2 Transport Planning
2.1 Transport Planning
Transport planning is a methodical process of planning for future transport
needs. It is a combination of art (creativity required) and science (solution
of highly technical problems).
It needs to be coordinated with other aspects of community planning, particularly
land use planning.
2.2 The Transport Modelling Process
Detailed transport planning depends on the use of complex mathematical
models of the transport system. These models are simplified representations
of the real world. They are used to explore the consequences of particular
transport policies or strategies.
The ideal model is one which produces accurate forecasts at minimum cost.
A critical decision in creating a transport model is deciding which parameters
to include and which parameters to ignore. The inclusion of the most relevant
parameters will optimise the accuracy of the predictions obtained from the
model. However it is always a difficult task to know which are the key parameters
in a particular situation. The accuracy of a model may be improved by the
inclusion of more parameters but this is usually at a greater financial cost
in creating and running the model.
The transport modelling process consists of the following steps:
- Definition of study area and creation of internal and external zones
- Data collection
- Forecast of parameters (eg land use)
- Preparation of mathematical model
- Calibration of model
- Modelling
- Evaluation of results and production of a physical plan.
The conventional mathematical model used is a four-step model which consists
of submodels:
- trip generation,
- trip distribution,
- modal choice, and
- traffic assignment.
The mathematical model is constructed using a number of mathematical algorithms.
Once constructed the model is adjusted, or calibrated, so that it is able
to model current transport conditions. The model is then used to predict
future transport conditions, using future modelling parameters for land use,
population, car ownership, etc.
LINKS TO SITES ON TRANSPORT MODELLING.
A very comprehensive (13 printed pages), introductory
paper on transport modelling, titled '
A Transport Modelling Primer
' is available on the Web. This paper discusses the basis, procedures and
limitations of transport modelling.
2.3 Survey Area
The size of the area to be studied depends on the purpose of the study.
If the impact of a new supermarket is to be examined the study area will
be relatively small, whereas the study area for an investigation of regional
transpot issues will be quite large.
The study area is defined by an external cordon. The area within the study
area is then broken down into internal zones. External zones are created for
travel from the study area to outer destinations. External zones will usually
be much larger than internal zones.
Zones should be established so that they are compatible with the zones
used in other relevant data collection exercises for the area (for example
population data from census collection). Zones should be relatively uniform
in land use, fairly uniform and regular in shape, and large enough to use
for a statistically reliable sample.
2.4 Data Collection
Data may be cross-sectional (ie establishing characteristics across the
area at one point in time- the usual case) or time series (establishing the
change of characteristics with time).
Census data often provides cross-sectional data on characteristics such
as population distribution and age characteristics. Home interview surveys
will also frequently be used in transport studies to supplement census data
and to obtain specific data on travel characteristics. Home interview surveys
generally sample about 5 to 20 % of households and obtain data on general
household characteristics (eg number of motor vehicles in the household),
characteristics of individual household members (eg age, drivers licence holding
or not, etc) and/or particulars of individual trips.
2.5 Forecasts of Future Planning Parameters
The predictions of future trips in an area is based on linking together
present trips with present land use, population, income distribution, etc,
and then forecasting future trips using predictions of the future levels
of these same parameters. It is therefore necessary to come up with future
predictions of the key chosen parameters such as:
- population,
- income distribution, and
- the spatial location of land use.
2.6 Trip Generation
A trip is defined as a one-way person movement by
one or more modes of travel.
Each trip has an origin and a destination.
Transport modelling studies usually consider trips
in two broad categories:
- Home-based (any trip having either the origin
or the destination of the trip as the home), and
- Non-home based (any trip having neither the trip
origin nor the trip destination as the home).
Trip Generation is the sub-model of the transport
planning model which predicts the total number of trips generated and attracted
to each zone in the study area.
The two methods which are commonly used to build
trip generation sub-models are:
- Multiple linear regression, and
- Category analysis.
2.7 Trip Distribution
The trip distribution sub-model is the second sub-model
used in the transport planning model. It uses the data produced by the trip
generation sub-model to predict the number of trips which will occur between
one zone and another.
In mathematical terms, the trip generation sub-model
will give a prediction of the total trips generated from zone i (Pi), and
the total trips attracted to zone j (Aj). The trip distribution sub-model
then allows the prediction of the interzonal trips between zones i and j
(Tij).
The methods used for trip distribution fall into
two groups:
- Growth Factor Methods, which include:
- The Constant Factor Method,
- The Average Factor Method,
- The Fratar Method, and
- The Furness Method.
- Synthetic Methods, of which the most widely used
is the Gravity Method.
2.8 Modal Choice
Trips may be made by differing methods or modes of
travel e.g. car, walking, bus, train, etc. The determination of the choice
of travel mode is known as mode choice or modal split.
