Trip Assignment Flashcards
What is Trip Assignment?
What are the aims, inputs and process and outputs?
By which routes will trips be undertaken?
Aim : To reproduce the pattern of vehicle passengers movement on the transport network
Inputs: trip matrix, network
Process: Select routes, load trips, modal trip matrices are assigned to appropriated transportation network
Output: link flows, turning movements, travel times and distances and travel paths
How are Productions and Attractions converted to Origin and Destination?
For a 24 hour trip matrix:
Tod = (Tpa + Tpa ^T) /2
For specific hour matrix:
Tod = alpha Tpa + ß Tpa ^T
What are the types of assignment models?
All or nothing
Stochastic
Capacity Restraint
What are the basic assumptions of the all or nothing approach?
- individuals select a route through the network that minimises generalised cost of travel
- > not always the case
What is the process of all or nothing approach?
Loads all of the trips onto the single minimum cost route between each origin and destination
What are the pros and cons for all or nothing approach?
Useful for uncongested networks
Cons:
overly sensitive to small changes in link costs
ignores link capacity and congestion
ignores variation in route choice behaviour
What is the stochastic method?
A method that emphasises variability in the individuals perceptions of cost
considers second best routes
simulation based methods
What are the pros of the stochastic method?
Often produce a reasonable spread of trips
Relatively simple to program
do not require speed flow relationships
What are the cons of the stochastic method?
result depends upon value of N chosen
ignores congestion effects
What is wardrop’s first principle?
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 a greater or equal cost
What are speed flow curves?
Assume delays occur along links not at junctions
incorporates congestion effects
Speed actual = Speed freeflow / {1 +0.15*(Flow/Capacity)^4}
What is the capacity restraint method - increment assignment?
- Split matrix into a number of fractional matrices
each is loaded onto the network in turn
link costs updated after each loading
typically fractional values
What are the pros of incremental assignment?
What are the cons?
very easy to program
algorithm analogous to the build up of congestion
good convergence properties when fractions are small
Cons:
once a flow has been loaded, it cannot be removed
What is the capacity restraint method - successive averaging?
- overcome problem of allocating too much traffic to low capacity links
current flow estimated 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 estimates
better to reduce the proportion of the new all or nothing assignment with each iteration
What are the pros of successive averaging?
Converges to Wardrops equilibrium (eventually)
