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Computer models of cities: an introduction
Computer models of cities: an introduction David Foot Depa~m~nt
of Geography,
University of Reading, UK
An urban model is a mathematical equation or series of equations representing the distribution of land use activities in an urban area (particularly population and employment), together with the movement about the region, for example the trip patterns from home to work or to school, shopping trips and commercial trips. A retail shopping model woutd deal with the shopping patterns and the distribution of shopping centres and the resident population; a residential location model would consider the journey-towork trip pattern and the distribution of population and employment; while a transportation mode1 would look at the trip pattern for all the different type of trips. The region would be divided into small zones for the collection of data. Mathematical equations would then represent the distribution of population, employment, housing, shopping facilities, offices, commercial premises and other activities, together with the population movements over the region for different trip purposes. Such mathematical equations vary in complexity, but even very simple models are necessarily solved on a computer because of the large numbers of zones and the repetitive calculations involved. A computer model of a city or a region is generally referred to as an ‘urban 0264-2751/84/050469-05$03.~
model’ and there is now a wide range of these models. Very simple models deal with aggregate activities, and describe total employment, total population and total retail trade for each zone. These models can be expanded and the activities disaggregated so that different sectors of population and employment and different types of retail goods are considered. More complex models can be developed that link several smaller models together to giye a model containing a large number of mathematical equations. The time taken to develop and operate the models varies considerably. Simple
‘simple models can be run on a microcomputer in a matter of minutes’ models can be run on a microcomputer in a matter of minutes with data assembled over a few day?, whereas the more complex models take months or even years for their data to be assembled and the model developed, and can take up considerable computer runtime to operate. The type of model to be developed will vary with the,nature of the particular planning proposal under investigation. A local authority considering a hypermarket application would probably use a
o 1984 Eu~e~~~h & Co (Publishers) Ltd
‘D. Foot, Operational urf_?aff#&e!s: An fnfroduction, Methuen, London, 1981.
‘Lewisham Borough Council, S~p~~~g in tewisham: Shopping Model, Planning Department, London 3orough of Lewisham, 1978.
3J. Moseiey, .$hopping in Berkshire: The Shoaping Mode!_ The Unit kr Retail Planning lnformation, Reading, 1977.
470
simple retail shopping model, whereas a Larger development would require a more complex model. The procedure for running an urban modet is first to divide the study region into zones and to collect data for some recent date, very often a census year. The model is the calibrated, which means that certain parts of the mathematical equations are gradually adjusted until the output from the modei is similar to the known base year distribution of the activities and trip pattern. This adjustment is generally in the form of the functions in the mathematical equations relating to distance, because of differences in the distances people travel to work. to school or to do the shopping, and atso because of var~~~tions in car ownership and income levels over a country or region. Once the model can be shown to reproduce some past point in time, then it can be used predictively to study the likely outcome of some future changes in the region; for exampfe, a new hypermarket. a new airport, a large new factory or the closure of a large industrial complex. Any number of future alterna-
‘retail shopping models are probably the most widety applied models’ tive planning policies can be considered and the outputs compared in order to investigate the effect new developments might have on the region. The results from the models must be interpreted in an intelligent way. It must always be remembered that the models are simplified rn~~~herna~~cai representations of a part of the urban system. The equations reflect the working of the real world and include what appear to be the most important variables and activities based on urban theory and empirical evidence. Obviously there are other factors that have to be taken into account in reaching final decisions, but the models can give a general indication of the likely consequences of certain policies. They do