The factors affecting a person's choice of mode
are numerous. The variables which are usually incorporated into a mode choice
model are:
- tripmaker characteristics - represented by factors
such as car ownership, income, and net residential density.
- trip characteristics - usually considered by
establishing differing modal split relationships for different trip types
e.g. home-work trips, home-school trips, etc.
- system characteristics - generally evaluated
by considering the cost of travel including time cost.
Various mathematical models are used for mode choice
modelling, with the most commonly used being disutility curves, and probit
and logit modelling techniques.
2.9 Traffic Assignment
At this point in the transport modelling process
the number of trips and their origins and destinations are known, but the
actual route through the transport network is not known. The process of allocating
trips to particular routes is known as traffic assignment.
Traffic assignment is usually confined to road traffic
as most other modes (except perhaps walking and cycling) are limited to a
particular route.
The basis of assignment is usually travel time,
and as future trips are assigned to the network travel times can be expected
to vary.
In the traffic assignment process it is not unusual
to find that the proposed road network becomes overloaded and that some
car trips may need to be restrained.
There are four common methods of traffic assignment:
- all-or-nothing assignment;
- assignment by the use of diversion curves;
- capacity restrained assignment; and
- multipath proportional assignment.
2.10 Evaluation
Once results from a transport model have been produced
they need to be evaluated. Four different forms of evaluation should take
place:
- Numerical evaluation. This is to confirm the
computational validity of the results. Two main sources of error which occur
are programming errors (i.e. errors in the computational algorithms used)
and input information errors.
- Operational evaluation. This is to confirm that
the predictions made, and solutions adapted, are physically achievable in
the real world. Types of problems which occur include the fact that modelling
is usually done with 24 hour flows but the network needs to be able to cope
with peak period flows, and traffic assignment may tend to leave some links
very heavily loaded while other alternate links have light flows predicted.
- Environmental evaluation. Any proposed transport
change must be perceived by the community as being acceptable, or at least
more acceptable than any alternative.
- Economic evaluation.The final stage of the evaluation
process is commonly known as cost-benefit analysis or economic evaluation.
The basic approach to economic evaluation is to compare the expected benefits
with the cost of making and operating any change.
2.11 Trends in Transport Planning
The theory and practice of transport planning have
developed rapidly since the early 1950's.
In the 1950's and 1960's most transport plans used
large household interview surveys to gather travel data, used the conventional
four-step transport planning model (trip generation, trip distribution, modal
split and traffic assignment), and produced a transport plan for some distant
planning horizon (usually 20 years). The results of the models were criticised
on many grounds, including that the process was cumbersome and expensive,
the process produced only one plan for some distant year, the modelling procedures
were unresponsive to policy needs or changes, and the process tended to rely
too much on historical data rather than being based on a true understanding
of travel behaviour.
In the late 1960's researchers started to model
transport demand based on behavioural factors. This required consideration
of individual travellers, or a "disaggregate" approach.
In the 1970's research into travel behaviour increased
substantially. In the late 1970's papers on 'activity analysis' appeared.
Activity analysis sees travel as but one type of human activity.
In the 1980's research tended to focus on dynamic
analysis of travel behaviour which focuses on looking at changes in travel
behaviour (which requires longitudinal time series data rather than the
more common cross-sectional data).
The rapid increase in the power of microcomputers
has meant that transport modelling has tended to shift away from main-frame
computers to microcomputers.
LINKS TO SITES ON TRANSPORT PLANNING.
The Queensland Department
of Transport have a section on Infrastructure Planning which is available at the
QT Planning
Infrastructure
. The site gives information on a number of transport planning projects around
Queensland.
2.12 Microcomputer Traffic Systems Design Models
A range of different transport planning software
packages are now available for microcomputers, which cover a range of different
levels of transport planning.
Commonly used packages in Australia include:
- Strategic Network Modelling packages - EMME/3, TRANPLAN, TRIPS, TRANSTEP and QRSII.
- Local Area Traffic Models - TRAFFICQ, NETSIM,
CONTRAM, SATURN and MULATM.
- Intersection Models - ARCADY, PICADY, SIDRA,
INSECT, SIMSET.
LINKS TO SITES ON MICROCOMPUTER TRAFFIC SYSTEMS
DESIGN MODELS.
As noted above a large number of different software
packages are available, and links in this section do not imply that these
systems are superior to any others.
The EMME/2 package is marketed by
INRO
, a Canadian firm, and a description of the product is available by accessing
their site and proceeding through 'Products' and then EMME/2. The software
comes in different sizes with the smallest size handling a maximum of 250
zones, and the largest size handling a maximum of 4000 zones.
The SIDRA (Signalised Intersection Design Research Aid) was developed under the guidance
of Dr Rahmi Akcelik when he was at the Australian Road Research Board in
Victoria. The product has been further developed and the Web site gives a good description of the product.
Page last modified 28 May 2009.