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10. Traffic Assignment

The chapter discusses traffic assignment, focusing on the allocation of trip interchanges to transportation systems in order to replicate observed vehicular movements. It details different methods of traffic assignment, including All-or-Nothing, User Equilibrium, and System Optimum Assignment, outlining their applications, assumptions, and limitations. The chapter also covers iterative methods for link flow calculation and touches on additional assignment methods such as Incremental and Capacity Restraint assignments.

Sections

Traffic Assignment

Traffic assignment is the process of allocating vehicle trips across a transportation network to reflect travel demands, focusing on estimating traffic volumes, interzonal travel costs, and identifying congestion.

10 Section Overview

Start current section content and materials

10.1 Overview

Traffic assignment is the process of allocating trip interchanges to transportation systems, aiming to reproduce vehicular movement patterns based on travel demand.

10.2 Link cost function

The Link Cost Function describes how travel time increases as link flow approaches capacity, impacting traffic assignment models.

10.3 All-or-nothing assignment

The all-or-nothing assignment method allocates trips from any origin to a single minimum-cost path, ignoring congestion and multiple route options.

10.4 User equilibrium assignment (UE)

User equilibrium assignment (UE) ensures that no driver can reduce their travel costs by changing routes, as defined by Wardrop's first principle.

10.5 System Optimum Assignment (SO)

The System Optimum Assignment (SO) aims to minimize total system travel time by directing drivers to optimal routes.

10.6 Other assignment methods

This section outlines various traffic assignment methods beyond user equilibrium, covering incremental assignment, capacity restraint assignment, stochastic user equilibrium assignment, and dynamic assignment.

10.6.1 Incremental assignment

Incremental assignment involves assigning traffic volumes in steps, adjusting link travel times after each step but does not yield a true equilibrium solution.

10.6.2 Capacity Restraint Assignment

Capacity restraint assignment aims to achieve an equilibrium solution by assessing traffic loads based on congestion functions.

10.6.3 Stochastic user equilibrium assignment

This section discusses Stochastic User Equilibrium Assignment, a traffic assignment method that accounts for variations in driver perception and allows for the selection of non-minimum cost routes.

10.6.4 Dynamic Assignment

Dynamic Assignment is a method that extends Wardrop's user equilibrium to include the concept of route and departure time optimization for drivers, considering disutility factors.

10.7 Summary

This section summarizes the key principles and assignment methods related to traffic assignment strategies in transportation.

10.8 Problems

This section presents exercise problems related to user equilibrium assignment in traffic assignment processes and provides hints for their solutions.

Learning Objectives

  • Traffic assignment is essential for estimating link traffic volumes and interzonal travel costs.

  • Different models, such as All-or-Nothing and User Equilibrium, provide frameworks for understanding travel patterns.

  • Assumptions play a crucial role in determining the effectiveness of various traffic assignment methods.

Key Concepts

Traffic Assignment

The process of allocating trip interchanges to a transportation network to reflect patterns of vehicular movement.

User Equilibrium Assignment (UE)

A traffic assignment method based on the principle that no driver can reduce their travel costs by unilaterally changing routes.

System Optimum Assignment (SO)

A model that minimizes total system travel time by suggesting optimal routes for drivers.

All-or-Nothing Assignment (AON)

An assignment method where trips are loaded onto a single minimum cost path, ignoring link congestion.

Practice Exercises

Total Questions

1

Estimated Time

2 min

Passing Score

70%

Instructions

  • Read each question carefully
  • You can use hints if you need help
  • Complete all questions before submitting
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