provide a great deal of quantitative information and also oblige the urban analyst to ask many questions about the policy under investigation. A large number of operational urban models have now been developed in many different countries.’ and three cxamptes wifl now be presumed in order to demonstrate how they have been applied. fif Rrrrrtl ~~~~~p~~~~~ mor2el. This model is probably the most widely applied urban model and in particular it has been used tn study the impact of a new hypermarket on the surrounding region. The retail shopping model postulates first that peoplc are attracted to a shopping centrc by the quantity and quality of the shops in the centrc. and second that there is a distance deterrent to travefling to a shopping centre. with a nearby shopping centre being far more attractive than a centre much further away. The London Borough of Lewisham use a model of this type to assess the likely proposal to redevelop Millwall football ground which would include a large hypermarket.’ Their model considers consumer and durable goods separately because of their different trip patterns. with people travelling shorter distances to purchase consumer goods and longer distances to purchase durable goods. The model can provide information on the tikefy retail sales to be attracted to the new centre, the extent to which nearby ccntres will be affected through losing part of their future retail sales, the trips to the new centre and hence an estimate of the expected traffic flows. More complex retail models have been developed. for example by Berkshire County Council” which divides the region into a large number of zones and deals not only with the shopping pattern for durahie and consumer goods, but also with different modes of travel, considering car, public transport, cycting and walking. This model requires far more data. uses much more computer time and is therefore run far less frequently than the Lewisham model. There are now
CITIES
August
1984
quite a large number of examples where retail models have been applied, both for impact studies and in local and
there
also been some attempts to test their effectiveness. One particular study in South Yorkshire4 developed a retail shopping model to estimate the likely impact of a new hypermarket in the area, and three years after the shopping centre was opened a survey was carried out to determine the actual effect of the new hypermarket on the area, providing information on the level of retail trade have
‘there will probably not be a single best policy, but the model can indicate preferences’ attracted to the new hypermarket, what part of that trade was captured from other shopping centres and the effect on the nearby centres. The two sets of results were very close, which seems to indicate that the retail model can be used effectively to assess the impact of a new hypermarket, although it can only deal with fairly aggregate shopping units and cannot consider individual shops. (ZI) Re.~~de~~~al ~o~ario~model. This model can be developed using a similar mathematical formula where employment is distributed from work zones to residential zones to provide the journey from work to home pattern and the population distribution over the region. Again this simple model can be extended to form a highly disaggregated model that includes population and employment forecasting and many aspects of the housing market. As an example, consider a model developed to investigate the urbanization effects of an expansion of Stansted airport.” The airport is 32 miles northeast of London and several times over the last 30 years has been considered a possible site as the third airport for CITIES August 1984
London. Despite several public inquiries Stansted has remained a small airport which at present handles about 2 million passengers per year, mainly charter flights. The present proposals are to accommodate 15 million passengers per year, with possible expansion to 50 million. This would lead to a large increase in population and employment in a region that is not designated a major growth area and where there are constrain& to development because of highquality agricultural land and the green belt policy around London. Several alternative schemes have been put forward to accommodate this new population and employment and a residential location model can test and compare their feasibility. For example, one proposal is to develop a new town west of the airport and expand the other centres along the Stort Valley to the south. Using the model to test the strategy indicates that a new town to the west of the airport is a good proposal, but that even with severe constraints on development around the airport it is still very difficult to attract the population to the expanded centres furthest away from the airport. People want to disperse themselves in all directions away from the airport, rather than in this very restricted area to the west and south-west. The feasibiIity of a number of other strategies can also be considered, such as totally unconstrained growth, the preservation of the green belt and highquality agricultural land, new town de-
%.T. Gilligan,P.M.
Rainf5rdand A~R.
Thorne, ‘The impact of out-of-town shopping’,
EuropeanJourna/of ~afkefi~9,“*I& ‘g74, pp
42-56.
‘over the last 30 years the population of Venice has more than halved’ 5M. Breheny,D. Foot, et al, ‘The proposedex-
expansion of existing towns and development to the east and west of the airport. These results in turn suggest further schemes that can also be tested. There will probably not be a single best planning policy for accommodating the new development. but the model can velopments,
pansionof StanstedAirport: the urbanization impact’,in K. Button and D. Gillingwater, eds, Transport, Locafion and Spafial Policy,
Gower, Aldershot, Hants,UK, 1983, pp
204-227.
471
indicate a preference for development in certain areas and show how other arcas are not really appropriate.
