How Density and Mixed Uses at the Workplace Affect Personal Commercial Travel and Commute Mode Choice

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1 Transportaton Research Record Paper No How Densty and Mxed Uses at the Workplace Affect Personal Commercal Travel and Commute Mode Choce Danel G. Chatman A hgh densty of shops and servces near the workplace may make t easer to carry out personal commercal actvtes on foot before, durng, and after work, enablng reduced vehcle use durng the rest of the day. Investgatng ths queston s an mportant addton to the current research, whch has focused on resdental neghborhoods. Data from the 1995 Natonwde Personal Transportaton Survey are used to nvestgate the nfluence of workplace employment densty and share of retal employment on commute mode choce and vehcle mles traveled (VMT) to access personal commercal actvtes. The analyss controls for socoeconomc characterstcs and accounts for the endogenety of commute mode choce and personal commercal VMT by employng a jont logt-tobt model. Employment densty at the workplace s found to be assocated wth a lower lkelhood of automoble commutng and reduced personal commercal VMT, whle the presence of employment n the retal category does not play a sgnfcant role. Workplace densty s more clearly related to reduced VMT and automoble commutng than to characterstcs of workers resdental neghborhoods and could have sgnfcant nfluences on personal commercal VMT and automoble commutng when ncreasng over a large area. The results suggest that land use planners should focus on encouragng employment densty to a greater extent than s the current practce, although further research s needed on the role played by correlated factors such as hgher parkng costs, ncreased road congeston, and better transt servce. Many urban planners want to facltate hgh-densty, mxed-use land development to make alternatve transportaton modes more attractve. The theory s that hgher development densty and reduced segregaton of land uses should reduce the dstance between the locaton of actvtes that requre travel. In turn, shorter trps are thought to be more lkely carred out on foot or va transt. Urban plannng polces ntended to effect denser, more mxed development nclude relaxaton of parkng requrements, so-called densty bonuses, mnmum floor-to-area rato requrements, and transt-orented development programs. Ths research nvestgates the relatonshps between the bult envronment characterstcs of the workplace, worker commute mode choce, and workers use of personal vehcles to carry out personal commercal actvtes. Shops and servces near the workplace may make t easy for workers to carry out actvtes on foot before, durng, and after the workday, such as buyng flowers or gfts, takng clothes to the dry cleaner, vstng the dentst, gong out to lunch, and catchng a move after work. Those who take advantage of ths pedestran accessblty may travel fewer vehcle mles to carry out such actvtes durng the rest of the day. Department of Urban Plannng, Unversty of Calforna, Los Angeles, 3250 Publc Polcy Buldng, Los Angeles, CA Workers who have a car avalable at the workplace are probably less lkely to patronze nearby shops and servces. At the same tme, the choce of whether to drve to work may be condtoned on the avalablty of those shops and servces. Understandng ths nterrelatedness s mportant for planners who clam that partcular land use polces have postve transportaton system benefts. Because workers make relatvely few dscretonary trps on workdays, the potental mpact of workplace land use on travel behavor mght be thought to be mnmal. But there are two mportant reasons to nvestgate the relatonshp. Frst, t may be pragmatcally and poltcally more acceptable to change polces n prmarly nonresdental areas, because the users of those areas may have fewer complants about more ntense development than resdental users typcally do. Furthermore, n employment clusters of large ctes, exstng polces may sgnfcantly constran development densty. Second, researchers can have greater confdence n analyss that focuses on how the bult envronment at the workplace affects travel. People who prefer to use alternatve modes may choose to lve n dense mxed-use neghborhoods, whch are often more walkable and have better access to transt. If so, typcal regresson methods relatng ther travel behavor to resdental land use characterstcs wll have results that must be nterpreted wth a great deal of cauton (1). But workers are less lkely to choose ther jobs based on workplace land use characterstcs. Thus, any relatonshps found n such analyss can be nterpreted wth greater confdence that they represent the ndependent effects of land use, nstead of self-selecton effects. PREVIOUS RESEARCH Much of the recent research on urban form and travel behavor has focused on the queston of whether people lvng n dense, mxed-use neghborhoods wth