A Longitudinal Household Travel Survey: Issues of Design, Nonresponse and Variation in Travel Behaviour

Copyright © IFAC Transportation Systems Chania, Greece, 1997

A LONGITUDINAL HOUSEHOLD TRAVEL SURVEY: ISSUES OF DESIGN, NONRESPONSE AND VARIATION IN TRAVEL BEHAVIOUR

Iossif Argiropoulos Anna Anastassaki A1exandros Deloukas

Attiko Metro A.E.

Abstract: A large longitudinal household travel survey conducted during the typical and the summer period in Attika for the needs of the Metro Development Study (l'vIDS) encountered complex design and execution issues. The paper discusses the variation of basic socioeconomic figures between periods as well as tentative results on travel characteristics for the typical period followed by a cross sectional comparison of travel patterns revealed within the frame of similar studies conducted in the past decades.

Key Words: Metro Development Study: Household Survey: Travel Behaviour: Panel

l.INTRODUCTION The estimation of travel forecasting models needs up-to-date travel demand data. Therefore a large household travel survey has been designed and conducted in the Attika region 1 . The household survey obtained relevant information to be used as input to the Metro Development Study, a comprehensive multi-modal transportation planning effort being undertaken by Attiko Metro.

design and the resulting response rates. In the third section, data processing, consistency checks and data base design issues are presented. In section 4 socioeconomic and tentative trip-related results are given. The paper ends \vith conclusions from the ,vork done so far and suggestions about further research.

This paper presents issues of design and execution of the household survey and discusses findings related to the travel behaviour of the population in Anika.

2. SURVEY DESIGN Studv area: The study area partitioned into 1233 zones covers 79 municipalities in the Greater Athens Area (essentially: Athens Basin and Thriasion Field) and 38 in the Rest of Attika region. For comparison purposes, it should be noted that the Athens Organisation of Public Transport (OASA) selyicc area covers the Greater Athens Area plus the municipalities of Kropia, Peania, Spata and Loutsa (Artemida).

The structure of this paper is as follows: The following section describes the household survey design, sampling issues, the survey questionnaire

1 Data collection and processing has been designed hv Attiko Metro and executed by ELLINIKI MELETON. STOHOS and EKOM firms.

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Survey method: The household survey has been designed as a home interview survey with face-toface interviewing with eligible household members i.e. persons older than 5 years. This interview method improves in general overall response rates and reduces as a consequence the survey bias.

logistical and cost efficiency of the survey. The strata are 855 zones of residential character. The population resides into approximately 70,000 building blocks. In a first stage a random sample of 3257 aggregate building blocks. depending on the 1991 zonal population. was chosen. The chosen areal units were then blocklisted to obtain a full list of household addresses within the blocks. An uncleaned number of 217.802 addresses is reported (before screening out mis-recorded invalid addresses of vacant or unfinished stmctures, overt workplaces etc.). The removal of such addresses reduced the coverage error of the sampling frame to an insignificant amount of covert workplaces or other units outside the target population. In the second stage. a random sample of households was drawn for interviewing from the already registered households. The strict sampling unit is the (address-based) home. whereby the household is the unit of measurement. In very few cases a single home contained more than one interviewed households. Unit of analysis is either the household or the personJhh member depending on the parameter analyzed.