“U. Pirrseniin, P. Costa ancl D. Foot, ‘The Venice problem: an approach by u&en modellhg’, f?egior~~~ Studies, Vol 12, 1976, pp 579-602.
{/Ii) ~~~~~~ ~otq&t~ modeis. As an example of how a more complex model can be applied, in the 1970s there was an investigation into the probfems of Venice.” The city has considerable environmental problems with water pollution, air poltutirtn
arrd winter
fkxx%ng
leading
to ;f
deterioration of historic buildings. Also, over the ias~ 30 years the poputation has more than hatved to a present total of under 100 Nkl. Population movement was initrafly to the tida and then onto the mainhmd. gradually moving further inland. This is partly the result of very high house prices in Venice and much cheaper and often superior housing on the mainland, These changes have left Venice with an aging popuIat~~~~_ but one with a high social class. The city has, however, retained a high level of employment, ~rticu~ar~~ tertiary, el~p~oyrnent~ and on the mainland at the end of the Venice causeway there is a very large industrial compfex at Marghera and Mestre. The distribution of population and emptoyment leads to a large inflow of people from the mainland to work in these lu-eas each day. tn order to study these socioeconomic relationships within an urban model of Venice, the study region must contain the areas that come within the sphere of infhmnce of the city, and this reaches at least as far as Padova and Treviso. The urban mwikl applied to ‘Venice combines a resrdential Iocation modei and a service location model which is similar to a retail shopping modef but deals with all services. not just retail trade. This modet was used to evaluate a series of alternative ~ndu~tr~a1, residential and transport policies for the region in order to study their effect on Venice. For Venice to survive and prosper it has to be regarded as a living city and maintain its population and employmem. Thus. the model deals with p~~pu~at~~~n~~mp~~~ment and transport movements over the region and
these in turn provide information that can be ~n~e~pr~ted in terms of the enviranmental effects on the city. Three out of a series of @arming policies that were tested are examined here. First, the hkely effects of ;I general increase rn transport costs due to a large increase in oif prices were tested. Such an increase in the price of oil would decreirse accessibility over the region and result in an increased demand for housing near to employment centres, particularly Venice and Marghera. There would be an increase in the refative cost of transport to housing which would deter people from travelhng !ong distances to rosidcntiaf arcas inland. This increased pressure for housing in Venice would lead to a further increase in the cost of housing and further emphasize the process of sociaf selection of its inhabitants. Second= im ~mp~#~/~?ent p&c!, that was tested by the urban model was a decentralization of part of the petrochemical complex from ~~~r~h~ra to a position further south on the lagoon at
policy tu increase Venice’s residential capacity woutd have to be dosely connected with control of house prices’ “any
Codcvig~.Not surprjsi~~~~,
if employment is moved then demand for residential accommodation would shift further south, particularty to areas inland from the new industrial plant. The effect on Mestre and Marghera would be a dccrease in dcmilnd for residential accommodation unless other forms of employment can be attracted to these areas. The effect on Venice in terms of population and employment changes would be very marginal, but there would be a definite improven~e~~ in its ~nv~ronm~nta~ problems and in particular air and water pollution. However, because of the s~~c~oec~~nom~c effects on Mestre arId Marghera it seems highly unlikclv that
this policy would ever be considered by the authorities. Third, a residential modei tested the effect of an increase in the residential capacity of Venice which would be achieved by the restoration of old underutilized houses. The results indicated that there would not be the demand from the local population to take up this extra housing in the city. The model showed that any policy to increase the residential capacity of Venice in the hope of attracting local population would have to be closely connected to control of house prices in the city, otherwise this housing would be taken up by people from outside the region capable of paying the high prices. ~~r~~e~ readirrg. These applications of urban models indicate how they can be used to provide information on the likely outcome of any number of alternative planning policies and their effect on the study area. Urban modets have now been quite widely applied throughout the world and numerous examples are contained in a recent book by the author on operational urban models.’ Another good description of how urban modeis can be applied is provided by Batty,” while a more advanced explanation is contained in Wilson.” There are a number of other books which cover a wider field; for example, statistical and
CITIES August 1984
quantitative techniques,” regional science techniques,’ 1 and genera1 planning techniques.” A number of more complex models than these described here have been developed (particularly land use transportation models) with varying degrees of success. Some. such as those developed
‘some US models in the 1960s were too complex, too costly and took too long’ by Echenique,‘” have been successfully applied in many different countries, but there have been others, particularly some developed in the USA in the late 196Os, which were just too complex, too costly and took too long to develop, and were therefore abandoned. It is therefore essentiaf to recognize the limitation of urban models. to use them intelligently and to develop the correct type of model for a particular probfem. It also has EObe recognized that some form of urban analysis is going to be undertaken and if urban models are not used, then the alternative analysis might be poorer. Now that computers are so readily avaiIable, urban models can easify be developed and can provide some good, useful information to assist the decision maker in reaching the most appropriate decision.