tradtonal street grds make fewer trps by prvate automoble (1 5). Recent mprovements n ths lne of research have ncluded correctng for endogenety of the resdental locaton decson and the tendency to use a car for trps (5, 6) and ncludng trp speeds and dstances as explanatory varables n a two-stage leastsquares procedure (7). There has been relatvely lttle recent research on how workplace land use affects travel behavor other than a study showng that commute mode choce can be affected to a modest extent by workplace transportaton demand management programs and ste-specfc land use mx (8). Age, sex, race, educaton, ncome, and household lfe cycle (e.g., presence of chldren n the household) have been found to be assocated wth partcular patterns of mode choce and total travel (9 13) and are ncluded n ths analyss. Includng nformaton on

2 194 Paper No Transportaton Research Record 1831 recent mmgrant status (14) and parental status (9) s not possble wth ths data set. The analyss presented here vares n other ways from earler research. Frst, the focus s narrowed to personal commercal travel, whch should be more drectly related to the accessblty of commercal actvtes than other types of nonwork trps. Second, the analyss accounts for the smultanety of commute mode choce and day-long partcpaton n personal commercal actvtes, whle correctng for the fact that the dependent varable s left-censored. DATA AND SAMPLE SELECTION The data set s drawn from the Person and Travel Day Trp fles of the 1995 Natonwde Personal Transportaton Survey (NPTS). The ntal data set conssts of 34,560 workers wth complete work perods and complete mleage nformaton. Wthn ths group the test sample s restrcted to those who dentfy themselves as drvers (95% of the sample), have at least one car avalable for every drver n the household (85%), and state that transt servce s avalable near ther resdences (64%). These restrctons are ntended to substantally ncrease the lkelhood that each ndvdual ncluded n the analyss has a complete set of mode choces for the commute. Also, respondents n the upper 1% of personal commercal vehcle mles traveled (VMT) (greater than about 50 m) are excluded. Based on an nspecton of the data set, the reportng appears to be error-prone for outlyng values. The omsson of ndvduals who report that bus servce s not avalable deserves comment. The omtted group may contan many who are gnorant of bus servce, perhaps due to a strong preference for drvng. However, comparng values of the ndependent and dependent varables before and after droppng these ndvduals does not reveal any obvous selecton bas. Dfferences between the ntal sample and the truncated estmaton sample are presented n Table 1. Table 2 presents the mean and varance of the ndependent varables n the estmaton sample for respondents who drove to work and those who dd not. Those who drove to work averaged almost a mle more per day n personal commercal VMT, and ther average workplace densty, at about 6,000 workers per square mle, s far lower than the nondrvng group average of 14,500 per square mle. METHODOLOGY Defnton of Personal Commercal VMT Personal commercal VMT s defned as the number of mles traveled n a personal vehcle for personal commercal purposes. The category ncludes shoppng (over half the total), medcal/dental, gong out to TABLE 1 Comparson of Samples Before and After Truncaton Before truncaton (N=30,587) After truncaton (N=14,478) Varable Mean St Dev Mean St Dev Descrpton pcdmtot vehcle mles traveled on personal commercal trps D drove to work (dummy) wtempldn employees per square mle, workplace Census tract (1,000s) wtndret employment share retal, workplace Census tract (percent) wtempz mssng workplace Census tract data (dummy) hbhresdn housng unts per square mle, resdental block group (1,000s) hteempdn employees per square mle, resdental Census tract (1,000s) htndret employment share retal, resdental Census tract (percent) hbtz mssng resdental Census block group data (dummy) dsttowk reported one-way dstance to work (m) wktm_tot total tme spent at work (mnutes) r_age age female female (dummy) afram Afrcan Amercan (dummy) asan Asan Amercan (dummy) oth_race other non-anglo race (dummy) nohs less than hgh school degree (dummy) somecoll more than hgh school degree (dummy) ncome household ncome (1,000s) ncomez mssng household ncome data (dummy) hh one-adult household (dummy) kds household wth chldren (dummy) carprdvr cars per drver n household hhsze household sze SOURCE: 1995 Natonwde Personal Transportaton Survey. Sample before truncaton conssts of all self-reported workers lvng n a CMSA wth complete work perods on the survey day. The after-truncaton sample conssts of the subgroup who report that bus servce s avalable, who lve n households wth at least one vehcle per lcensed drver, who are self-reported drvers, and who travel 50 mles or less for personal commercal purposes.

