important in setting policy and the prioritization
audiences. No single measure will satisfy all the needs, and of transportation improvement projects, pavement design-
no single measure can identify all aspects of mobility and re- ing, responding to developer requests for improvement, and
liability--there is no "silver bullet" measure suited to every performing many other analyses.
application or question. Mobility and reliability are com- The focus of this section is those measures most applicable to
plex and, in many cases, requires more than one measure, the individual traveler. Key characteristics about each mobility
and reliability measure are summarized after the measures are
presented. Summarizing the measure characteristics illustrates
Time
the flexibility of mobility and reliability measures based on time
and person or freight movement.
The delay per person or delay per peak-period traveler (in
daily minutes or annual hours) can be used to reduce the
travel delay value to a figure more useful in communicating
to nontechnical audiences. It can normalize the impact of
Level
mobility projects that handle much higher person demand
than other alternatives, where a measure of total delay might
lead to different conclusions about the benefits of a solution.
Delay for the primary route or road, in these alternatives, may
be higher due to this higher volume, but this also indicates the
Location:
The volume of the box is the amount of mobility and reliability provided. need to examine the other facilities or operations within the
A change in volume of the box indicates the reliability of the system. corridor included in the "before" case. To the extent possible,
Larger volume is better.
the initial analysis should include as much of the demand
Exhibit 2.3. Components of mobility (2). that might move to the improved facility, route, or road.
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Individual Measures1
Actual FFS or PSL
Delay per Delay per Traveler 250 weekdays hour
Travel Time Travel Time
Traveler annual hours year 60 minutes
minutes minutes
Travel Time Actual Travel Rate Length Vehicle Volume Vehicle Occupancy
Travel Time
person minutes minutes per mile miles vehicles persons/vehicles
Actual Travel Rate
Travel Time minutes per mile
Travel Time Index
Index 2 FFS or PSL Travel Rate
minutes per mile
95th Percentile Travel Time Average Travel Time
Buffer minutes minutes
Buffer Index % 100%
Index 2 Average Travel Time
minutes
95th Percentile Travel Time
Planning Planning Time Index minutes
Time Index 2 no units FFS or PSL Travel Time
minutes
Area Measures 1
Actual FFS or PSL
Total Segment Delay Vehicle Volume Vehicle Occupancy
Total Delay = Travel Time - Travel Time
person - minutes vehicles persons/vehicle
minutes minutes
Congested
Congested Congested Travel Vehicle Volume
= Segment Length
Travel vehicle - miles vehicles
miles
Actual FFS or PSL
m Vehicle Vehicle
Travel Travel
- Volumei Occupancyi
Percent Timei Timei
i 1 vehicles persons/vehicle
Percent of of minutes minutes Each congested segment
Congested 100
Congested n Vehicle Vehicle
Travel Actual Travel Ratei Lengthi
Travel Volumei Occupancy i
minutes per mile miles
i 1 vehicles persons/ve hicle All segments
Area Measures1
Congested Congested Roadway Congested Segment
=
Roadway miles Lengths miles
Objective Fulfillment Opportunities
Accessibility
Accessibility = e.g., jobs , Where
opportunities
Travel Time Target Travel Time
1
"Individual" measures are those measures that relate best to the individual traveler, whereas the "area" measures are more
applicable beyond the individual (e.g., corridor, area, or region). Some individual measures are useful at the area level when
weighted by Passenger Miles Traveled (PMT) or Vehicle Miles Traveled (VMT).
2
Can be computed as a weighted average of all sections using VMT or PMT.
Note: FFS = Free-flow speed, and PSL = Posted speed limit.
Exhibit 2.4. Quick reference guide to selected mobility and reliability measures.
