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17
Total Delay Actual FFS orPSL
Vehicle Volume VehicleOccupanc cy 1hour
= Travel Time - Travel Time × × × (Eq. 2.7)
(persons - hours) (vehicles) (persons /vehicle)
(minutes) (minutes)
60minutes
Congested Travel Congested Vehicle Volume
= SegmentLength ×
(vehicles)
(Eq. 2.8)
(vehicle - miles)
(miles)
m Actual Travel FFS or PSL Vehicle Vehicle
Time i - Travel Timei × Volumei × Occupancy i
Percent of i =1 ( minutes ) ( minutes ) ( vehicles ) ( persons/vehicle ) Each congested segment
Congested = × 100
Vehicl e Vehicle (Eq. 2.9)
Travel
n
Actual Travel Ratei Length i
× × Volumei × Occupancy i
i =1 ( minutes per mile ) ( miles )
( vehicles ) ( persons/vehicle ) All segments
also is difficult to combine route or trip travel times into a of road, hours per 1,000 miles traveled, or hours per 1,000
subarea or citywide average. travelers might be more meaningful to agencies at the corri-
Several other statistical measures of variability have been dor level, but the public may not understand these measures
suggested to quantify travel-time reliability, such as standard since it is difficult to relate to key travel decisions or travel
deviation and coefficient of variation. These are discouraged experience.
as performance measures, as they are not readily understood Congested travel is a measure that captures the extent of
by nontechnical audiences nor easily related to everyday congestion. It estimates the extent of the system affected by
commuting experiences. The 90th or 95th travel time, or the congestion. Equation 2.8 illustrates the computation of
indexes such as the BI or Planning Time Index, are recom- congested travel in vehicle-miles as the product of the con-
mended as simpler ways to express the variability of expected gested segment length and the vehicle volume summed across
travel time in a way that travelers can relate more directly to all congested segments.
their travel expectations or experience. The percent of congested travel is an extension of the con-
gested travel measure. It also measures the extent of congestion.
When speed and occupancy data are available for each roadway
2.5 Area Measures
segment, this measure can be computed. It is computed as the
The mobility and reliability measures described in the pre- ratio of the congested segment person-hours of travel to the total
vious section mainly relate to the individual traveler making person-hours of travel. Equation 2.9 shows the computation.
a particular trip. The measures described in this section are Congested roadway is another measure of the extent of
area measures where the area may be a corridor or region. congestion. It is the sum of the mileage of roadways that op-
These measures may be better suited to large scale system erate under free-flow or posted speed limit conditions. This
planning analysis. is shown in Equation 2.10.
The total delay (in person- or vehicle-hours) for a transit or
roadway segment is the sum of time lost due to congestion. Congested Roadway Congested Segment (Eq. 2.10)
=
Delay can be expressed as a value relative to free-flow travel or ( miles ) Lengths ( miles )
relative to the posted speed limit. Total delay in a corridor or Accessibility is a measure that often accompanies mobility
an urban area is calculated as the sum of individual segment measures. It quantifies the extent that different opportunities
delays. This quantity is used to estimate the impact of im- can be realized. These might be accessibility to jobs, a transit
provements on transportation systems. The values can be used station, or other land use or trip attractor of interest. Acces-
to illustrate the effect of major improvements to one portion sibility is satisfied if the travel time to perform the desired ac-
of a corridor that affects several other elements of the corridor. tivity is less than or equal to the target travel time as indicted
The quantity is particularly useful in economic or benefit/cost in Equation 2.11.
analyses that use information about the magnitude of the mo-
bility improvement for cost-effectiveness decisions. Objective Fulfillment
Accessibility
Opportunities ( e.g., jobs ) , Where (Eq. 2.11)
Equation 2.7 shows the computation of delay in person-
(opportunities )= Travel Time Target Travel Time
hours. In addition, using a delay measure of hours per mile