Lec 12 TD-Part 5: ch5.4.4, H/O, pp.498,
Intro to trip assignment
Lecture Objectives



Know the purpose of trip assignment
Know a few names of trip-assignment
procedures
Know the difference of minimum-path
techniques and minimum-path with capacity
restraints
Trip Assignment
Trip assignment is the
procedure by which the
planner predicts the paths the
trips will take.
The planner can get realistic
estimates of the effects of
policies and programs on travel
demand.
Minimum-path techniques (+ “all-ornothing” trip loading)
Assumption: Travelers
want to use the minimum
impedance route between
two points.
Given impedance values,
assignment algorithms find
minimum paths (or shortest
paths) to get from point A to all
other locations (to which trips
are distributed).
A skimtree is created from 1 to
all other nodes (see next slide).
Minimum-path techniques (+ “all-ornothing” trip loading) (cont)
Travel time
Previous node
A skimtree is created from 1 to
all other nodes.
In this method, all trips between a given origin and
destination are loaded on the links comprising the
minimum path and nothing is loaded on the other links.
 No consideration of the link capacities.
Minimum path with capacity restraints
Once you reach this point
travel time exponentially
increases.
Travel time increases as traffic
volume on the link increases
because of interaction between
the drivers and their perception
of safety because they slow
down as volume increases.
Capacity restraints
attempts to balance the
assigned volume, the
capacity, and the related
speed (translated to
travel time).
Several methods are
available, but the most
popular one is the Bureau
of Public Roads model.
The BPR model

 Q
TQ  T0 1   

 Qmax







Where TQ = travel time at traffic flow Q
T0 = “zero-flow” or “free-flow” travel time
= travel time at practical capacity x 0.87
Q = traffic flow (veh/hr)
Qmax = practical capacity = ¾ x saturation flow
alpha & beta = parameters (need calibration)
See the example on page 500. Note that this example
assumes that the ratio of daily flow/capacity is equal to hourly
flow/capacity.
Two other methods discussed in the text
Davidson’s model
1  1   Q Qmax
TQ  T0
1  Q Qmax
Tau = LOS parameter, 0.1-0.2 for freeways, 0.4-0.6 for
urban arterials, 1.0-1.5 for collector roads
Greenshields’ model
us  u f 
uf
kj
k
q  us k  u f k 
qmax 
uf
kj
k2
k ju f
4
(See Example 13)