Accessibility under competition: An implementation in a GIS environment

1
Accessibility under competition An
implementation in a GIS environment
POEC 6389 GIS Masters Project

• By Binod Adhikari

2
Introduction

• What is Accessibility?
• The degree to which a system is usable by as many
  people as possible
• Study of potential activity between facilities
  and those who will use them
• It is an indicator
• Social indicator shows the availability of
  social opportunities (jobs, health care)

3
Introduction Types of Accessibility Studies

• Transportation and Planning
• Expansion of road network (Ahmed et al 2006)
• Traffic congestion study (Geurs, B Wee 2004)
• Operating speed (Geurs, B Wee 2004)
• Labor Market
• Spatial study between workers and jobs
• Study of spatial mismatch
• Real Estate
• Property value estimation
• Apartment rent estimation
• Location choice

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Scenario

• Retail Site Selection
• What is the story?
• Where to locate a retail facility
• Competition with other retail facilities
• Competition for patrons (population)
• Seeking sites that provide access to population
  in the context of retail competition

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Introduction Accessibility Under Competition

• People do not access opportunities in a vacuum
• There are competitors seeking to attract
  population
• Competition requires consideration of both demand
  and supply factors
• This research asserts that an approach that
  considers competition gives a more reasonable
  result for retail site selection

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Objective

• Implement two different methods of determining
  accessibility under competition in a GIS
  environment
• No such implementation exists
• Design a GIS extension
• Compare the results of the different methods
• Target audiences
• Geographic Information Scientists
• Real Estate developers

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Literature Review - Overview

• Accessibility has two components
• Location and Transportation
• Location defines the attractiveness or the
  opportunities that are present
• Transportation defines the impedance in order to
  access those opportunities
• Example of opportunities
• Job locations, shopping centers, day care
  centers, recreation centers etc.
• The population itself can be an opportunity if
  you are locating a facility, and other facilities
  are the competition

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Literature Review Methods of computing
accessibility

• Regional Accessibility and Population Potential
• Demographic elements are considered as the
  opportunities
• E.g. population, household income, home owners
  etc.
• Note in the following slides the symbols i, j,
  and k refer to locationsor zones where things
  are located, where there are n zones
• John Q. Stewart (1947) was the first to coin the
  term population potential

population of area j
distance between location i and area j
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Lit Review Methods cont.

• Mathematical formulation of Accessibility
• Distance based
• Simplest form of accessibility measurement
• Coined by Ingram
• Ingram refers to as the relative
  accessibility and the average (Ai ) as the
  integral accessibility
• Bigger the number less accessible is i

distance between location i and j
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Lit Review - Methods cont

• Mathematical formulation of Accessibility
• Gravity based
• Based on the classical law of the Newtonian
  Gravity Model
• Accessibility measure depends upon the magnitude
  of the opportunities, and how far these
  opportunities are located from each other

Ej Opportunities in zone j
Impedance function
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Lit Review - Methods cont

• Impedance Function
• Different mathematical forms
• Simple negative function
• Drawback Division by Zero when i j
• Opportunities in zone i are excluded from the
  calculation
• Gaussian function
• Negative exponential function

v is an empirical parameter
is a distance decay parameter
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Lit Review - Methods cont

• Mathematical formulation of Accessibility
• Cumulative Opportunities Based Method
• Opportunities are cumulated based on travel time
  or travel distance
• The travel time based method is also known as the
  isochrone method or Drive-time method

Ej Opportunities in zone j Tj Cut-off time (0
or 1). If greater than drive-time 0, otherwise 1
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Lit Review - Methods cont

• Accessibility with competition
• Gravity Based
• Traditional accessibility measures consider only
  the supply side and not the demand side
• Shens formula explains the demand side

Di Demand Potential Pk Population seeking
opportunities in zone k
Impedance function
Ai Competitive accessibility
Ej Opportunities in zone j
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Lit Review - Methods cont

• Accessibility with competition
• Isochrone based
• Competitive accessibility based on the Isochrone
  method is a very rare topic to find in the
  accessibility literature
• Two-step floating catchment area (Luo, 2003)
  resembles the isochrone based competitive
  accessibility measurement
• First step for every opportunity, measure the
  ratio of opportunity to opportunity seekers
  within an isochrone
• Second step for every zone the ratios from the
  first step are accumulated to every isochrone

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Lit Review - Methods cont
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Lit Review - Methods cont

• Accessibility with competition
• Isochrone based
• Why is the isochrone method interesting?
• Opportunities are defined based on the actual
  road network
• Opportunities beyond the travel time threshold
  are excluded from the analysis
• Less complicated and pretty straight forward
  method of analysis

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Lit Review - Methods cont

• Retail Trade Analysis
• Accessibility analysis-
• Doesnt take into account consumer behavior of
  choosing stores while shopping
• Doesnt consider the attractiveness or the stores
• Huff model and Intervening Opportunity model try
  to overcome these deficiencies in Accessibility
  analysis

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Lit Review - Methods cont

• Huff Model
• Introduced by David Huff in 1963
• A probabilistic model
• Gives the probability that a customer will shop
  in a certain shopping center

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Lit Review - Methods cont

• Intervening Opportunity Model
• Introduced by Samuel A. Stouffer in 1940
• the number of persons going a given distance is
  directly proportional to the number of
  intervening opportunities
• nearer opportunities compete with more remote
  ones and thus the intervening opportunities act
  as a barrier