The survey runs in two distinctive periods (waves). Wave I covers the typical period (mainly Spring 1996) and Wave 2 covers the summer period (mainly July 1996). In order to observe the seasonal variation of the travel behaviour with higher precision and lower cost, a panel design has been selected. In the summer wave the same (sub)sample of respondents of the typical period are re-interviewed. It is a short panel because only a couple of months lie between the waves. In such a controlled experiment design, the socioeconomic characteristics of the sample remain essentially the same over time, and the variation refers solely to travel characteristics. Sample size: The net sampling rate (after replacements) amounts to 2% of the population in the typical period (27696 households, including 1786 replications) which is more than adequate for the modeling of trip patterns of the 4-million metropolitan area inhibitants considered. It should be noted, that such a large household survey scopes not only to a transport model development but also reliable and current population estimates. Census information is not only (1991) dated, but also is believed to include inaccuracies (more than 10% of the study area population move deliberately to their place of origin to be captured there during the census day) . All small zones were oversampled to allow for an analytical variation of their base characteristics. The minimum sample size required was 25 households per zone, thus resulting in 3448 additional households interviewed. In the sunm1er wave the rate in effect was 0,5% in the Greater Athens Area (i.e. every fourth of the households sampled in sampled in Wave 1). A special emphasis was placed on the Rest of Attika region (2%-rate). The overall summer sample size was 8194 households. including 585 replications. Small zones were oversampled in the Rest of Attika region only, resulting in 393 additional households interviewed. The replicated households have been selected according to certain pre-defined rules. The replication step was necessary in order to get a selfbalanced sample and to expand directly sample data to the population. Statistically invalid intervievvs refering to over-replacements or nonsample interviews were removed from the net sample.

The list of households sampled in Wave I was used as the sampling frame for the summer wave. As a consequence of the mixed panel design selected, households nonparticipating in the summer wave (dropouts) were replaced either by remaining households who participated in the typical period or, as in the case of the Rest of Attica region. by first contact households in order to maintain the sample size. The broad majority of households residing in the Greater Athens Area remains the same over both survey periods. In the summer period though, a spatial dissolution of certain households together with a seasonal formation of ne" households is observed, whereby a household migrates to a summer residence in the Rest of Attika region (outmovers) while the breadwinners typically remain in Athens. Even members of other households may join the fom1er (in-movers). Entire households may relocate to summer residences within Attika as well. An increase of the mean household size in the Rest of Attika region. respectiYely a decrease in the Greater Athens Area. is expected during the summer period (see section 4) . Questionnaire design. The questionnaire follows

the current standard. namely the KONTIV-design and consists of three parts. The household as well as the person data part were answered by a spokeperson older than I X years. The above parts exhibit in the summer survey a reduced number of questions. The third part is a trip-based diary recording the chronological sllccession of a complete chain of trips of each person during the

Sampling design: A mUltisage stratified sampling design has been chosen in order to increase the

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previous 24-hour period. Only weekday periods were reported. Its clear layout discourages trip underrecording. More reliable reporting of trips was intented through a "previous day" recall of travel activities. Trips on foot are recorded only if they exceed 500 meters. Walk access/ transfer/egress trips to/from travel modes were not recorded as separate trips. However, because of memory lapses or of a certain conviction of what constitutes an "important" trip, short or walk trips may be underreported.

contacts (20%). which is of a limited extent indeed. As a result. 92% of the sample in the Greater Athens Area and 36')1" of the sample in the Rest of Attika region had participated in both periods (74% of total sample - 5924 out of 8002 households). The non-response issue requires more discussion. A first comment is. that the non-contact rate exceeded the refusal rate in the study area. Another comment is that. in the Greek context. interviewer incentives and fieldwork monitoring influence the non-contact rate more that the household mobility ("'not-at-home") per se. Less controlled interviewers paid by interview tend to select "easy" addresses or sampled households. even when a maximum daily amount of (5 in this case) interviews is set. As a general remark, female interviewers were found to perform better in that sense. The extent of trip under-reporting due to the interviewer effect. and (even) pro:\~' effect. may be estimated by comparison with more reliable control information (on trip rates and socioeconomic correlates) from well-monitored interviewer work.

The survey instrument consists of a single form of paper, which costs less than a booklet form of diary but encourages third persons to give travel information about other household members (pro:\) effect). Present persons have reported their own trips. In case of absentees, the informed spokeperson reported their trip data. The latter case resulted into some trip under-reporting in general, and especially of short and walk as well as NHB trips, or even inaccurate reporting of trip characteristics of the absent household members (because of a certain knowledge gap of the respondent).

