Transport Study Methodology

In a review of Land use - Transport studies in the U.K. Solesbury and Townsend (1970) identified two main influences in their development. The first was a growing political awareness of urban transport problems during the early to mid 1960's, which led the government to encourage the establishment of land use/transport studies[1] . The other was the development of early U.S. transport studies which produced packages of techniques for transport analysis. The first studies, which were started in the mid '60s, were for the large conurbations. During the late '60s the sub-regional studies produced, as one of their outputs, a transport plan for the area under study.

The general objective of transport studies has been to ensure that the transport system will cope adequately with future travel demand. To this end an analysis is made of the present transport system and travel pattern. From this analysis a model of the transport system is built up. The inputs to the model are those factors which are considered to determine the travel pattern which exists at present. These Inputs are generally a description of the land use pattern, the socio-economic characteristics of the population and the transport system. The output from the model is a description of the resultant travel pattern. The model is initially built to represent present conditions. It is assumed that the relationships in the model will also hold in the future, so that given the changed inputs, the future travel pattern can be predicted.

Having built the model it is generally used to produce a description of the transport pattern for some date 20 to 30 years in the future, for the predicted change in land use and socio-economic conditions but for no change in the transport system. This will help in the analysis of where improvements could be made in the transport system for the target date. From this, a series of possible improvements are drawn up. The effect of these improvements is then predicted by re-running the model having first amended the transport network description to take account of the improvements. Predictions are produced for each set of improvements. A choice is then made between the possible sets of improvements,aided by cost benefit and operational analyses of each plan. Having decided on the improvements which are to be made, detailed designs of the new facilities must be produced and a decision made as to when, during the period between the present, and the design date, that they are to be built.

Of this process one of the most important elements is the transport model.

Transport Model

A transport model has as its input, a description of the transport systems, land use, and the socio-economic characteristics of the population, in the area under study. The model can be used to predict the resultant pattern of movement of people and goods.

The movement of people, which is the element upon which there has been most emphasis in transport studies, is assumed to be dependent upon people's needs and desires for such things as getting to work, to shops and services, to recreation and to suchlike, the disposition of housing and the places where these activities occur, and the availability of different transport modes, such as private car, bus or train. The relationships are expressed as equations which are generally calibrated from present day data which is obtained from a sample survey of households and traffic patterns. The household survey collects details on such things as family size, ages, income, oar ownership, and, as part of the traffic survey, details of the Journeys that every member of the family makes dally. The details of these journeys which are collected are, where a Journey was to and from, what its purpose was, and what mode was used. The traffic survey also collects details of the flow of people and vehicles in the area of study.

For the purposes of modelling travel patterns the area is divided into small zones. All movements are assumed to start or finish at the centre of a zone. The transport systems are represented by networks of links which meet at nodes. Each zone is connected to nearby nodes by 'notional' links.

Having produced and calibrated the transport model it operates by; forecasting where individual trips will go from and to, i.e. their 'origin' and 'destination', known as 'trip generation'; forecasting for the set of trips starting at each origin, how many will go to each of the various destinations, known as 'trip distribution'; forecasting for each interzonal set of trips the proportion which will use each of the available modes, known as 'modal split'; and forecasting for each mode of transport the routes by which each set of interzonal trips will be made, known as 'trip assignment'. There are generally sub-models for different types of trip purpose, such as Journey to work, to school, to shops, etc. These are kept separate until the trip assignment stage. Figure 1. gives a schematic representation of the stages involved.

Figure 1 Schematic Representation of a Transport Model

Socio-economic and Land use projections.
Description of Transport System (and Value(s)[2] of time[3] ).
Car ownership (by group).
Person trip ends, by purpose (and group).
Interzonal time, distance, cost or generalised cost.
Person trip ends by mode (purpose[4] and group).
Interzonal person trips by mode (purpose[4] and group).
Skim trees.
Flows on each link of each transport system.
Interzonal person trips.
Capacity restraint, iterative feed-back.

E and I can be combined to produce total distance, time cost or generalised cost by mode.

Trip Generation. Most trips are usually found to be home based, that is either starting from or ending at the person's home. The home is considered the origin, and the other end the destination. The number of trip origins form a zone is related to the socio-economic characteristics of the population living in the zone. The number of trip destinations in any zone is related to the intensity of land use, such as the number of jobs or floor area of shops.

Trip Distribution. The distribution of trips between zones is usually determined by a gravity model, though some other models have been used. The gravity model works on the basis that the number of trips between any two zones will be proportional to the number of trip origins from the one, and the number of trip destinations at the other, and inversely proportional to a function of distance between them. This function need not necessarily be of distance, it can be the interzonal travel time, interzonal travel cost or interzonal generalised cost[5] , which is used in the trip assignment process. Corrections have to be applied to ensure that the total number of trips to each zone from all others is equal to the number of destinations for that zone and vice versa for origins.

Modal Split. The splitting of trips between the various transport systems available is sometimes carried out before, sometimes after and sometimes at the same time as the trip distribution stage. It is generally carried out by assigning all non-car owners to public transport[6] , then splitting car owners trips between private and public transport. The split is made on the basis of some type of diversion curve which relates the proportions using each mode to the relative difference in accessibility by each mode, or of generalised cost, or some such function.

Assignment. The assignment of interzonal trips is usually made on the basis of an "all or nothing" assignment. That is all trips between two zones are assumed to go by the shortest route. (Other criteria can also be used such as quickest, cheapest, lowest generalised cost, etc.). Any other type of assignment becomes computationally very complex. The routes used between zones are found by a programme which generates a tree of shortest routes to all other zones known as a "skim tree". This programme also compiles a note of which interzonal movements are carried on each link of the transport system. It is then a simple matter to find the volume of trips made on each link by summing all the interzonal trips which use that link.

Once the route loadings have been found it is generally necessary to check that each link can carry the indicated volume of trips. If it can not, a "capacity restraint" has to be applied so as to reduce the volume of traffic. This will involve in the case of roads, a decrease in the speed on the link so increasing the time of travel. This may remove some of the interzonal trips by making other routes quicker. If it does this will also have an effect on the distribution and modal split.

To use the transport model to predict future travel patterns, estimates have to be made of all the inputs to the model - the future land use pattern, the socio-economic characteristics of the people living in each zone and the transport systems.

The evaluation of transport plans has generally been on the basis of a cost-benefit analysis and an operational evaluation. The basis of the cost-benefit analysis is outlined in Chapter 2.

The operational analysis is a detailed examination of the volumes of movement on each link of the transport systems. For example on roads - what congestion exists? are any roads so badly overloaded that stop-go conditions will exist on them for any significant time during the day? On public transport there is usually less necessity to apply capacity restraint procedures but there is a necessity to check that the assumed volumes of flow used to determine the frequency of service are approximated, to ensure that the operation is financially viable. It is not possible to reduce levels of service if volumes are too low without affecting the distribution, assignment, and especially the modal split.

[1] Solesburg and Townsend cite the issuing of Ministry of Transport & Ministry of Housing and Local Government (1965) which recommended comprehensive land used transport studies. The M.o.T. undertook to pay 50% of the cost of any studies made.

[2] can be used if separate groups (perhaps by income) are used in behavioural modelling.

[3] required if generalised cost is used.

[4] between Distribution and Assignment all purposes are combined to give the total interzonal person trips.

[5] Generalised cost is a composite function of the time and monetary costs of a Journey, which is used in an attempt to model behavioural patterns.

[6] This is assuming that only two modes, private and public transport, are being considered.