3 Chatman Paper No TABLE 2 Comparson of Subgroups: Automoble Commuters and Other Workers (Drvers wth Complete Work Perods, Car Access, and Transt Access) a Dd not drve to Drove to work work (N=1,075) (N=13,403) Varable Mean St Dev Mean St Dev Descrpton pcdmtot vehcle mles traveled on personal commercal trps b wtempldn employees per square mle, workplace Census tract (1,000s) wtndret employment share retal, workplace Census tract (percent) wtempz mssng workplace Census tract data (dummy) hbhresdn housng unts per square mle, resdental block group (1,000s) hteempdn employees per square mle, resdental Census tract (1,000s) htndret employment share retal, resdental Census tract (percent) hbtz mssng resdental Census block group data (dummy ) dsttowk reported one-way dstance to work (m) wktm_tot total tme spent at work (mnutes) c r_age age female female (dummy) afram Afrcan Amercan (dummy) asan Asan Amercan (dummy) oth_race other non-anglo race (dummy) nohs less than hgh school degree (dummy) somecoll more than hgh school degree (dummy) ncome household ncome (1,000s) ncomez mssng household ncome data (dummy) hh one-adult household (dummy) kds household wth chldren (dummy) carprdvr cars per drver n household hhsze household sze Bold type sgnfes exogenous varables used only n the logt equaton (step 1 of model 2). All other varables are used n both equatons. NOTES: a Car accessblty defned as one or more personally owned vehcles per drver n household. Bus accessblty defned usng "bus_avl" varable n Travel Day Trp Fle (nonresponses also dropped). b Personal commercal trps defned as NPTS trp purposes "shoppng," "medcal/dental," "gong out to eat," and "other socal/recreatonal." Varable calculated usng "trpmles" and "trptrans," varables n NPTS Travel Day Trp fle. c Varable calculated usng actvty duraton ("dweltm2") and trp purpose ("whytrp95") data from Travel Day Trp Fle. SOURCE: 1995 Natonwde Personal Transportaton Survey. eat, and other socal/recreatonal trps. Ignorng return trps home, personal commercal trps made up 39% of trps n the unweghted natonal sample. Independent Varables for VMT Models Independent varables for the VMT models are presented n Table 2. The test varables are workplace census tract employment densty and share of retal employment. In the NPTS, ths nformaton s represented categorcally. Here, the nformaton has been recoded at center ponts of the categores and treated as a contnuous varable. Smlarly, three varables proxyng land use at the resdental locaton are ncluded: block group housng unt densty, employment densty, and share of retal employment. Control varables nclude sex, age, race/ethncty, household ncome, educaton, household sze, and dummy varables ndcatng whether the household has only one adult and whether chldren under age 17 are present. The dstance from the resdence to the workplace and the amount of tme spent at work are also ncluded. Increased dstance to the workplace s expected to ncrease the relatve convenence of an automoble for the commute and hence the lkelhood that an automoble wll be used both for the commute and for carryng out personal commercal actvtes. By reducng dscretonary tme, the length of the work perod s expected to decrease the amount of personal commercal actvty and hence personal commercal VMT. Treatment of Mssng Data Lke most data sets, the NPTS lacks complete responses for some varables. To account for respondents wth mssng nformaton, nstead of merely deletng cases from the analyss, the mssng ndcator method s used (15). A dummy varable, created to ndcate mssng values for each relevant varable, s set equal to 1 for respondents mssng the data and to 0 otherwse. Ths procedure s followed for household ncome [represented by the varable (ncomez)], workplace land use characterstcs (wtempz), and resdence area land use characterstcs (hbtz). The problem s most

4 196 Paper No Transportaton Research Record 1831 extensve wth the workplace census tract employment data, wth mssng values for 26% of the estmaton sample (see Table 2). Addtonal Varables for Car Commutng Model In addton to the ndependent varables used n the VMT models, varables are ncluded n the model for whether a motorzed vehcle was drven to work. Frst, race and Hspanc status are ncluded. These characterstcs are typcally seen as drectly related to automoble use even when controllng for ncome (16 ). However, n some accounts, once automoble ownershp s controlled for, race effects dsappear (17). In ntal testng, the race varables were not strongly related to personal commercal VMT, but