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Equations 2.1.a and 2.1.b illustrate the computation of delay can be applied to various system elements with different
per traveler in annual hours. Equation 2.1.a is appropriate for free-flow or posted speeds. Travel rate (in minutes per mile)
a single section of highway where the delay (i.e., actual travel is a direct indicator of the amount of travel time, which
time minus free-flow travel time) for a number of travelers makes it relevant to travelers.
over the same segment can be averaged and then expanded to The measure can be averaged for freeways and arterial
annual hours. Equation 2.1.b applies the same concept to a streets using the amount of travel on each portion of the
situation involving multiple highway segments of different network. An average corridor value can be developed using
lengths, and can be used to estimate average delay per trav- the number of persons using each facility type (or modes)
eler over a number of segments, routes or a system. In this to calculate the weighted average of the conditions on adja-
case, the number of vehicles and occupants per vehicle is used cent facilities. The corridor values can be computed for
in the numerator to expand and sum the total individual hourly conditions and weighted by the number of travelers
traveler delay over the various segments, and again in the de- or person-miles traveled to estimate peak period or daily
nominator to reduce the summed traveler delay to an average index values.
amount per traveler. The TTI in Equation 2.2 compares measured travel rates to
The Travel-Time Index (TTI) is a dimensionless quantity free-flow or PSL conditions for any combination of freeway
that compares travel conditions in the peak period to travel and arterial streets. Index values can be related to the general
conditions during free-flow or posted speed limit conditions. public as an indicator of the length of extra time spent in the
For example, a TTI of 1.20 indicates that a trip that takes 20 transportation system during a trip. Equation 2.2 illustrates a
minutes in the off-peak period will take 24 minutes in the peak relatively simple version of the calculation using VMT, but
period or 20 percent longer. The TTI can be quickly and easily PMT also could be used, as could a value of time calculation
interpreted by most users in both an absolute sense (e.g., a TTI that incorporates person and freight travel.
of 1.5 means a free-flow 200-minute trip will take 30 minutes) Travel Rate Index (TRI) is similar to the TTI in that it also is
or a relative sense (the trip will take 50 percent longer.) This a dimensionless quantity that compares travel conditions in
dual mode is useful because for a very short trip even a rela- the peak period to travel conditions during free-flow or PSL
tively large percent increase in travel time may be insignificant. conditions. The TRI measure is computed in the same way as
Conversely, for a longer trip, a relatively small percent increase the TTI, but does not include incident conditions. A typical ap-
in travel time may be significant in terms of late arrival. plication of the TRI would be calculating congestion levels
TTI reflects travelers' perceptions of travel time on the from a travel demand forecasting model, because incident con-
roadway, transit facility, or other transportation network ditions are not considered in the model's forecasts. In contrast,
element. This comparison can be based on the travel time continuous data streams allow for the direct measurement of a
increases relative to free-flow conditions (or PSL) and com- TTI that includes incidents. For some analysis applications,
pared to the target conditions. Thus, the same index formula however, incident conditions would intentionally be excluded.
Actual FFS or PSL
Delay per Traveler 250 weekdays hour
= Travel Time - Travel Time × × (Eq. 2.1.a)
(annual hours ) ( minutes ) ( minuttes ) year 60 minu tes
Actual FFS orPSL
Vehicle Vehicle
Travel - Travel × Volume × Occ cupancy ×
250 week days
×
hour
Time Time (vehicles) (persons /vehicle) year 60 minutes
Delay per (minutes) (minutes)
Traveler = (Eq. 2.1.b)
(annual hours) Vehicle Volume (vehicles) × Vehicle Occupancy (persons /vehicle)
Principal Arteria al Princiipal
Freeway Peak Street TravelRate
Freeway TravelRate ArterialStreet
× Period + ×
Freeway Free-flow or Principal Arterial Peak Period
VMT
Posted Speed Limit Rate Stree et Free-flow or VMT
Travel Time Index = PostedSpeedLimit Rate (Eq. 2.2)
Freeway Peak Period VMT +Principal Arteri
ial Street Peak Period VMT
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For example, when travel time runs are performed for a corri- scales. The Planning Time Index is computed as the 95th
dor study, those runs affected by incident conditions are nor- percentile travel time divided by the free-flow travel time as
mally removed. This provides an estimate of the nonincident shown in Equation 2.4.
