Trip Assignment
The following excerpt was taken from the Transportation Planning Handbook
published in 1992 by the Institute of Transportation Engineers (pp. 115-117).
The traffic assignment process is somewhat different from the mathematical models used
for trip distribution and mode choice. Traffic is assigned to available transit or roadway
routes using a mathematical algorithm which determines the amount of traffic to allocate
to each route. The traffic allocation is usually based on the relative time to travel
along each available path, computed from the transit and roadway networks.
All or nothing
Historically all trips between two zones were assigned to the route having the minimum
travel time, regardless of the available capacity; this is termed an
"all-or-nothing" assignment. Such an approach is still used for identifying
travel desire corridors as an initial step in locating new and improved transportation
facilities. For most transit assignments the all-or-nothing approach is still used since
there are rarely closely competing transit routes in an efficiently designed transit
system. Similarly the all-or-nothing approach is used for assigning high occupancy vehicle
trip assignments.
Capacity Restraint
More common today for roadway assignments is the "capacity-restrained"
assignment, a strategy which assigns traffic in steps. One option in this approach is
"proportional" assignment, which allocates a portion of the trips between every
origin-destination zone pair to the network at each step. An alternative is the
"incremental" assignment, which allocates all of the trips between a subset of
zone pairs at each step. In either case the travel times between all zone pairs are
recalculated after each assignment step, considering the traffic already assigned, to
adjust the speeds on all network links. The revised speeds on all links are determined by
a speed-volume function that indicates the maximum speed likely for a particular
volume/capacity ratio. . . . Another assignment step is then computed considering the
revised travel times, after which the link speeds are again adjusted as previously. This
process is iterated until all trips have been assigned. Additional fully iterated
assignments may be necessary to reach an equilibrium in which there is little change in
speeds throughout the network at each assignment step.
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