Trip Assignment Analysis
Once you have determined the number of trips that will enter and leave each zone, as
well as the transportation modes that the travelers will use, you can identify the exact
roadways or routes that will be selected for each trip. Trip assignment involves assigning
traffic to a transportation network such as roads and streets or a transit network.
Traffic is assigned to available transit or roadway routes using a mathematical
algorithm that determines the amount of traffic as a function of time, volume, capacity,
or impedance factor. There are three common methods for trip assignment: all or nothing,
diversion, and capacity restraint.
All-or-Nothing
All-or-nothing is often referred to as the minimum path algorithm. The minimum
path, or tree, represents the minimum time path between two zone centroids and is assigned
all of the traffic volume between the zones in question. As volumes and travel times
increase, the results of this method become more unreliable.
As an example of this method, imagine that zones A and B are connected by ten separate
routes. Route 3.0 has the shortest travel time which means that, according to this model,
all trips from A to B will use route 3.0.
Diversion is the allocation of trips to two or more possible routes in a designated
proportion that depends on some specified criterion. In most cases the criterion that is
used is time, although some also use distance and generalized cost. Diversion is very
similar to the all-or-nothing method, except that portions of the total number of
trips are allocated to different routes, with fewer trips being given to those routes with
longer travel times.
Capacity Restraint
Many different capacity restraint equations have been developed and tested and are
available for use. There are two basic characteristics common to capacity restraint
models; (i) they are non-linear relationships and (ii) they use the volume-capacity ratio
or v/c as a common factor. The underlying premise of a capacity restraint model is that
the travel time on any link is related to the traffic volume on that link. This is
analogous to the level of service (LOS) criterion, where LOS A corresponds to a low v/c
and a higher vehicle speed. LOS E and the corresponding v/c = 1 represents capacity.
Capacity restraint models assign traffic to possible routes in an iterative manner:
- A portion of the total traffic volume is assigned to the link with the shortest travel
time.
- Travel times for all possible links are calculated again, since volumes have changed.
- Another portion of the traffic volume remaining to be assigned is allocated to the link
that now has the shortest travel time.
- The travel time for all links are calculated and revised if changes result.
- The process of incremental assignments, followed by calculation of revised shortest
travel times, by link, continues until all trips have been assigned.
The capacity restraint model used by FHWA is applied in an iterative manner. The
adjusted link speed and/or its associated travel impedance is computed using the following
capacity restraint function:
T=To[1+0.15(V/C)4]
Where:
T= balance travel time (at which traffic V can travel on a highway segment)
To= free flow travel time: observed travel time (at practical capacity) times
0.87
V= assigned volume
C = practical capacity
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