Lec 12 TD-Part 5: ch5.4.4, H/O, pp.498, Intro to trip assignment - PowerPoint PPT Presentation

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Lec 12 TD-Part 5: ch5.4.4, H/O, pp.498, Intro to trip assignment

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... (+ all-or-nothing trip loading) (cont) Minimum path with capacity restraints The BPR model Two other methods discussed in the text Trip assignment is the ... – PowerPoint PPT presentation

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Title: Lec 12 TD-Part 5: ch5.4.4, H/O, pp.498, Intro to trip assignment


1
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

2
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.
3
Minimum-path techniques ( all-or-nothing 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).
4
Minimum-path techniques ( all-or-nothing trip
loading) (cont)
A skimtree is created from 1 to all other nodes.
Travel time
Previous node
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.
5
Minimum path with capacity restraints
Once you reach this point travel time
exponentially increases.
Capacity restraints attempts to balance the
assigned volume, the capacity, and the related
speed (translated to travel time).
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.
Several methods are available, but the most
popular one is the Bureau of Public Roads model.
6
The BPR model
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.
7
Two other methods discussed in the text
  • Davidsons model

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

(See Example 13)
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