Pekol (Transport Demand Modelling - Trip Distribution) Flashcards
What does trip distribution attempt to answer?
Where trips start and end
What is a trip matrix?
Output of trip distribution Two dimensional n*n array where: -n=no. zones in the study area -rows=trip productions/origins -columns = trip attractions/destinations
Tij=trips from zone i to zone j
Diagonal Tij=intra-zonal trips
What is the generalised cost of travel?
Linear summation of the “cost of travel. (can be in terms of minutes or dollars)
What does the generalised cost of travel include?
- in-vehicle (travel) time
- vehicle operating cost (distance based)
- walking time and waiting time
- fares, tolls or parking charges
- interchange penalty and modal penalty
What is the Value of Time (VOT)?
Weights are used to convert each attribute to appropriate measure of time or money
What are the methods of Growth Factoring?
- Uniform growth factor
- Singly constrained growth factors
- Doubly constrained growth factors
What is the main assumption in Uniform Growth Factoring?
Assumption of uniform growth
>very unrealistic! Except for short time spans
What are the key components of singly constrained growth factoring?
Apply origin or destination specific growth factors
What are the key components of doubly constrained growth factoring?
- Different origin and destination growth rates
- Application of an “average” growth rate
What are the limitations of growth factoring?
- Need a starting matrix and expected trip growth
- Dependant upon the accuracy of the starting matrix
- Suitable for only short-term forecasts
- Ignores changes in transport costs
When should a gravity model be applied?
Appropriate when changes in transport supply need to be considered
Does the gravity model rely on an existing trip matrix?
NO.
-synthetic model
What is the gravity model analagous to? (This may surprise you)
Newton’s law of gravity
Who is the most handsome person in the the Davo Appreciation society?
Well obviously Gareth.
What is the formula for the gravity model?
T=aibjf(cij)
How are skim matrices produced?
Produced by “skimming” the network to determine the cost of travel on the cheapest route from zone i to zone j
What do Skim matrices vary by?
Time of day and mode
mode (car, bus, etc.)
Time of day.. come on mate.. you can work that out.
What are blended skims (not the milk)
Weighted average travel cost:
- over a day
- over a network
What are deterrence functions?
Used in gravity models to reflect propensity for cost saving in travel decisions
(Under assumption people don’t want to spend any more money or time than they have to.. note: this excludes Sunday drivers)
What is the assumption once a deterrence function is calibrated?
Hold it constant
What deterrence functions depend on?
- Vary by trip purpose and study area
- Shape of curve determined through gravity model calibration process
What are the inputs of the Gravity model calibration?
- zonal trip productions and attractions (from trip generation model)
- skim matrix (fixed)
- observed trip length frequency distribution
What are the practical considerations of a Synthetic Gravity model?
-Significant proportion of trips start or end outside the study area
-Cost of these trips essentially undefined
Solutions to this include:
-surround study area with a coarser buffer network
-deal with external trips separately from internal trips
What are the benefits of Matrix Estimation?
- simple mechanical process
- can reproduce observed traffic flows quite well
- minimal data needed for small areas
What are the issues with Matrix estimation?
- errors from inconsistent traffic count data
- tends to overestimate short distance trips
- can only deal with a single trip purpose (because method simply tries to match up trips with a purpose)
- We don’t get purpose information
What is the key input in a synthetic gravity model?
Generalised cost of travel
