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Operationalising a new concept of accessibility to personal activities

Scott Le Vine, Aruna Sivakumar, John PolakCentre for Transport Studies

Imperial College London

Martin Lee-GosselinUniversité Laval & Imperial College London

RGS-TGRG, 31st August 2011

With thanks to the RAC Foundation for Motoring

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Outline

• Motivation & concepts

• Preparation of empirical data

• Findings

• Implications

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Motivation• Understand and predict which mobility resources a

person owns

• MR: any product, service, status, or information that enables or facilitates travel in some way

• Some methods of travel may require one or several

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Motivation (2)

• Little rigorous analysis except w.r.t. car ownership (this is beginning to change)

• Typically:

owning a car = f(income, gender,

employment status, age, residential location, etc.)

What about how you expectyou’d use it?

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Accessibility concept

• Perceived activity set:

the array of activities which a person views as encompassing their travel needs when making decisions that structurally affect

their accessibility

• Hypothesis:

owning MRs = f(perceived activity set)

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Accessibility concept (2)

• “I need [want] to get to…”

– Park for recreation (each morning)

– Grocery shop (twice a week in afternoons)

– Granddaughter’s school (once a year, in the evening)

– Specific government office (bi-annually, at times which vary)

– Physician’s office (once every two months, varying times)

– etc.

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Accessibility concept (3)

• Activities defined by:

– Temporal attributes

– Spatial attributes

– Qualitative (motivation for activity, need to carry cargo, flexibility of scheduling & location, etc.)

• Activities in a person’s PAS:

– Need not be conscious needs/wants

– Need not be observable as revealed behaviour

– May be forward or backward-looking

– May vary in importance

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Mobility resources

• MRs not exclusive – depending on definition, may own none, one or several

• Some methods of travel require advance commitment MRs, others may not: e.g. taxi, walking

• Specify: a person [HH] composes a portfolio of MRs, weighing the benefits (in terms of ease of access to activities in their PAS) against the cost of owning MRs

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Specification• For some portfolio d of MRs...

• “Of the various methods of travel it enables, how well does the ‘best’ one get me to activity j”...

• Person i repeats this J times, once for each activity in their PAS, and then weighs the value against the costs of acquiring the MRs in portfolio d

𝑈𝑑𝑖 = 𝑉𝑑𝑖,𝑛𝑜𝑛−𝑡𝑟𝑎𝑣𝑒𝑙 + 𝑉𝑑𝑖,𝑡𝑟𝑎𝑣𝑒𝑙 + 𝜀𝑑𝑖 𝑉𝑑𝑖,𝑡𝑟𝑎𝑣𝑒𝑙 = ቌ 𝛾𝑗𝑖

𝐽𝑖𝑗𝑖=1 ∗ln 𝑒ቀ𝑉𝑚𝑗𝑖𝑖,𝑡𝑟𝑎𝑣𝑒𝑙 ቁ𝑀

𝑚∈𝜇𝑑ቍ

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Empirics• System of three market-traded,

durable MRs

• Thus 23 = 8 portfolios:

– A, B, C, AB, AC, BC, ABC, None

• Six methods of travel

– Drive car – Ride bicycle – Take Public transport – Ride as car passenger – Walk – Take taxi

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Modes of Travel

Drive Car

Public Transport

(Free)

Ride Bicycle

WalkTake

Taxicab

Public Transport

(PAYG)

Ride as car pax.

Portf

olio

s of r

esou

rces

Own Car 1 -- -- 1 1 1 1

Own PT Season Ticket

-- 1 -- 1 1 -- 1

Own Bicycle -- -- 1 1 1 1 1Own Car + PT Season Ticket

1 1 -- 1 1 -- 1

Own Car + Bicycle

1 -- 1 1 1 1 1

Own PT Season Ticket + Bicycle

-- 1 1 1 1 -- 1

Own Car + PT Season Ticket + Bicycle

1 1 1 1 1 -- 1

Own none of these

-- -- -- 1 1 1 1

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Example• Consider a person with a very simple PAS:

‘I need to get to the food store’‘I need to visit my daughter’

• If walking is ‘good’ for accessing both activities, (s)he is likely to own none of the MRs

• If a car is ‘good’ for both, he may just own a car

• If a bicycle is ‘good’ for getting to one, and a car for getting to the other, he may choose to own both a car and bicycle

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Example (2)• Consider a person with a very simple PAS:

‘I need to get to the food store’‘I need to visit my daughter’

• Bear in mind he may view his need to access one activity to be more important than the other

𝑉𝑑𝑖,𝑡𝑟𝑎𝑣𝑒𝑙 = ቌ 𝛾𝑗𝑖𝐽𝑖

𝑗𝑖=1 ∗ln 𝑒ቀ𝑉𝑚𝑗𝑖𝑖,𝑡𝑟𝑎𝑣𝑒𝑙 ቁ𝑀𝑚∈𝜇𝑑

ቍ

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Empirical data• PAS: weeklong trace of activity-travel pattern

• 300 London households from 2004/05 National Travel Survey (c.700 people / 10K journeys)

• Information on alternative journey itineraries: online travel planning services (Journey Planner and Transport Direct)

• We need to know the ease of access for each journey by various methods of travel

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Online travel planning services

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Findings• Model estimation successful (but slow)

• Various parameters estimated to have intuitive signs and magnitudes (see paper)

• Correlations between observations and predictions:

Car ownership: 0.41PT season ticket: 0.20Bicycle ownership: 0.06

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Findings (2)

-1,250

-1,240

-1,230

-1,220

-1,210

-1,200

-1,190

-1,180

-1,170

-1,160

-1,150

-1,140

-1,130

-1,120

-1,110

-1,100

0 10 20 30 40 50 60

Log-

likel

ihoo

d

Number of journeys 'observed'

• Large gains from using long-duration diaries rather than one- or two-day diaries

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Findings (3)• Relative magnitude of salience parameters

Work / education: 0.34Shopping / personal business / other: 0.23Leisure: 0.19Social: 0.13Escort: 0.08

• So – access to each work activity is found to more strongly correlate with MR holdings than access to other activity types, but only moderately

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Findings (4)• Correct predictions

People observed to hold no MRs: 30%People observed to hold 1 MR:

30%People observed to hold 2 MRs:

18%People observed to hold 3 MRs:

6%

• Explanatory power inversely proportional to complexity of a person’s MR holdings

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Findings (5)

• Observed correlations in top right, predicted in bottom left

• In sum: Relatively weak both observed and predicted, but pattern not well replicated

Car

ownership

Public transport season ticket

ownership

Bicycle ownership

Car ownership -.07 .07

Public transport season ticket ownership .01 -.14

Bicycle ownership -.05 -.03

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Conclusions

• PAS concept found to be tractable

• Tight coupling between MR holdings & mobility patterns

• Several distinctive patterns in findings, mostly encouraging

• Broader aim is better predictions of responses to policy or market stimuli

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Conclusions (2)• PAS is latent, specifying it as revealed behaviour

may well introduce endogeneity and bias

• Directions for further research:

Address this issue of endogeneity

Subtler specification of MRs (car body styles, different PT season ticket offers, etc.)

Servicised MRs (car club subscription/use)

Thank you – contact: slevine@imperial.ac.uk