‘Foot, op tit, Fief1. ‘M. Batty. ‘Urban models in the ~~annjngprocess’, in D.T. Herbert and R.J. Johnston, eds, Geosraphy and Ihe Urban ~nviron~nt~ Progress in Research and Applications, Vol 1, Wiley, Chichester, ff K, 1980. ‘A.G. Wilson, Urban and Regiona/ Models in Geography and Pfanning, Wiiey, Chichester, UK, 1974. “‘R.S. Baxter, Computer and S$a%ficat Te&niques far Planners, Methuen, London, 1976. “N. ~ppenhe~m, Applied Models in Urban and Regional Analysis, Prentice-Halt, Engtewood Cliffs, NJ, f 960. “W. Helly, Urban Systerns Models, Academic Press, New York, 1975. ‘3M.H. Echeniqueand I.N. Williams, ‘Developing theoretically-based urban models for practical studies’, in The Application of Cornput& in Architecture, Building Des&t3 and Urban Pfanning, AMH, Berlin, 1979, pp 365376.
473
of Geography,
University of Reading, UK
An urban model is a mathematical equation or series of equations representing the distribution of land use activities in an urban area (particularly population and employment), together with the movement about the region, for example the trip patterns from home to work or to school, shopping trips and commercial trips. A retail shopping model woutd deal with the shopping patterns and the distribution of shopping centres and the resident population; a residential location model would consider the journey-towork trip pattern and the distribution of population and employment; while a transportation mode1 would look at the trip pattern for all the different type of trips. The region would be divided into small zones for the collection of data. Mathematical equations would then represent the distribution of population, employment, housing, shopping facilities, offices, commercial premises and other activities, together with the population movements over the region for different trip purposes. Such mathematical equations vary in complexity, but even very simple models are necessarily solved on a computer because of the large numbers of zones and the repetitive calculations involved. A computer model of a city or a region is generally referred to as an ‘urban 0264-2751/84/050469-05$03.~
model’ and there is now a wide range of these models. Very simple models deal with aggregate activities, and describe total employment, total population and total retail trade for each zone. These models can be expanded and the activities disaggregated so that different sectors of population and employment and different types of retail goods are considered. More complex models can be developed that link several smaller models together to giye a model containing a large number of mathematical equations. The time taken to develop and operate the models varies considerably. Simple
‘simple models can be run on a microcomputer in a matter of minutes’ models can be run on a microcomputer in a matter of minutes with data assembled over a few day?, whereas the more complex models take months or even years for their data to be assembled and the model developed, and can take up considerable computer runtime to operate. The type of model to be developed will vary with the,nature of the particular planning proposal under investigation. A local authority considering a hypermarket application would probably use a
o 1984 Eu~e~~~h & Co (Publishers) Ltd
‘D. Foot, Operational urf_?aff#&e!s: An fnfroduction, Methuen, London, 1981.
‘Lewisham Borough Council, S~p~~~g in tewisham: Shopping Model, Planning Department, London 3orough of Lewisham, 1978.
