Transport Demand - Modal Split Flashcards
What is modal spilt?
By which mode will trips be undertaken?
What does modal spilt vary by?
zone
purpose
time
What factors influence mode choice?
Characteristics of Trip maker
Characteristics of the journey
Characteristics of the transport system
Characteristics of trip origin and destination
What is the factor based approach?
Uniform factoring:
Tij ^k = F^k Tij
where F^k = proportion of trips by mode k
Sector Based factoring:
Tij ^k = F^k Tij
where F^k = proportion of trips by mode k between zones i
What are the pros and cons of factor based approach?
Pros:
Simple to develop and apply
suitable for areas with no or little likelihood of significant change to existing mode choice behaviours
Cons:
ignores all factors influencing mode choice
how should these factors change for the forecast years
What is modal penalty?
The penalty associated with the preference of one mode over another (Even though there is a difference in cost)
What is the binary logit model?
Pij^c = exp (-bCij^c)/(exp(-bCij^c) +exp (-b(Cij^B +d)))
How is calibrating a binary logit model?
By using Ln(P^C / 1- P^c)) = b (C^B - C^C) + bd
in the form y = mx +c
What is a Multinomical logit model?
When more than two modes are used -
P^1 = proportion of trips by mode 1 = exp (-bC^1) /∑(exp (-bC^N))
N way structure
Assumes alternatives have equal weight
What is hierarchical logit model?
Modes with common elements are combined into composite modes
Relies on composite costs which depart further from reality as one moves up the hierarchy
What is the pivot point model?
P^1 ij = proportion of trips by mode 1 from zone i to zone j
= Tij / (1+exp(aC^2 ij - bC^2 ij + mij)
a and b are modal constants
m = combined socio economic variable m = ∑ wx = ∑(weight x socio economic variable)
must have pre existing matrix for each mode
must have skim matrice for each mode
find a and be and ma through iterative process
useful for estimating the impact of incremental changes in modal costs
What are direct demand models?
Combines trip generation, distribution and mode split into a single well defined model:
T^k ij = f (origin attributes, destination attributes, LOS)
What are the pros of direct demand models?
What are the cons?
Overcomes theoretical problems of 4 step process
proved useful inter-urban studies
Cons:
large number of parameters to calibrate
requires lots of input data
few successful urban applications
What is a discrete choice model?
Aggregate modesl based on observations of group behaviour
Probability of an individual choosing a given option = f
Attractiveness = utility
utility usually defined as a linear combination of key variables
What are the four different model used for modal spilt?
Discrete Choice model Direct Demand model Pivot Model Logit Model S-shaped mode split curves factor based models
What is utility?
the perceived ability of something to needs or wants
represents satisfaction experience by the consumer of a good
What is the utility maximisation principle?
People choose the alternative that has the highest perceived utility for them
Since the perceived utilities are not exactly known, the choice models incorporate an error term that accounts for the stochasticity of choices
What does the assumptions of logit model?
Assume that error terms are independent, identically and Gumbel distributed
What is maximum likelihood estimation (MLE)
- We use MLE to estimate the coefficients of a utility function
- MLE searches among all possible values of utility coefficients and finds the ones that best describe the choice observations
How is maximum likelihood calculated?
The produce of the likelihood of individuals observations in the smaple
What is Newton - Raphson method?
Used to find the best model coefficients
- use row^2 to measure the goodness of fit
p^2 = 1 - LL(ß*)/LL(O)
When p^2 = 1 happens in the ideal situation when LL(ß8)= 0
p^2 = 0 model does not tell us anything
ß = maximum likelihood
ln O = likelihood if not preference is given