Afrcan Amercan status was negatvely related to the lkelhood of drvng to work even when controllng for transt access. Therefore, the race varables were ncluded as exogenous varables, reflectng the hypothess that cultural dfferences for whch racal/ethnc status s a proxy affect commute mode preferences. Second, a varable denotng the number of cars per drver n the household s ncluded. Recall that the sample s restrcted to those n households wth at least one car per drver. The car per drver varable thus can be nterpreted as ndcatng a household taste for cars nstead of strctly as an ndcator of access to the automoble mode. Fnally, four other varables relatng to transportaton nfrastructure are ncluded to model the commute mode choce: whether the ndvdual must pay to park at work (7% of the sample); a dummy varable ndcatng that the nearest transt stop s more than 2 m from the resdence (17%); dstance to the nearest transt stop (whether bus, streetcar, commuter ral, or subway) for stops wthn 2 m of the resdence; and a dummy varable ndcatng whether streetcar or subway servce s reported avalable, whch may ndcate a hgher-qualty metropoltan transt system (9%). The transt system varables are plausbly exogenous to personal commercal VMT because transt s rarely used for personal commercal trps; the prmary hypothess s that dense, mxed-use workplaces enable walk trps to be substtuted for drvng trps. Hypothess Testng Two methodologcal complcatons arse n testng the relatonshp between workplace land use and personal commercal VMT. Frst, only 52% of respondents on the survey day drve to access personal commercal actvtes (Fgure 1). Because tme s lkely to be scarce on a workday, there s a latent tendency to conserve tme by travelng less for other purposes. At ts maxmum, ths tendency can be expressed only by not travelng at all. As a result, the dependent varable, personal commercal VMT, s left-censored, and estmatng an ordnary least-squares model on the sample wll yeld based estmates. Second, an mportant predctor of personal commercal VMT by workers s whether a car was used to commute to work. However, for workers wth access to cars, the decson to commute to work n turn depends on the planned extent of partcpaton n personal commercal actvtes and the antcpated locaton of those actvtes (Fgure 2). The analyss addresses these complcatons by two approaches. To correct for a based error term due to the censored dependent varable, a Tobt regresson s used. To account for the smultanety of the commute mode choce and personal commercal VMT, a selectvty correcton approach, essentally equvalent to two-stage least squares, s also used. These approaches have rarely, f ever, been used together n the academc lterature. For the purpose of llustraton, a straghtforward Tobt model s estmated frst, n whch personal commercal VMT s specfed as a functon of the ndependent varables. The Tobt s used to account for 50% 45% 40% 35% 30% 25% 20% 15% 10% 5% 0% >15 Personal commercal VMT FIGURE 1 Personal commercal VMT by share of workers.

5 Chatman Paper No Workplace densty and mxed use reduce reduce Automoble commutng ncrease Personal commercal VMT FIGURE 2 Conceptual model. the fact that the dependent varable s left-censored. Second, a jont logt-tobt model s estmated, n whch personal commercal VMT and the choce of whether to drve to work are understood as smultaneous, followng the selectvty correcton procedure n Tran (18). By ncludng ths term n the Tobt model, t s corrected for endogenety. The procedure s smlar to that followed by Gulano and Lave (19). Smple Contnuous Model wth Tobt Correcton The frst model can be thought of as two separately estmated equatons on each of two subsamples: those who dd not drve to work, and those who dd. The dependent varable y s personal commercal VMT. The model can be represented wth an equaton for each subsample: ynca = βxnca + nca ( 1a) yca = αca + βxca + ca ( 1b) where y nca and y ca = vectors of dmenson 1 by n nca and 1 by n ca, respectvely; n ca = subset of ndvduals who drove to work; n nca = subset who dd not drve to work; X s = an n s -by-k matrx of k ndependent varables for n s ndvduals; β=vector of coeffcents to be estmated; α ca = fxed effect of havng a car avalable at work on personal commercal VMT; and s = vectors of normally dstrbuted error terms wth each term equvalent, under the dentcally and ndependently dstrbuted assumpton. Ths model can then be represented n a pooled form: y = γ Z + ( 2) where y = vector of dmenson 1 by m formed by stackng y ca and y nca (where m = n ca + n