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Lit Review - Methods cont

• Intervening Opportunity Model

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Data

• Boundary data (Block Groups) US Census Bureau

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Data

• Road Network Street Map USA, ESRI

Street Map USA comes with the speed limit data
for the US roads. Otherwise, Census Feature Class
Codes (CFCC) is used to estimate the speed limit
for the roads.
Note Streets not shown in the map
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Data

• Demographic Data US Census Bureau

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Data

• Retail Data Research Department, Archon Group

Major Retailers Wal-Mart, Target and H-E-B
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Methodology

• Implement two methods
• Accessibility using the isochrone method
• Primary focus of the research
• First implementation of accessibility with
  competition for the isochrone method
• Accessibility using the gravity-based method
• Implementation of Shens method
• Helps to compare the results from the isochrone
  based method

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Methodology cont

• Isochrone based method.
• Based on the floating catchment area method (Luo,
  Wang 2003)
• Implemented in two steps
• First Step
• Voronoi diagram (catchment) is drawn around each
  competitor
• For each voronoi polygon j all the zones that lie
  inside are identified and the turnover population
  is calculated
• for
  all i within j
• New population at zone i is
• Ei is calculated for all the zones i that are
  within the catchment j.

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Methodology cont

• Isochrone based method.
• First Step cont

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Methodology cont

• Isochrone based method.
• Second Step
• An iscochrone is drawn around each zone
• Zone centroids lying inside the isochrone are
  identified
• The population of each zone calculated in the
  first step is accumulated to the isochrone
• Alternative approach using Intervening
  Opportunity Model.
• The model could be implemented in the first step
  of the Isochrone method.
• Instead of Voronoi diagrams Intervening
  opportunity model could give more realistic
  figures in terms of population turnover for each
  zone.

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Methodology cont

• Isochrone based method.
• Second Step cont

Accessibility at zone 8 69.153.372.385.9190.2
125.6100.1 696.5
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Methodology cont

• Gravity based method.
• Implementation of the Shens formula
• Huff Model.
• The outcomes from the Accessibility analysis are
  not capable of pinpointing the new location
• Once the potential areas are identified, Huff
  Model could be run for these areas to pinpoint
  the suitable location

Ej is the population in zone j. Pk is the retail
stores present in zone k. f(Cij) and f(Ckj) are
the impedance between the zones
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Implementation in GIS Env
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Implementation in GIS Env

• Geodatabase.
• Designed in ArcCatalog
• Four Feature classes
• Isochrone (Stores Isochrones)
• Network ( Stores Road Network data)
• ODMatrix (Table with Origin and Destination data)
• Zones ( Stores boundary data- Block Groups)

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Implementation in GIS Env

• Isochrone Generation
• Network Analyst Service Area feature
• Impedance Minutes
• Isochrone generated for each block group centroid
• Analysis Setting
• Do not allow U-Turns
• Polygon type - Generalized
• Do not trim Polygons

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Implementation in GIS Env

• OD Matrix Generation
• Network Analyst OD Cost Matrix feature
• Impedance Street Length (Miles)
• OD Matrix generated between all the block group
  centroids
• 765 block groups, 765 x 765 585225 total
  records
• Analysis Setting
• Do not allow U-Turns

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Implementation in GIS Env

• Visual Basic Code
• Arc Objects
• Perform all the spatial related operations
• Isochrone generation
• Spatial join
• Copy objects
• Voronoi diagrams

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Implementation in GIS Env

• Visual Basic Code
• Microsoft ActiveX Data Object (ADO)
• Perform all the attribute data related operations
• Computation of distance decay parameter, ß
• Computation of Shens formula
• Why use ADO?
• Better performance
• Direct access to the geodatabase and manipulate
  the data
• Geodatabase can be accessed from other
  application, like Microsoft Excel

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Result
(4 min Isochrone)
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Result
(ß 0.2)
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Result

• Comments on the maps
• Symbology based on Natural Breaks
• Central part of the city has higher accessibility
  values
• Better road network
• More people live in the central part
• Both methods give almost same accessibility
  values for the outer part of the city.
• Yellow color represents accessibility lt 8300

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Result

• Statistical analysis of accessibility measures

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Result

• Statistical analysis of accessibility measures

42
Result

• Statistical analysis of accessibility measures

43
Result

• Statistical analysis of accessibility measures

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Result

• Statistical analysis of accessibility measures
• Pearson Correlation coefficients

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Result

• Statistical analysis of accessibility measures

46
Screenshots

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Conclusion

• Attempt has been made to implement two methods of
  competitive accessibility for retail site
  selection
• Both the methods gave similar results except for
  the inner part of the city
• Isochrone method tends to give higher
  accessibility values where there is better road
  network
• The correlation between the two methods is high
  but the median value and the standard deviation
  value did not match that well
• Block level data is recommended for Isochrone
  method

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Future Work

• Include marketing and social variables
• Differentiate population with vehicle and without
  vehicle
• Differentiate population with socioeconomic and
  ethnicities
• Compare competitive accessibility for different
  time span

49
Reference

• Ahmed M. El-Geneidy David M. Levinson 2006,
  Access to Destinations Development of
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  Transportation.
• Bhat, C., Handy, S., Kockelman, K., Mahmassani,
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• de Jong, 1996, Location profile-based measures as
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50
Reference

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• Huff, 1964, Defining and Estimating a Trading
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