Household concept: Due to the atypical composition of the summer population of the study area. the concept of the household has been redefined for the scope of the summer survey. In the summer period. membership to a household is given when a person spent the last but cne night before the interview day at the sampled housing unit. As a consequence of the household redefinition. person estimates of the summer and the typical survey are directly comparable. but household estimates of the summer survey could be better interpreted after a more detailed (disaggregate) analysis of the links between newly formed and antecedent households.

Survey execution: People who do not respond to travel surveys have different travel characteristics than the overall population. Increasing the response rate means reducing survey bias. Several actions were envisaged in order to reduce the nonresponse bias and the number of replacements. The first noncontact was followed by a re-contact at different times of day, and in the summer ",ave even by telephone callbacks in order to increase the contact probability. Seasonal absenses (e.g. for holidays) during the summer wave -as opposed to accidental absenses- were considered as a sample loss and were not replaced. Low refusal rates were intended bv means of a wide publicity campaign (TV, radio, posters. infoffilation brochures) and targeted actions of approaching people (prior notification vvith a contact letter. hot line, interviewer identification. gift) . The base actions involved a sound interviewer training and instruction as well as telephone controls and daily supervision of fieldwork performance. The overall effect of the above actions on the response rate and the attrition of the summer wave, due to respondents fatigue. was encouraging and influenced positively the resulting data quality: the response rate in the typical period was 69% and in the summer period 72%. Regarding the households that participated in Wave L the response rate in Wave 2 amounts to 68% (64% in the Greater Athens Area. 90% in the Rest of Attika region, totally 3664 out of 5419 households). The rest of the non-responding households are either dropouts (12%) or non-

3. DATA BASE DESIGN

Data editing: Control routines allowed edit and skipping checks during the data entry. Mispunching was corrected after a comparison of two independent data entry nms . Incorrect infoffilation due to mis-reporting or mis-recording \\"as detected in batch mode \"ia logical control routines. Inconsistencies were corrected partly automated. partly manually. follo\\"ing certain deterministic rules and presening the distribution of the \"ariables. Missing \alues of certain critical \"ariables due to nonreponing or nonrecording were imputed. Variables such as "age class" and "dri\"er licence status" \\ere handled \\ith the hot-deck methodology. A calibrated regression function was used for missing \alues of the continuolls "rent"· variable. whereby a qualitati\"e response (logit) function was used for the categorical "household

437

income class". In any case, the nonresponse rate of the survey exceeds missing data rates by far.

only one third of the summer residences is seasonally occupied. The sUI"Yey does cover weekday travel patterns. but not weekend travel. It is, however. expected that during the weekend the occupation rate of the second homes increases significantly.

Data base structure: The data base contains three interrelated hierarchical levels (tables), namely a household table, a person table and a trip table. Out of the source files an analytical data base was created containing all the information collected through the questionnaire and the corresponding trip end zones. Furthermore, an aggregated data base was developed where the following procedures were performed: linked trips were consolidated into journeys. the continuous variables have been discretized into class variables, and trip modes and trip purposes were aggregated into main modes and main purposes respectively. Finally, a working data base has been created where linked journeys were consolidated to home-based tours, characteristics by a primary activity, and the trip chains were classified according to a chain typology ranging from less to more complex.

Pu,lUJ"tiun Gr.Athens Rest of Attika Study Area

~-------

% change T\iQ!cal Sunun~r 37)<01<00 31931<00 -15 .5% - - - - - - ~------ ---::---25X600 40X750 )1<.1 % 4039400 3602550 -10.1<%

Houselwlds TUJical C'rr.Athens r-.------Rest of Attika Study Area

Sununer

~o

change

1131<200 -12.5% -1301150 - - - - - r-124050r------1<1650 13X21<00

1262250

51.9% -x,7%

Household Size TYI'ical

Summ~r

% change

r-~~~~--- -~~~--r-~~~--r-~~~-Rest of Attika Study .-\rea

3.17 2.92

3.30 2.1<5

4.0% -2.3%

4. EMPIRICAL FINDINGS Regarding the residence type and status. a significant difference is noticed between the two geographical areas and time periods surveyed: most of the households in the Rest of Attika region leave in owned detached or semi-detached houses. while most households in the Greater Athens Area reside in apartments while the percentage of households renting a house rises to 25'%.

