Pekol (Transport Demand Modelling - Trip Assignment)

Question 1

What is the purpose of trip assignment?

Answer 1

Determine which routes that trips will be undertaken on

Question 2

What is the aim of trip assignment?

Answer 2

To reproduce the pattern of vehicle/passenger movement on the transport network

Question 3

What are the inputs of travel assignment?

Answer 3

• trip matrix

- Network

Question 4

What is the output of trip assignment?

Answer 4

• Link flows
• Turning movements
• Travel times and distances
• travel paths

Question 5

How do we convert P’s and A’s to O’s and D’s

Answer 5

Well, first we find out what robbie meant by those random letters… then we simply use a matrix.

Question 6

What are the three basic types of assignment?

Answer 6

• all-or-nothing
• stochastic
• capacity restraint

Question 7

What is the basis of the three basic types of assignment?

Answer 7

• All or nothing

- Necessary starting point for all other/more complicated methods

Question 8

What is the basic assumption in All-Or-Nothing assignment?

Answer 8

Individuals select a route through the network that minimises their generalised cost of travel (Note: this is not always the case.. some people like a scenic route.. others are like guthrie and attempt to impress people with their thorough knowledge of back-roads)

Question 9

How does All-Or-Nothing assignment work?

Answer 9

Loads all of the trips onto the single minimum cost route between each origin and destination. (note. costs are fixed for all links)

Question 10

When does the All-Or-Nothing assignment Shine (when is it most useful)?

Answer 10

• uncongested networks

- the basis of more complicated assignment models

Question 11

What are the issues of All-Or-Nothing assignment?

Answer 11

• overly sensitive to small changes in link costs
• ignores link capacity and congestion
• ignores variation in route choice behaviour

Question 12

If you had to travel with ANYONE in your car who would it be? Please keep in mind on this trip you would want many dad jokes, and a nice smelling man.

Answer 12

Robbie

Question 13

Why would we use Stochastic methods?

Answer 13

• recognises in reality, not all people go on the shortest route.
• Emphasise variability in the individual’s perceptions of cost

Question 14

What are the benefits of Stochastic methods?

Answer 14

• often produce a reasonable spread of trips
• relatively simple to program
• do not require speed-flow relationships

Question 15

What are the issues with Stochastic methods?

Answer 15

• result depends upon value of N (no. of routes) chosen

- ignores congestion effects

Question 16

What is Wardrop’s equilibrium?

Answer 16

Under equilibrium conditions, traffic arranges itself in congested networks such that all routes used between any origin and destination have equal and minimum costs, while all unused routes have greater or equal costs.

Question 17

What is Incremental assignment?

Answer 17

• split matrix into a number of fractional matrices

- each loaded onto network in turn

Question 18

What are the benefits of incremental assignment?

Answer 18

-very easy to program
-algorithm analogous to the build-up of congestions
-good convergence properties when fractions are small
(surely we don’t have to know this.)

Question 19

What are the issues with incremental analysis?

Answer 19

Once a flow has been loaded, it cannot be removed. (Some would say, it cannot go with the flow.)

Question 20

What is successive averaging?

Answer 20

• overcome problem of allocating too much traffic to low-capacity links
• ‘current’ flow estimate is a linear combination of the ‘previous’ flow estimate plus a proportion of a new all-or-nothing assignment, with speeds adjusted in line with ‘previous’ flow estimate

Question 21

What is the benefit of successive averaging?

Answer 21

-converges to Wardrop’s equilibrium (eventually)

Question 22

What are the application issues of Traffic assignment?

Answer 22

Limitations in the node-link model:
-most delays actually occur at the nodes (intersections)
-dynamic nature of queues and its effect on route choice
Errors in defining average perceived costs
Assumptions of perfect information
Variations in demand (day to day and time of day)