travel time along the corridor. In these conditions, the com-
95th Percentile
puted measure would be a TRI rather than a TTI. Planning Time Index Travel Time (minutes)
Buffer Index (BI) is a measure of trip reliability that = (Eq. 2.4)
(no units) Travel Time Based
expresses the amount of extra buffer time needed to be on time on Free-Flow or
for 95 percent of the trips (e.g., late for work on one day out of PostedSpeed (minutes)
the typical 20-work-day month.) As with the TTI, indexing the
measure provides a time- and distance-neutral measure, but On-Time Arrival estimates the percentage of time that a
the actual minute values could be used by an individual traveler traveler arrives on time based on an acceptable lateness
for a particular trip length or specific origin-destination (O-D) threshold. A value in excess of the travel rate mean, say 10
pair. With continuous data, the index is calculated for each percent to 15 percent, is used to identify the threshold of
road or transit route segment, and a weighted average is calcu- acceptable lateness or being "on time." Required data include
lated using vehicle-miles or, more desirably, person-miles of a sample distribution of trip times, whether for transit or
highway trips. The On-Time Arrival percent is computed
travel as the weighting factor. Travel rates for approximately
according to the following formula:
5-mile sections of roadway provide a good base data element
for the performance measure. The BI can be calculated for each %OnTime = PercentTripTimes <[1.10 * MeanTime] (Eq. 2.5)
road segment or particular system element using Equation 2.3.
Note that a weighted average for more than one roadway sec- where
tion could be computed using VMT or PMT on each roadway %OnTime = Percent On-Time Arrivals;
section. The measure would be explained as "a traveler should PercentTripTimes = Percent of measured trip times; and
allow an extra (BI) percent travel time due to variations in the MeanTime = The computed mean of the measured
amount of congestion and delay on that trip." travel time.
95thPercentile AverageTravel Percent Variation is closely related to the Planning Time
Travel Time - Time
Buffer (minutes) Index. It is expressed as a percentage of average travel time
(minutes)
Index = × 100% (Eq. 2.3) and is distance/time neutral. Multiplying the average travel
(%) AverageTravel time by the percent variation yields the total travel time
Time needed to be on time 85 percent of the time (one standard
(minutes)
deviation above the mean). Higher values of percent variation
The buffer time concept appears to relate particularly well indicate less reliability. It is computed according to the fol-
to the way travelers make decisions. Conceptually, travel lowing formula:
decisions proceed through questions, such as: "How far is it?"
std.dev.
"When do I need to arrive?" "How bad is the traffic likely to %V = * 100% (Eq. 2.6)
be?" "How much time do I need to allow?" "When should I Mean
leave?" In the time allowance stage, there is an assessment of where
how much extra time has to be allowed for uncertainty in the
travel conditions. This includes weather, incidents, construc- %V = Percent Variation;
tion zones, holiday or special event traffic, or other disrup- Std.dev. = The standard deviation of measured travel time;
tions or traffic irregularities. and
Planning Time Index represents the total travel time that Mean = The computed mean of the measured travel
should be planned when an adequate buffer time is time.
included. Planning Time Index differs from the BI in that The 90th or 95th percentile travel time is perhaps the sim-
it includes typical delay as well as unexpected delay. Thus, plest measure of travel-time reliability for specific travel
the Planning Time Index compares near-worst case travel routes or trips, which indicates how bad delay will be on the
time to light or free-flow traffic travel time. For example, a heaviest travel days. The 90th or 95th percentile travel times
planning time index of 1.60 means, for a 15-minute trip in are reported in minutes and seconds and should be easily un-
light traffic, the total time that should be planned for the derstood by commuters familiar with their trips. For this rea-
trip is 24 minutes (15 minutes * 1.60 = 24 minutes). The son, this measure is ideally suited for traveler information.
Planning Time Index is useful because it can be directly This measure has the disadvantage of not being easily com-
compared to the travel-time index on similar numeric pared across trips, as most trips will have different lengths. It