3J. Moseiey, .$hopping in Berkshire: The Shoaping Mode!_ The Unit kr Retail Planning lnformation, Reading, 1977.
470
simple retail shopping model, whereas a Larger development would require a more complex model. The procedure for running an urban modet is first to divide the study region into zones and to collect data for some recent date, very often a census year. The model is the calibrated, which means that certain parts of the mathematical equations are gradually adjusted until the output from the modei is similar to the known base year distribution of the activities and trip pattern. This adjustment is generally in the form of the functions in the mathematical equations relating to distance, because of differences in the distances people travel to work. to school or to do the shopping, and atso because of var~~~tions in car ownership and income levels over a country or region. Once the model can be shown to reproduce some past point in time, then it can be used predictively to study the likely outcome of some future changes in the region; for exampfe, a new hypermarket. a new airport, a large new factory or the closure of a large industrial complex. Any number of future alterna-
‘retail shopping models are probably the most widety applied models’ tive planning policies can be considered and the outputs compared in order to investigate the effect new developments might have on the region. The results from the models must be interpreted in an intelligent way. It must always be remembered that the models are simplified rn~~~herna~~cai representations of a part of the urban system. The equations reflect the working of the real world and include what appear to be the most important variables and activities based on urban theory and empirical evidence. Obviously there are other factors that have to be taken into account in reaching final decisions, but the models can give a general indication of the likely consequences of certain policies. They do
provide a great deal of quantitative information and also oblige the urban analyst to ask many questions about the policy under investigation. A large number of operational urban models have now been developed in many different countries.’ and three cxamptes wifl now be presumed in order to demonstrate how they have been applied. fif Rrrrrtl ~~~~~p~~~~~ mor2el. This model is probably the most widely applied urban model and in particular it has been used tn study the impact of a new hypermarket on the surrounding region. The retail shopping model postulates first that peoplc are attracted to a shopping centrc by the quantity and quality of the shops in the centrc. and second that there is a distance deterrent to travefling to a shopping centre. with a nearby shopping centre being far more attractive than a centre much further away. The London Borough of Lewisham use a model of this type to assess the likely proposal to redevelop Millwall football ground which would include a large hypermarket.’ Their model considers consumer and durable goods separately because of their different trip patterns. with people travelling shorter distances to purchase consumer goods and longer distances to purchase durable goods. The model can provide information on the tikefy retail sales to be attracted to the new centre, the extent to which nearby ccntres will be affected through losing part of their future retail sales, the trips to the new centre and hence an estimate of the expected traffic flows. More complex retail models have been developed. for example by Berkshire County Council” which divides the region into a large number of zones and deals not only with the shopping pattern for durahie and consumer goods, but also with different modes of travel, considering car, public transport, cycting and walking. This model requires far more data. uses much more computer time and is therefore run far less frequently than the Lewisham model. There are now
CITIES
August
1984
quite a large number of examples where retail models have been applied, both for impact studies and in local and
there
also been some attempts to test their effectiveness. One particular study in South Yorkshire4 developed a retail shopping model to estimate the likely impact of a new hypermarket in the area, and three years after the shopping centre was opened a survey was carried out to determine the actual effect of the new hypermarket on the area, providing information on the level of retail trade have
‘there will probably not be a single best policy, but the model can indicate preferences’ attracted to the new hypermarket, what part of that trade was captured from other shopping centres and the effect on the nearby centres. The two sets of results were very close, which seems to indicate that the retail model can be used effectively to assess the impact of a new hypermarket, although it can only deal with fairly aggregate shopping units and cannot consider individual shops. (ZI) Re.~~de~~~al ~o~ario~model. This model can be developed using a similar mathematical formula where employment is distributed from work zones to residential zones to provide the journey from work to home pattern and the population distribution over the region. Again this simple model can be extended to form a highly disaggregated model that includes population and employment forecasting and many aspects of the housing market. As an example, consider a model developed to investigate the urbanization effects of an expansion of Stansted airport.” The airport is 32 miles northeast of London and several times over the last 30 years has been considered a possible site as the third airport for CITIES August 1984
London. Despite several public inquiries Stansted has remained a small airport which at present handles about 2 million passengers per year, mainly charter flights. The present proposals are to accommodate 15 million passengers per year, with possible expansion to 50 million. This would lead to a large increase in population and employment in a region that is not designated a major growth area and where there are constrain& to development because of highquality agricultural land and the green belt policy around London. Several alternative schemes have been put forward to accommodate this new population and employment and a residential location model can test and compare their feasibility. For example, one proposal is to develop a new town west of the airport and expand the other centres along the Stort Valley to the south. Using the model to test the strategy indicates that a new town to the west of the airport is a good proposal, but that even with severe constraints on development around the airport it is still very difficult to attract the population to the expanded centres furthest away from the airport. People want to disperse themselves in all directions away from the airport, rather than in this very restricted area to the west and south-west. The feasibiIity of a number of other strategies can also be considered, such as totally unconstrained growth, the preservation of the green belt and highquality agricultural land, new town de-
%.T. Gilligan,P.M.
