Introduction
Exposition 1
Origin of accessibility in the planning profession
Forms of Accessibility Measurement: 1950's to 1990's
Introduction
Land transport, as we begin the third millennium is increasingly expected to provide good outcomes in the communities economic productivity and social well being, while at the same time providing a better fit to environmental capacity.
In strongly urban communities, the land transport systems are paramount in shaping the effectiveness and liveability of the urban form itself. In less urban communities, the transport relationship to those in neighbouring regions, is also of great importance to the survivability of the communities continued existence.
On a corporate level, the suppliers of transport services and equipment are increasingly expected to improve the system�s efficiency. With the increasing maturity of existing transport technology and the development and commercialisation of newer technologies, many opportunities are becoming available for innovation in re engineering and integration of transport supply systems and their relationship to the spatial form of the communities they serve.
In 1998 while attending the 8th World Conference on Transport Research in Antwerpen, Belgium, the idea was seeded to give effort towards better representing the transport supply characteristics of hybrid guided transport technology systems, into the planning process. The thought here, was to increase the sensitivity of the traditional planning tools to better discern the benefits of technology systems to large scale transport tasks. Experience in planning studies carried out for State Rail in the 1980�s -1990�s and for Transrapid Australia in the late 1990�s demonstrated the difficulty in bringing these system benefits into account.
It was while in Europe in 1998 and in subsequent discussions on this subject with Professor John Black that the resurgent interest in the concept of accessibility came to my attention.
Exposition 1
Throughout planning thinking around the world is the increasing attention to sustainability in our communities, largely coming from the increased awareness of environmental and social issues, over the last 20 years. This focus on sustainability is opening up interest in accessibility concepts. The portrait of a region painted by accessibility measures, relate closely to the objectives expected when we talk of sustainability. Examples are minimising total vehicle kilometres and total trips and consequentially air, greenhouse and noise pollution.
To further our understanding of these directions, it is useful to look at the meaning of accessibility and how it has been applied in the second half of the twentieth century. The methodology employed in this paper is to begin with the essential meaning of the concept of accessibility and look back at the first use in society. This is followed by a review of the type of accessibility indicators, categorised in chronological order from the 1950�s to the 1990�s. The paper then steps above the subject, to attempt to chronologically look at the question�s practitioners have been seeking to answer, when applying the accessibility concept.
Dictionary definition of accessibility is the �quality of being accessible� (Patterson, 1982) or �quality of being accessible or admitting approach�(Oxford,1989) First usage of the word appears to be by Langhorne between 1766 and 1802. (Oxford,1989) Accessible, itself is defined as �easy to come near to� or �approachable� or �open to the influence of � (Blair, 1982), or � capable of being reached, easy of access, such as one can go to, come into the presence of � ( Oxford,1989). The earliest usage of the word in this context is 1642 in England (Oxford,1989).
It relates but differs from the term access which is defined as the �act or right of coming to� (Blair, 1982) or �the action of going , coming to , coming into the presence of or into contact with�. The first usage of access in this meaning, appears in English in 1528. (Oxford,1989). Access also has the relevant meaning of the �way or means of approach�. This form of usage dates from 1642 . (Oxford,1989). Approach similarly is defined as �the act of coming nearer, or of drawing near, in space� First usage of this form dates from 1555. ( Oxford, 1989 )Approach is also defined as �movements towards the establishment of personal relations with one� This usage dates from 1642 ( Oxford, 1989 )
The connection between these words can be clearly seen in The Bible. In Ephesions 2: 14-22 we are reminded that through the work of Jesus himself, we are given through God�s Grace the right of coming to God the Father through the Holy Spirit. That is, through Jesus we have been given the way or means of approach to God the Father. Through this, God is accessible for us. It is easy for us to come near to God when we accept the access he has provided for us. Furthermore, when we accept this access we are in relationship with God in union with Christ . This is the highest quality of accessibility and open to the influence of God in our lives. In addition we are fellow citizens with all other Christians and in relationship to them (Carson, 1994).
From this basic definition, this account illustrates that good accessibility is only possible if access is made available. However, the opportunity provided through being accessible may be taken up or not taken up, depending on the acceptance or otherwise by the recipient of the accessibility. Most importantly, the opportunity that is offered, by being accessible, can be viewed in terms of both personal and community relationships.