This section provides basic socioeconomic figures regarding the typical and the summer period 1996 as well as tentative results on travel characteristics for the typical period (since data analysis is underway). Finally a cross-sectional analytical comparison is performed of the 1973 (Smith Study), 1983 (OAS) and 1996 (AM) travel patterns in the study area. ~.

1. Socioeconomic characteristics Detached house Semi-detached Apartment

Seasonal variation: The following tables show population figures (population, number of households, household size) for the typical and the summer period, where the percentage change is provided. As expected, the summer population reduces in the Greater Athens Area and increases significantly in the Rest of Attika. Furthemlore. it is noted that about 150,000 persons are seasonally leaving during the week at the summer residence. \\hile the population in the Study Area reduces overall by approximately 440,000 persons (due to movers outside Attika. mainly holiday makers. and possibly due to the sampling error difference between the two periods). This means that seasonal movers outside Attika are three times more than movers \Vitllin Attika during the summer period. Similarly. the household size reduces from 2.92 to 2.85 in the summer period. Finally. it has to be marked that according to the information collected during the typical period, about 120.000 households own a summer residence \vithin Attika. The net increase of 42.400 households in the Rest of Attika region indicates that during the week

Owned Rent

Fr""

Residence T'l!.c Typical P~riodSummer PeriodR~st of Attika Greakr Athens 20~0 x5% 7°/'0 15° 0 x% 65°'0 Residence Status Summer PeriodTypical PeriodGreater Athens Rest of Attika 90% 3%

TVplCO/ period charaClerislics: This subsection provides information regarding socioeconomic characteristics influencing short -term and longterm mobility in the study area. A long-term mobility indicator is the ~ 'ears of residence in the present municipality . As it can be seen from the following table. Cl rather small percentage of the population has recently mO\ed in the mUl1icipali~ (usually younger peoplc) while more than half of the population resides in the samc mUl1icipali~ for more than 2() years.

438

o·

rI

.1 Gr.Athens Rest Attika Studv Area

f-l.2 3

Years in the Municipalitv ( %) 5· 10· 15· :20· 9 .14 .19 .30 11 12 _..1.9_ r-12 11 10 15 12 11 12 19

2· A 7

-- --~

7

_JL

employment status and the sector of economic activity of the employed. Age follows a rather typical distribution while the employment rate amounts to 39.1'% of the population over 6 years old. though actually the active p~pulation comprises ages over IS years old. Regarding the sector of economic activity of the employed (> 15 years). it is clear that the vast majority works in the tertiary sector.

30+

I--~41 35

According to the outcome of the household survey, the car ownership rate (defined as the number of private cars per 1000 inhabitants) in the study area is 248. The figure is higher in the Greater Athens Area than in the Rest of Attika region (249 and 232 respectively). Compared to the OAS 83 study, the car ownership rate has increased by 45% (from 172 to 249) in the Greater Athens Area. It has to be noted that the relevant figure from the Wilbur Smith Study (1973) was 100 cars per 1000 inhabitants.