nca ), = vector of dmenson 1 by m ( ca and nca are stacked to make of dmenson 1 by m) (and ca s assumed to equal nca for all n), and Z = m by (k + 1), composed of X ca and X nca plus a column vector consstng of a dummy varable set equal to 1 for those who drove to work and 0 otherwse. Ths censored dependent varable requres estmaton wth censored regresson technques. The Tobt regresson can be carred out wth the followng form (20) n whch s an observaton subscrpt: y The rato n the second term can be estmated by usng a separate probt regresson to get values of ϕ and Φ, whch are the normal probablty densty functon and cumulatve densty functon, respectvely, evaluated at γ z /σ. The dependent varable n the probt s a dummy equal to 1 f personal commercal VMT s 0 and equal to 0 f personal commercal VMT s postve. The ndependent varable s the same vector used n the ordnary least-squares (OLS) equaton, whose coeffcents are equal to γ/σ. Equaton 3 can then be estmated as an OLS regresson n whch σ s a coeffcent. Stata s Tobt command s used to estmate σ. Ths preprogrammed routne uses a maxmum lkelhood procedure (21, pp. 138, 145, 146), whch s asymptotcally equvalent to estmatng Equaton 3 by ordnary least squares but s more effcent. Jont Dscrete/Contnuous Tobt Model The jont model s sequentally estmated as follows. Frst, a logt regresson s calculated wth a dependent varable equal to 1 f the ndvdual drove to work and 0 otherwse. The logt model uses all the ndependent varables used n the VMT model as well as a number of addtonal ndependent varables, as explaned n an earler secton. In the second step, a selectvty correcton term, E( s ), s added: y = γ z + σ φ + () 3 Φ = βx + E( ) + η ( 4a) nca nca nca nca y = α + βx + E( e ) + η ( 4b) ca ca ca ca ca

6 198 Paper No Transportaton Research Record 1831 where E( s ) s nonzero and vares by person, assumng that the choce of commute mode and how many mles to drve for personal commercal actvtes are related. For each person E( ca ) s estmated by the followng equaton by property of the logt model (18, Equaton 5.11): Pnca P E( ca) = 2 6σ ln ρca π 1 P where σ 2 = varance of, ρ ca = mutual correlaton of car ownershp and mles traveled wth unobserved characterstcs of the person, and P ca = probablty of drvng to work. In the bnomal case, E( nca ) s a symmetrc expresson wth the precedng (18, p. 4), and ρ nca s just 1 ρ ca. The term n brackets s calculated by usng estmated probabltes from the logt model and s therefore stochastc, but t s treated as a nonstochastc ndependent varable n the estmaton of the next step. Therefore, the reported standard errors wll be too small. Smlar to the smple contnuous model, the data are pooled as follows: y = γz + E( ) + η ( 6) In the pooled sample, the bracketed term n Equaton 5 vares dependng on whch subsample the ndvdual belongs to. Followng Gulano and Lave (19, p. 158), ths selectvty correcton term (SEL) can be wrtten as follows: C E( ) ( 1 C) E( ) = SEL ( 7) where C s a dummy varable set equal to 1 for ndvduals n households wth at least one vehcle per drver and 0 otherwse. Then, E( ) = ζ SEL ( 8) where ζ s the coeffcent to be estmated as part of Equaton 6, equal to the term outsde the brackets n Equaton 5. The fnal addton to the model s to add a term reflectng the correcton for left-censorng of the dependent varable. The resultng equaton has two correcton terms: the selectvty correcton term that accounts for the smultaneous choce of whether to drve to work and how much personal commercal travel to carry out by car, and the term correctng for the censored dependent varable: y ca = γ z + ζ SEL + σ φ + η Φ Equaton 9 s estmated by usng Stata s Tobt command. Test of Robustness of Selectvty Correcton Approach nca nca nca + ln Pca ( 5) The selectvty correcton approach s unbased but neffcent because t s sequentally estmated. Its robustness can be determned by assumng that the true model s not endogenous and testng whether the approach mght ncorrectly support the null hypothess. Ths was ( 9) done wth a Monte Carlo smulaton: settng the coeffcent on SEL equal to zero; usng other coeffcents from the emprcal model to predct automoble ownershp and personal commercal VMT; calculatng estmates of the emprcal mean and standard devaton of ζ, the coeffcent on SEL; and testng whether the calculated coeffcent was sgnfcantly dfferent from zero. For a sample of 20 replcatons, the estmated emprcal mean was 0.24, wth an estmated standard devaton of 0.32, yeldng a t-statstc of about Thus, the coeffcent s not sgnfcantly dfferent from zero wth any level