# Traffic Assignment Part I - PowerPoint PPT Presentation

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Traffic Assignment Part I. CE 573 Transportation Planning Lecture 16. Objectives. Define traffic assignment assumptions Mathematically define relationship between OD trips and network Load traffic onto the network. Network Loading.

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Traffic Assignment Part I

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## Traffic Assignment Part I

CE 573 Transportation Planning

Lecture 16

### Objectives

• Define traffic assignment assumptions

• Mathematically define relationship between OD trips and network

• Load traffic onto the network

Michael Dixon

The basic objective is to assign traffic in a reasonable fashion that approximates, on the aggregate scale, how traffic uses the transportation network.

• Assign traffic (vehicle trips) to the links

• Approximates traffic use of network

• Assumptions:

• driver’s informationperfectly informed

• driver response to informationperception of cost

• driver objectivesminimize cost

• Traffic assignment resultUser Equilibrium

• no driver can reduce their travel costs from i to j by changing routes

Michael Dixon

Zone A

Zone B

Michael Dixon

• Trip matrixconvert from person trips to vehicle trips By trip purpose

• HBW: 1.1 person trips/veh trip

• HBO: 1.6 person trips/veh trip

• Network components

• centroid connectors

• nodes

• Route selection criteria/rules

• Cost function

• Minimize cost

Michael Dixon

### Route Selection Criteria/Rules

• Routing concerns

• stochasticdifference in motorist perceptions (quality of information and sensitivities to costs)

• congestedcapacity constrained

• Classification scheme for traffic assignment algorithms

Michael Dixon

### Basic Steps of Traffic Assignment Methods

• Identify routes

• stored in tree

• output from tree building algorithm

• Assign trip matrix

• to routes

• Check for convergence to user equilibrium

Michael Dixon

### Assigning the Trip Matrix to Routes

• Use Dijkstra’s algorithm to build the minimum cost path trees

• Have min cost path tree for all origins

• Let’s use a link index to represent these path trees

• a  index for each link

• i  index for the origin zone

• j  index for the destination zone

• Let’s put all of the link indices () in matrix form, link choice matrix (P)

• One dimension is O-D pairs

• Now cumulatively assign all of the O-D pair volumes to their respective shortest path links

Michael Dixon

Michael Dixon

Michael Dixon

Michael Dixon

### Assigning O-D Pair Volumes

• Cumulatively to their respective shortest path links

• This is called All-or-Nothing Assignment

• no representation of traffic effects on travel costs

• Only one path per O-D pair

• Just like our link choice matrix

Michael Dixon

Michael Dixon

### Assigning O-D Pair Volumes

• Assume a vehicle occupancy of

• 1 person trips/veh trip

Michael Dixon