0-5 _0~t!:e.!!:,

Rest of Attika Studv.-\rea

Gr.Athens - - - - - - r"}2:~Rest Attika 38.8 Study .-\rea 38.7

2-car

Primarv s~ctor Secondar,' 'ector Industry Electricity Constmction Tertiar~v sl.!dor

Pri\'ak (*) Puhlic Total

r'!'±'- r-...!.·~-1.1 1.1

( *)

Female 35% ----73%-1 - 25 % 76% 36%

G6A

15 .0

69. 1

11 .6

%)

39. 1 60 .9 100.0

r------------

<}o

. lncl udlll::':

IIOlIlt: - work~l ~

')600 242550 1(,0200 2D50 57000 1244250 93G 7 50 )075 00 1496400 cLlld

0.6 16.2 10.7 1.7 3.8 X3 .2 62.6 20.6 100.0

t rallSI)l ll t ;;l tio t l - ro.!ht t ~ c\

clllp loymcnt

4.2 Tvpica/ period Irip pallerns

The total number of trips in the study area are (j.g million trips (incl. escorted children <5 years). \vhich corresponds to (j.2 million journeys (i.e. 1.1 unlinked trips per journey). The following information concerns journeys (linked trips) . It has to be noted that in the following table illustrating trip purpose distribution. the return home trips have been distributed according to the purpose of the previous journey. As can be seen from this table. "\vork" is the dominant trip purpose followed by journeys for recreation and social activities which is typical of the e:-;trO\erted local lifestyle. It can also be noted that a significant share of journeys is made for "education"

Licenced Drivers « 17 Years) Gr. Athens -----Rest of Attika Study .-\rea

65 + 11.3

Sl'ctllr Ill' ECUlllJlllic Activity

Furthermore, the number of licenced drivers amounts to 55% of the population. It has to be noted the significant difference between male and female population ,.vhich is higher in the Rest of Attika region. Compared to the OAS 83 study. it is noted that the number of licenced drivers has doubled (increase from 28% to 55% overall), mainly due to the dramatic rise of female drivers and of the car ownership rate.

1-.-Iale 76%

D .9 14.2

Pt.:rs ons

3-car

9. 1 9.0

4. 7 5.1

I\;rsons 1496400 _ ~m.£I~:? ___ --2:135150 Non-Emplow d ))<)1550 Total

Car O\\'nin~ Households (%) I-car 53. 1 --51 ,0 51,2

18-64

Emplu)ll1Cllt Status

As far as the current distribution of households in terms of car ownership is concerned, it has to be noted that 51.2% of the households have one private car while 10, I % of the households own more than one car. The relevant figures from the OAS 83 study were 48% and 4% respectively, thus resulting in a lower car ownership rate overall.

O-car

6-17

_____ ~~ __ 2..42 __ 32;::"

Total 55 %

--------49 ~;'

55%

Another finding concerning PT-oriented persons refers to the transit pass ownership rate (defined as the number of transit passes per 1000 inhabitants) which in the Greater Athens Area is III (out of which 64 are permanent and 47 occasional holders). The relevant figure from the OAS 83 study "vas 77 passes per 1000 inhabitants (actually 57 with paid and 20 with free passes).

Trip Purpose Work Education Shoppin g Personal Recreati on Soci al Bu s in~ss

Other

Other socioeconomic characteristics provided below. concern the age distribution. the

439

42 .6°;' 14.)0·" 11.7% 11.3%

According to the survey findings, the number of trips per person (over 5 years old) is 1,69 and the mobility rate (mobile persons per 100 persons) reaches 56%. The latter percentage compared to western european figures is rather low, the immobility reflecting the lower labour force participation and car ownership rates in Greece. However, trips per mobile person are directly comparable with international figures. In the Rest of Attika region the trip rate is lower (1,34) and that is due to the increased number of trips made on foot «500 meters) that are not recorded by the survey. The corresponding figures by the OAS 83 and the Smith Study 73 were 1,63 and 1,06 respectively, indicating a stability of the trip rate during the last decade. Car ownership increase is traffic counterbalanced by the worsening of conditions and this may explain this phenomenon.

0:..;~~~_ Rest Attika Study Area

Trips per person - -1,69 --1.34 1.67

Mobility rate

Trips per mohile person

42% 55%

3. 18 3.05

Regarding the development of trip purpose shares, education marks a significant increase, possibly due to the expansion of private schooling while shopping share remains rather stable over time.