of confdence for ths sample. Wth a sample of 50 replcatons, the estmated emprcal mean was 0.36, wth an estmated emprcal standard devaton of 0.363, yeldng a t-statstc of about 1.0, agan statstcally ndstngushable from zero. The results of the test support the fndng that the choce to commute to work by car and the amount of personal commercal VMT are smultaneously determned. MODEL RESULTS Table 3 reports regresson results for the uncorrected Tobt model, n whch the decson to drve to work (D = 1) s treated as an ndependent varable that s unrelated to workplace densty, share of retal, and other ndependent varables ncluded n the model. Controllng for other correlates, the results show that those who drve to work travel, on average, 1.6 m more per day va personal vehcle for personal commercal purposes. Hgher workplace employment densty s assocated wth slghtly reduced personal commercal VMT at a hgh level TABLE 3 Personal Commercal VMT Regressed on Land Use and Demographc Varables (Smple Contnuous Model wth Tobt Correcton) Varable Coef. Std. Err. z P> z 95% Conf. Interval D wtempldn wtndret wtempz dsttowk hbhresdn hbtz hteempdn htndret wktm_tot r_age nohs somecoll ncome ncome ncomez female hh kds hhsze _cons _se (sgma-hat) Log-lkelhood -33, ; 14,478 observatons; lkelhood rato test wth χ 2 (20) = ; Prob > χ 2 = ; Pseudo-R 2 = Observaton summary: 6,998 left-censored observatons at pcdmtot 0; 7,480 uncensored observatons. Note: Bolded varables are sgnfcant at the 95 percent confdence level.

7 Chatman Paper No of statstcal sgnfcance but wth relatvely modest magntude. The coeffcent of 0.05 means that each addtonal 10,000 employees per square mle n workplace densty (an addton of about 15 employees per gross acre) s assocated wth an average half-mle reducton n personal commercal VMT per capta. Although the share of retal at the workplace s also negatvely related to personal commercal VMT, the relatonshp s weak and not statstcally sgnfcant. Other varables n the model have the expected sgns. Hgher resdental densty decreases personal commercal VMT: every addtonal 1.5 housng unts per gross acre (1,000 unts per square mle) s assocated wth a 0.2-m reducton n personal commercal VMT. However, nether employment densty nor the share of retal n the resdental census tract s related to personal commercal VMT n a statstcally sgnfcant sense. Total tme spent at work and the presence of chldren n the household are both assocated wth reduced personal commercal VMT, suggestng that reduced dscretonary tme for such actvtes may be a cause. By the same token, ndvduals n one-person households, wth fewer household responsbltes, travel sgnfcantly more for personal commercal purposes than others. Hgher ncome ncreases personal commercal VMT, wth a declnng ncrease reflected n the sgnfcant coeffcent on the ncome squared varable (ncome2), whle those lvng n older households have lower personal commercal VMT. Fnally, women drve farther than men for personal commercal purposes, whch s generally consstent wth lterature relatng gender roles to household responsbltes. The jont logt Tobt model s presented n Tables 4 and 5. Table 4 presents logt regresson results from the commute choce stage. The dependent varable s a dummy varable set equal to one for those who drove to work. Two of the nstruments have an ndvdually sgnfcant effect on the lkelhood of drvng to work: subway/streetcar avalablty (ral) sgnfcantly decreases the lkelhood, and, unexpectedly, havng to pay to park (paypark) s correlated wth an ncreased lkelhood of drvng to work. Most of the hghly sgnfcant varables n the contnuous model also are mportant n the second model; clearly, there s reason to beleve the choces are endogenous. Table 5 presents results for the second stage, ncorporatng the selectvty correcton term n a Tobt regresson on personal commercal VMT. Taken together, the results from Tables 4 and 5 are nstructve, pontng to two separate effects of workplace densty. Frst, workplace employment densty s assocated wth a lower lkelhood of car commutng, as reflected n the coeffcent on wtempldn n Table 4, whch suggests that each ncrease of 1.5 employees per gross acre (.e., 1,000 employees per square mle) at the workplace decreases the probablty of usng an avalable car for commutng by about 3%. Second, as ndcated n Table 5, workplace employment densty s assocated wth reduced personal commercal VMT regardless of whether a car was used to commute to work. The corrected VMT relatonshp s half as strong as that found by the uncorrected Tobt approach: an ncrease of