Rainf5rdand A~R.
Thorne, ‘The impact of out-of-town shopping’,
EuropeanJourna/of ~afkefi~9,“*I& ‘g74, pp
42-56.
‘over the last 30 years the population of Venice has more than halved’ 5M. Breheny,D. Foot, et al, ‘The proposedex-
expansion of existing towns and development to the east and west of the airport. These results in turn suggest further schemes that can also be tested. There will probably not be a single best planning policy for accommodating the new development. but the model can velopments,
pansionof StanstedAirport: the urbanization impact’,in K. Button and D. Gillingwater, eds, Transport, Locafion and Spafial Policy,
Gower, Aldershot, Hants,UK, 1983, pp
204-227.
471
indicate a preference for development in certain areas and show how other arcas are not really appropriate.
“U. Pirrseniin, P. Costa ancl D. Foot, ‘The Venice problem: an approach by u&en modellhg’, f?egior~~~ Studies, Vol 12, 1976, pp 579-602.
{/Ii) ~~~~~~ ~otq&t~ modeis. As an example of how a more complex model can be applied, in the 1970s there was an investigation into the probfems of Venice.” The city has considerable environmental problems with water pollution, air poltutirtn
arrd winter
fkxx%ng
leading
to ;f
deterioration of historic buildings. Also, over the ias~ 30 years the poputation has more than hatved to a present total of under 100 Nkl. Population movement was initrafly to the tida and then onto the mainhmd. gradually moving further inland. This is partly the result of very high house prices in Venice and much cheaper and often superior housing on the mainland, These changes have left Venice with an aging popuIat~~~~_ but one with a high social class. The city has, however, retained a high level of employment, ~rticu~ar~~ tertiary, el~p~oyrnent~ and on the mainland at the end of the Venice causeway there is a very large industrial compfex at Marghera and Mestre. The distribution of population and emptoyment leads to a large inflow of people from the mainland to work in these lu-eas each day. tn order to study these socioeconomic relationships within an urban model of Venice, the study region must contain the areas that come within the sphere of infhmnce of the city, and this reaches at least as far as Padova and Treviso. The urban mwikl applied to ‘Venice combines a resrdential Iocation modei and a service location model which is similar to a retail shopping modef but deals with all services. not just retail trade. This modet was used to evaluate a series of alternative ~ndu~tr~a1, residential and transport policies for the region in order to study their effect on Venice. For Venice to survive and prosper it has to be regarded as a living city and maintain its population and employmem. Thus. the model deals with p~~pu~at~~~n~~mp~~~ment and transport movements over the region and
these in turn provide information that can be ~n~e~pr~ted in terms of the enviranmental effects on the city. Three out of a series of @arming policies that were tested are examined here. First, the hkely effects of ;I general increase rn transport costs due to a large increase in oif prices were tested. Such an increase in the price of oil would decreirse accessibility over the region and result in an increased demand for housing near to employment centres, particularly Venice and Marghera. There would be an increase in the refative cost of transport to housing which would deter people from travelhng !ong distances to rosidcntiaf arcas inland. This increased pressure for housing in Venice would lead to a further increase in the cost of housing and further emphasize the process of sociaf selection of its inhabitants. Second= im ~mp~#~/~?ent p&c!, that was tested by the urban model was a decentralization of part of the petrochemical complex from ~~~r~h~ra to a position further south on the lagoon at
policy tu increase Venice’s residential capacity woutd have to be dosely connected with control of house prices’ “any