Work Education Shopping Personal (ind.health) SociallRe~Teation

Return home Change mode Other (ind.Business. persons)

~S l.:ort

Trip Purpose OAS -lO AM-96 IX.O% 20.3% 4.5% G.7% 4.1% 3.7% l<.3% 5.9% 7. 1% 10"<% 3 9.6% 42.0% 11 .5% 9.1 % :>.2% 5.2% 100°'0

100%

From the above table in conjuction with the current PT share (32'1.•. i.e. 2.2 million unlinked trips). it may be derived that about 30% of the PT trips involve at least one transfer (totally 620.000 trips aiming "change of mode" ). It should be noted that the absolute amount of recorded PT trips includes also "free riders". Concerning a modal subset of PT trips, about 58% of the ISAP-Metro trips (i.e more than every second passenger) involve at least one transfer.

--??~-- ~ ___ ~Oi ___

Compared to the typical period, the summer trip rate in the Greater Athens Area remains virtually stable while in the Rest of Attika region the trip rate increases slightly by 5%.

5. FURTHER SUGGESTIONS 4.3 Cross sectional comparison by mode and by purpose (Smith Study-73 vs. OAS-83 vs. A/I,1-96)

It has to be noted that in the context of the data processing that is underway. cross-tabulations of the socioeconomic data will be provided at the traffic zone, municipality and sector level as well as trip rates by socioeconomic data will be provided at the traffic zone. municipality and sector level as well as trip rates by socioeconomic segment. Furthermore. trip tables will be provided by time of day . main mode of transport and trip purpose.

For comparison purposes over time, the following tables indicate the modal split and trip purpose distribution at the trip level (unlinked trips) . Regarding the development of modal shares over time, a dramatic increase of private car use coupled with the parallel decline of public transport share is noticed. This is mainly due to the increase of car ownership rate and the insufficient level of PT service. The relevant figures from the Smith Study 73 referred to aggregate modes of transport. For comparison purposes the OAS 83 and the AM 96 figures have been similarly aggregated. It may be derived that the rate of decline of public transport share has rather decelerated in the last decade compared to the 1973-1983 period. Other modes such as vans are obviously much less used for person trips than in the 70 's.

-

Pri vate Car Motorcyde PuhlicTran.'port Special Bus Taxi + Other motorized

Smith -73 24

In a future exploitation of the household travel panel data . it is suggested to analyze : •

•

Modal Split (%) OAS-83 A\1-96 44 55

62

47

39

14

9

6

100

100

100

•

the net seasonal \'ariation of thc travel patterns as well as possible compensation effects. which otherwise could be \\Tongly appeared as stability the characteristics of thc pcoplc who refused to report again in the summer \\'a\,e (selfselectivity). being in the ad\'antageous position to know thcir socioeconomics already from Wave I (typical period) . in order to compare them with acceptors and in\,estigate the attrition effects effects of the dynamic household definition of the longitudinal suney .

A strong point of panels is that they measure mobility changes at the indi\'idual lc\'e1 0vertime.

440

Their usual weak point, i.e. their small sample size and lack of representativeness, does not apply in the case of the large Attika panel which is both representative of the population and of the seasons.

ACKNOWLEDGEMENTS The authors are indebted to I.Kelperis, for his spirited and more than valuable statistical consultancy. and to G.J.Nellas for his critical contribution. The study was fully funded by Attiko Metro.

REFERENCES Wilbur Smith and Associates, (1973). Traffic and Transportation Study of the Greater Athens Area, YoU . Ministry of Public Works, Traffic Division. Akritidis N., Drosis Th., Yoskoglou N. , (1984). Transportation Policy for Greater Athens Area, Presentation of the 1983 OAS Origin Destination Study. EVEA, Athens.

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