employment densty of 15 employees per acre (10,000 employees per square mle) reduces per capta personal commercal VMT by 0.25 m. Apparently, much of the nfluence of employment densty on personal commercal VMT s actually ndrect, medated by ts effect on the choce of whether to drve to work. For the same reason, the coeffcent on D (equal to 1 for those drvng to work) has ncreased a great deal n comparson to Table 3, up to more than 9 m per day, reflectng the fact that several nfluences of ndependent varables n the VMT model are medated through commute mode choce. TABLE 4 Car Commute Choce Regressed on Land Use, Demographc, and Instrumental Varables (Logt, Frst Stage of Jont Dscrete/Contnuous Model) Varable Coef. Std. Err. z P> z [95% Conf.Interval] afram asan oth_race Ihh_h_ Ihh_h_ carprdvr parkpay trnsdst trnsdsz farstop ral wtempldn wtndret wtempz dsttowk hbhresdn hbtz hteempdn htndret wktm_tot r_age nohs somecoll ncome ncome ncomez female hh kds hhsze _cons Log-lkelhood ; 14,478 observatons; lkelhood rato test wth χ 2 (30) = ; Prob > χ 2 = ; Pseudo-R 2 = Note: exogenous varables appear n the upper ter. Bolded varables are sgnfcant at the 95 percent confdence level. A smlar effect occurs for housng densty on the resdental sde. An addtonal 1.5 unts per gross acre (1,000 unts per square mle) s assocated wth a 12% lower lkelhood of car commutng, but the drect effect of resdental densty on personal commercal VMT s statstcally ndstngushable from zero. Ths s a very nterestng result t suggests that resdental densty may be more mportant n determnng commute mode choce than n drectly nfluencng overall personal commercal VMT, at least on workdays. Note that the pseudo-r 2 statstc for the VMT models s extremely low, at about Because travel behavor s largely dosyncratc and can vary a great deal on a day-to-day bass, ths s not altogether surprsng. Fnally, respondents mssng data on workplace land use and household ncome are systematcally dfferent from the rest of the sample. Indvduals wth unreported workplace land use data (wtempz equal to 1) are substantally more lkely not to use a car to go to work, and those refusng to report ther ncome travel substantally more mles for personal commercal purposes (see Tables 3 and 5). Ths descrbes a caveat wth the data set that would go unreported f those cases were smply deleted, as s the current practce, and suggests a need for further research, as dscussed n the followng secton.

8 200 Paper No Transportaton Research Record 1831 TABLE 5 Personal Commercal VMT Regressed on Land Use and Demographc Varables (Tobt, Second Stage of Jont Dscrete/Contnuous Model) Varable Coef. Std. Err. z P> z D SEL wtempldn wtndret wtempz dsttowk hbhresdn hbtz hteempdn htndret wktm_tot r_age nohs somecoll ncome ncome ncomez female hh kds hhsze _cons _se (sgma-hat) Log-lkelhood -33, ; 14,478 observatons; lkelhood rato test wth χ 2 (21) = ; Prob > χ 2 = ; Pseudo-R 2 = Observaton summary: 6,998 left-censored observatons at pcdmtot 0; 7,480 uncensored observatons. Note: Bolded varables are sgnfcant at the 95 percent confdence level. CONCLUSIONS 95% Conf. Interval Compared wth a smple contnuous model, the jont dscrete-tobt model provdes a better understandng of how employment land use affects personal commercal VMT. Frst, hgher employment densty s assocated wth a lower lkelhood that a worker wll drve to work, whch n turn s assocated wth lower personal commercal VMT. Second, workplace densty s also drectly assocated wth reduced personal commercal VMT, regardless of commute mode choce. These effects are modest but hghly sgnfcant n a statstcal sense, and there s lttle reason to suspect that the result s an artfact of a selfselecton process, both because the workplace nstead of the resdental locaton s the subject of nvestgaton, and because all workers n the data set have at least one car per drver n ther households. The net effect over a large number of ndvduals n a cty s lkely substantal. Assume that a 1-m 2 area of a cty contans 5,000 employees. The analyss suggests that an ncrease n densty of about 3,000 employees n ths area would be assocated wth a 9% decrease n the automoble commute mode share and a reducton n personal commercal VMT of a bt more than a mle per day per worker, or almost 9,000 VMT per day wthout even accountng for lower VMT from reduced automoble commutng. There may be concomtant ncreases n personal commercal VMT on weekends or by other household members, phenomena that are not nvestgated here. Nevertheless, f nothng else, ths research strongly supports the noton