Codcvig~.Not surprjsi~~~~,
if employment is moved then demand for residential accommodation would shift further south, particularty to areas inland from the new industrial plant. The effect on Mestre and Marghera would be a dccrease in dcmilnd for residential accommodation unless other forms of employment can be attracted to these areas. The effect on Venice in terms of population and employment changes would be very marginal, but there would be a definite improven~e~~ in its ~nv~ronm~nta~ problems and in particular air and water pollution. However, because of the s~~c~oec~~nom~c effects on Mestre arId Marghera it seems highly unlikclv that
this policy would ever be considered by the authorities. Third, a residential modei tested the effect of an increase in the residential capacity of Venice which would be achieved by the restoration of old underutilized houses. The results indicated that there would not be the demand from the local population to take up this extra housing in the city. The model showed that any policy to increase the residential capacity of Venice in the hope of attracting local population would have to be closely connected to control of house prices in the city, otherwise this housing would be taken up by people from outside the region capable of paying the high prices. ~~r~~e~ readirrg. These applications of urban models indicate how they can be used to provide information on the likely outcome of any number of alternative planning policies and their effect on the study area. Urban modets have now been quite widely applied throughout the world and numerous examples are contained in a recent book by the author on operational urban models.’ Another good description of how urban modeis can be applied is provided by Batty,” while a more advanced explanation is contained in Wilson.” There are a number of other books which cover a wider field; for example, statistical and
CITIES August 1984
quantitative techniques,” regional science techniques,’ 1 and genera1 planning techniques.” A number of more complex models than these described here have been developed (particularly land use transportation models) with varying degrees of success. Some. such as those developed
‘some US models in the 1960s were too complex, too costly and took too long’ by Echenique,‘” have been successfully applied in many different countries, but there have been others, particularly some developed in the USA in the late 196Os, which were just too complex, too costly and took too long to develop, and were therefore abandoned. It is therefore essentiaf to recognize the limitation of urban models. to use them intelligently and to develop the correct type of model for a particular probfem. It also has EObe recognized that some form of urban analysis is going to be undertaken and if urban models are not used, then the alternative analysis might be poorer. Now that computers are so readily avaiIable, urban models can easify be developed and can provide some good, useful information to assist the decision maker in reaching the most appropriate decision.
‘Foot, op tit, Fief1. ‘M. Batty. ‘Urban models in the ~~annjngprocess’, in D.T. Herbert and R.J. Johnston, eds, Geosraphy and Ihe Urban ~nviron~nt~ Progress in Research and Applications, Vol 1, Wiley, Chichester, ff K, 1980. ‘A.G. Wilson, Urban and Regiona/ Models in Geography and Pfanning, Wiiey, Chichester, UK, 1974. “‘R.S. Baxter, Computer and S$a%ficat Te&niques far Planners, Methuen, London, 1976. “N. ~ppenhe~m, Applied Models in Urban and Regional Analysis, Prentice-Halt, Engtewood Cliffs, NJ, f 960. “W. Helly, Urban Systerns Models, Academic Press, New York, 1975. ‘3M.H. Echeniqueand I.N. Williams, ‘Developing theoretically-based urban models for practical studies’, in The Application of Cornput& in Architecture, Building Des&t3 and Urban Pfanning, AMH, Berlin, 1979, pp 365376.
473