Module 3.1: Transport Demand Flashcards
What is transport supply?
Transport supply:
- is a service
- requires number of fixed assets
- requires set rules for operation
What is transport demand?
Transport demand:
- is a derived demand
- takes place over space
- is highly differentiated
- has strong dynamic elements
What is transport planning?
Transport planning aims to achieve equilibrium between demand and supply
What is a transport model?
Simplified representation of the real world transport network
What are zones?
Aggregations of individual land parcels into manageable divisions
What is a network?
Made up of nodes & links that represent roads & routes, lower order roads/routes may be ignored/aggregated
What are the 3 main areas of land use data?
- Population (age, income, car ownership, activity status)
- Employment (occupation, industry)
- Education (enrolments by type or level)
What areas does travel data involve?
- number of trips per day
- average trip length
- trip purpose
- mode
- time of day
- socio-demographic
What are the types of travel surveys?
Observation: traffic count, boardings/alightings, number plate, bluetooth
Interview: roadside, on-board, household
What info does a household travel survey involve?
- household: number of persons, vehicles
- vehicles: type, ownership, age
- persons: age, gender, disabilities
- trips: in a nominated travel day
What is the 4-step transport model?
- Trip generation
- Trip distribution
- Modal Split
- Assignment
What percentage of trips at home-based?
85%
What is a trip production?
- home end of a HB trip
- origin of a NHB trip
What is a trip attraction?
- non-home end of a HB trip
- destination of a NHB trip
What factors affect trip productions & attractions?
Productions: household size, income & car ownership
Attractions: population, employment, enrolments
What are the 3 model forms?
- Growth factor modelling
- Category analysis
- Regression analysis
What is growth factor modelling?
Crude but simple
Relies on pre-existing estimates & a measure of growth
Suitable for developed areas, short term (≤3 years), external trips
Ti = Fi.ti
Ti: future trips, ti: existing trips, Fi: growth factor
What is category analysis?
Popular in US and UK
Estimates average trip rates as function of household attributes
What is regression analysis?
Dependent variable (e.g. attractions) is estimated from independent variables (e.g. population, employment) Yi = ∑bX + c + ei b: coefficient X: independent variable c: constant, ei: error
What is a dummy variable regression?
Variables take on a value of 0 or 1
e.g. male: 0, female: 1
Describe how Productions & Attractions are equated?
No guarantee that ∑P = ∑A
Factor A’s by a factor: ƒ = ∑P / ∑A
This assumes that the production models are ‘better’
What do Trip generation models ignore?
- impact of transport supply
- congestion
- accessibility on trip making
What is doubly constrained growth factoring? What are the limitation?
Different growth rates for origin & destination, requires iterative solutions
Tij = tij x ai x bj
Limitations:
- needs a starting matrix & expected trip growth
- suitable only for short term 1-2years
- zero cells remain zero cells
What is a gravity model?
- used when there are changes in a network
- doesn’t rely on existing trip matrix (synthetic)
Tij = ai x bj x ƒ(cij)
cij: generalised cost, ƒ(cij): deterrence function
What are the 3 forms of deterrence functions?
Power: ƒ(c) = c^a
Exponential: ƒ(c) = e^(ßc)
Gamma: ƒ(c) = c^a x e^(ßc)
What is matrix estimation? What are the pro’s & con’s?
Use traffic counts as ‘targets’ that are reproduced by changing the values in matrix cells
Pro’s: simple, good reproduction of traffic flows, minimal data required
Con’s: errors from inconsistency, over-estimates, single trip purpose only,
Describe a model split curve (a.k.a. diversion curve).
Probability of using Mode 1 vs. Cost difference (C2-C1)
‘S’-shaped
Modal penalty: ∂ - some modes are preferred over others, even when the cost is equal
What are the 3 types of trip assignment?
- All or nothing (AON)
- Stochastic
- Capacity restraint
What is the All-or-Nothing assignment?
Assumptions: - route chosen that minimises cost of travel Useful for: - un-congested networks Con's: - ignores link capacity & congestion - sensitive to small changes in cost
What is Stochastic assignment?
- considers 2nd, 3rd, 4th best routes Pro's: - reasonable spread - relatively simple Con's: - ignores congestion effects
What is Wardrop’s Equilibrium?
Under equilibrium conditions, traffic arranges itself in congested networks such that all routes have equal and minimum costs
i.e. traffic arranges itself such that no individual can reduce their costs by switching route
What is Capacity Restraint method?
- incremental assignment
- typical values: 40%, 30%, 20%, 10%
Pro’s: - good convergence
Con’s - cannot remove loading of flow
