Pekol (Transport Demand Modelling - Trip Distribution)

Question 1

What does trip distribution attempt to answer?

Answer 1

Where trips start and end

Question 2

What is a trip matrix?

Answer 2

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

Question 3

What is the generalised cost of travel?

Answer 3

Linear summation of the “cost of travel. (can be in terms of minutes or dollars)

Question 4

What does the generalised cost of travel include?

Answer 4

• in-vehicle (travel) time
• vehicle operating cost (distance based)
• walking time and waiting time
• fares, tolls or parking charges
• interchange penalty and modal penalty

Question 5

What is the Value of Time (VOT)?

Answer 5

Weights are used to convert each attribute to appropriate measure of time or money

Question 6

What are the methods of Growth Factoring?

Answer 6

• Uniform growth factor
• Singly constrained growth factors
• Doubly constrained growth factors

Question 7

What is the main assumption in Uniform Growth Factoring?

Answer 7

Assumption of uniform growth

>very unrealistic! Except for short time spans

Question 8

What are the key components of singly constrained growth factoring?

Answer 8

Apply origin or destination specific growth factors

Question 9

What are the key components of doubly constrained growth factoring?

Answer 9

• Different origin and destination growth rates

- Application of an “average” growth rate

Question 10

What are the limitations of growth factoring?

Answer 10

• 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

Question 11

When should a gravity model be applied?

Answer 11

Appropriate when changes in transport supply need to be considered

Question 12

Does the gravity model rely on an existing trip matrix?

Answer 12

NO.

-synthetic model

Question 13

What is the gravity model analagous to? (This may surprise you)

Answer 13

Newton’s law of gravity

Question 14

Who is the most handsome person in the the Davo Appreciation society?

Answer 14

Well obviously Gareth.

Question 15

What is the formula for the gravity model?

Answer 15

T=aibjf(cij)

Question 16

How are skim matrices produced?

Answer 16

Produced by “skimming” the network to determine the cost of travel on the cheapest route from zone i to zone j

Question 17

What do Skim matrices vary by?

Answer 17

Time of day and mode
mode (car, bus, etc.)
Time of day.. come on mate.. you can work that out.

Question 18

What are blended skims (not the milk)

Answer 18

Weighted average travel cost:

• over a day
• over a network

Question 19

What are deterrence functions?

Answer 19

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)

Question 20

What is the assumption once a deterrence function is calibrated?

Answer 20

Hold it constant

Question 21

What deterrence functions depend on?

Answer 21

• Vary by trip purpose and study area

- Shape of curve determined through gravity model calibration process

Question 22

What are the inputs of the Gravity model calibration?

Answer 22

• zonal trip productions and attractions (from trip generation model)
• skim matrix (fixed)
• observed trip length frequency distribution

Question 23

What are the practical considerations of a Synthetic Gravity model?

Answer 23

-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

Question 24

What are the benefits of Matrix Estimation?

Answer 24

• simple mechanical process
• can reproduce observed traffic flows quite well
• minimal data needed for small areas

Question 25

What are the issues with Matrix estimation?

Answer 25

• 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

Question 26

What is the key input in a synthetic gravity model?

Answer 26

Generalised cost of travel