Origin of accessibility in the planning profession
Accessibility, as it has been used in planning, has been described as a �slippery concept�, being used by many, assumed to be understood, but application often pointing to a varied understanding of the concept in reality. This has prompted some practitioners to ask the question, accessibility for whom? and to what? as being key to providing some understanding behind the various outworkings of accessibility applications.
In a simple definition for accessibility Hansen (1959)and Martellato(1998) define it as the potential of opportunities for interaction. Geurs and van Eck ( 2001) track back the concept of potential to the social physics school, citing Carey in 1858 as the earliest reference, followed by an application by Stewart (1947,1948) to a study on population distribution. Moseley (1979) gives an insightful description which delineates the difference between accessibility and mobility. He notes that � mobility relates simply with people�s ability to move, it depends on for example a person�s physical attributes and disabilities, monetary resources, availability of transport and infrastructure, but not upon the opportunities which may or may not present themselves as a result of moving. Accessibility alone incorporates this latter feature.�
Accessibility has a number of dimensions. Hagerstrand (1974) identifies a social and a physical side. There must be the physical location and connections to the destination, but the social situation of individual people will impact on the demand for and limitations on people�s actual use of the physical accessibility available to a person. Moseley (1979) reflects on the physical ( or spatial dimension ), commenting that there are �the units of separation and the utility of the various destinations� to be taken into account.
Forms of Accessibility Measurement: 1950's to 1990's
The most influential original work on accessibility concept was done in 1959 by Hansen ( 1959, pp74 ) who defined �accessibility at point 1 to a particular type of activity at point 2 as directly proportional to the size of the activity at point 2 and inversely proportional to a function of the distance separating the two points. The total accessibility at point 1 to the activity is the summation of the accessibility to each of the points around point 1�.
Measures of accessibility became aspects of debate throughout the 1970�s. Authors such as Pirie (1978), Black (1977) and Moseley (1979) began to identify different types ranging from the simple distance and topological types to the popular gravity (composite) types ( based on Hansen�s work) that aggregated accessibility.
A fourth type known as cumulative opportunity (comparative) measures was identified, which indexed based on the number of opportunities that can be reached from an origin within a specified distance or time. An inverted type of the latter is the work by Sherman (1974) which indexed based on zones within given travel times and distance from a destination. A case study of Sapporo, Japan by Black, Kuranami and Rimmer (1981) utilised these two types of cumulative opportunity measures. Doust (1987) similarly utilised the inverted cumulative opportunity measure in defining the accessibility of off airport terminals for a second Sydney Airport.
A further development of the measures (Pirie, 1978) was to disaggregate by applying separately to different social situations ( eg. ages, occupation, income ) and trip purpose, travel times etc. This stratification was also complimented by including perception factors ( eg perceived travel time). The purpose of these refinements was to shift from a measure of place accessibility to person accessibility.
The introduction of the trip chain typology into accessibility was discussed by a number of authors from the early 1970�s. Southworth (1985) commented in his paper that the incorporation of trip chaining into the accessibility measure, recognises the spatially cooperative trips as well as the competitive trips giving new insights into mixing activities within urban areas.
Martinez (1994) developed a parallel conceptualisation of access and accessibility. The approach taken is from a economic perspective where he builds off the work by Neuburger(1971) and Williams (1976) who interpret Hansen�s accessibility measures in terms of consumer surplus. Martinez ( 1994) defines access as the economic benefits derived from the interaction between two activities, i.e. the net benefit from making the contact less the interaction (transport) cost. Two types of access are defined. The first, accessibility is defined as the benefit obtained from visiting other activities, as perceived by the traveler. The second is the attractiveness, defined as the benefit the visited activity perceives from the visits. The accessibility between the zones is termed relative access and the summation of all these is termed the integral access measure, which conceptually parallels the gravity definition of Hansen (1959). The concept of attractiveness parallels the inverse cumulative opportunity measure discussed earlier.. Martinez(1994) notes that trips are made if the benefits derived from making contact exceed the transport generalised cost. In contrast to the conclusions of. Hanson & Scwab (1986), he notes that there does appear to be a relationship between access and trip generation rates, based on the theoretical appreciation, citing the studies by Koening(1980) and Dalvi & Martin (1976) as providing some evidence for this.
In his study on accessibility indices for rural NSW, Nichols (1994) identified three key elements to accessibility. Namely, the supply and geographic distribution of desired activities; the impedance measured as distance or time or cost in reaching the desired activity and the ability of the individual to overcome variations of location and different levels of impedance (as a function of many economic and social variables eg. life cycle, wealth).
Evaluation of social implications, by using accessibility measures indicating the opportunities to reach destinations is according to Cervero, Rood & Appleyard (1999) gaining increasing attention as a compliment to more traditional mobility based measures of transport performance. It is beginning to be used as a tool to measure equity issues of past infrastructure investments and tracking such issues as spatial mismatch from jobs. In a study of employment & housing opportunities in the San Francisco Bay Area, Cervero, Rood & Appleyard (1999) identify two types of indicator, namely �gravity� type and isochronic ( cumulative opportunities) type. The choice of model in their study is a gravity type, favoured because it considers all trip - end possibilities, avoiding the introduction of arbitrary spatial boundaries. This model was then stratified by occupational class.
During the 1999 Short Course on new Directions in Urban Transport Planning Practice, Black (1999), notes the role of accessibility in integrated landuse/transport modeling, citing the Lowry type model. Black(1999) notes that this is an iterative example of allocation of landuse activities. Allocation is based on optimising residential activities, accessibility to basic employment and service employment to the market ( the residents).
An, optimisation model reviewed by ISGLUTI (1990) known as Saloc does not use a trip matrix but instead an accessibility measure of ease of travel from each zone to a composite set of destinations. Osaka is an example of a predictive model. Osaka as for Saloc does not use the trip matrix, but instead uses ten accessibility indices to measure the ease of access to ten different types of activity for each zone.
While Osaka and Saloc are not designed to forecast trip matrices, their accessibility indicators include assumptions about potential trip patterns. Osaka uses accessibility summed across all other zones, weighted according to the amount of activity of the appropriate type in the zone and inversely, via an entropy like formulation, with the generalised cost of travel to the zone.
Bruinsma & Rietveld (1998) reviewed a number of approaches to accessibility measurement in Europe, developing 11 categories of type of indicator or process. The majority of these are further breakups of the categories defined by Pirie (1978) , Black (1977) and Moseley (1979). Putting aside the distance and topological type, the remainder consists of one comparative type, two gravity type and one quasi gravity type where the mass of the destination is represented by a weighting factor applied to the average travel cost between the node and all destination nodes. A second quasi gravity model is identified as a function of the inverse of the balancing factor in a singly or doubly constrained spatial interaction model.
Three types of accessibility indicator are identified by Schurmann, Spiekermann, Wegener (1997pp), who recognise a region�s gross domestic product as a function of endowment indicators( suitability of a region for economic activity) and accessibility indicators. A key insight is that accessibility is a construct of two functions. One represents the activities or opportunities to be reached, the other the effort, time, distance or cost needed to reach them. Further more they identify accessibility as an important factor of the quality of life. The draft Socio- Economic and Spatial Impacts of transport infrastructure investments project (SASI) for the European commission is founded on this work. The three indicator types (Travel cost, daily accessibility and potential) were the basis for applying raster contour maps of accessibility, developed up using GIS techniques to calculate continuous accessibility profiles of regions. Schurmann, Spiekermann, Wegener (1997) have further developed measures of distribution of accessibility across regions, in line with an objective of the European Union to provide a fair distribution of accessibility to all its regions. Profiles of accessibility distribution have become known as cohesion indicators.
Schurmann, Spiekermann, Wegener (1997) observe that virtually all accessibility indicators used to date concentrate on network nodes or centroids representing cities or regions, so ignoring internal spatial organisation within regions. Bruinsma & Rietveld (1998) made similar observations with regard to intracity treatment of accessibility and recommends minimising the problem through the use of small areas as in the case of a grid system. This is in fact the solution applied by Schurmann, Spiekermann, Wegener (1997) in dealing with the issue, when they develop disaggregate accessibility indicators using the raster based GIS. The application of GIS, is one of the most current developments in accessibility .
Lee (1999) in a study on Kwangju, Korea utilises a transport simulation model based on the work of Wegener & Spiekermann (1996), together with a GIS process of disaggregating data spatially to assess the environmental impact of landuse/ transport scenarios. This application is a guide on how the direct connection between accessibility based analysis and assessment of environmental sustainability can be achieved. Spatial contour maps of eight environmental indices are displayed*. This information can be aggregated to give an equivalent to a cumulative opportunity accessibility index for environmental effect e.g cumulative distribution of population effected by emissions.
* = loss of open space, sealing of land, loss of biotype, heat islands, ventilation, noise. Air pollution NO2 and energy consumption
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