that urban planners should pay attenton to the characterstcs of downtowns, job centers, and other prmarly nonresdental parts of urban areas, because plannng polces n such areas may have stronger nfluences on mode choce than resdentally based nterventons. The results also suggest that resdental densty s correlated wth transt and walkng convenence for the commute but does not drectly nfluence personal commercal VMT. However, because the model does not correct for the potental endogenety of resdental locaton choce, resdental densty, and commute mode choce, the relatvely large effect of the relatonshp wth commute mode choce s probably overstated. Retal share, ntended as a proxy for mxed-use development, does not enter sgnfcantly nto the model on ether the workplace or the resdental sde. Two possble explanatons for ths fndng come to mnd. Frst, the retal employment share may be a poor measure of accessblty of actvtes at the workplace, partcularly f very hgh concentratons of retal shops drve out nonretal actvtes that mght be more valued by workers durng the work day, such as restaurants, banks, and dry cleaners. Second, the strongest effects of development densty may not be related to ncreased accessblty of actvtes va walk and transt. Instead, other forces may be at work. A number of researchers n ths area have noted that development densty n analyss of ths sort may be a proxy for unobserved varables such as metropoltan-wde transt and walkng accessblty, hgh road congeston, and hgh parkng costs (22). Parkng avalablty has been argued to be a partcularly mportant determnant of travel choces (23). Because most of the hgh-densty workplaces n the sample are n places lke New York and Chcago, where these characterstcs are lkely common, such a correlaton may be at work n ths data set. If so, ths analyss suggests that urban polces to relax densty constrants n prmarly nonresdental areas are lkely to be most successful when accompaned by concomtant changes n those assocated factors. SUGGESTIONS FOR FUTURE RESEARCH As noted, the sgnfcant relatonshp found between workplace densty and personal commercal VMT may be partally due to factors correlated wth, or even caused by, nonresdental development densty. Ths suggests several mportant research questons for further examnaton. Frst, to what extent s workplace development densty assocated wth hgher parkng costs, better pedestran walkng envronments, hgher-qualty transt, greater road congeston, or other possble nfluences on travel? Second, assumng such correlatons are strong n U.S. ctes, to what extent do these factors account for the apparent downward nfluence of employment densty on VMT? Fnally, to what extent can one expect the lftng of plannng constrants on nonresdental development densty (such as mnmum floor-to-area rato requrements) to result n ncreases n employment densty along wth these correlated factors? Because the NPTS lacks an explct spatal component, t s mpossble to enrch the data set wth addtonal spatal nformaton below the metropoltan statstcal area level. Replcatng ths research by usng regonal travel dares s essental to understandng the relatonshps more deeply, although some results are lkely to be regon specfc. For example, ths could make t possble to nclude all ndvduals n the data set wth nearby transt access, not relyng on respondents to accurately report such avalablty. Fnally, the treatment of mssng data n ths analyss ndcates a larger problem wth prevous analyss of ths sort that could be addressed n future research. The mssng ndcator method reveals that deletng respondents wthout complete ncome or workplace

9 Chatman Paper No nformaton would sgnfcantly bas the results. More sophstcated methods, such as data mputaton, are preferable when the mssng ndcator method shows that subgroups wth mssng nformaton appear to be dfferent n some way (24, 25). Such treatment of mssng data should be an ntegral part of the future analyss of travel data sets and apparently has rarely been consdered to date. ACKNOWLEDGMENTS Ths research was supported by a Dwght D. Esenhower Graduate Fellowshp from FHWA. Helpful comments on prevous drafts were provded by Kenneth Small, Randall Crane, and three anonymous revewers from the TRB Commttee on Transportaton and Land Development. REFERENCES 1. Boarnet, M. G., and R. Crane. Travel by Desgn: The Influence of Urban Form on Travel. Oxford Unversty Press, New York, Cervero, R. Mxed Land-Uses and Commutng: Evdence from the Amercan Housng Survey. Transportaton Research A, Vol. 30, No. 5, 1996, pp Cervero, R., and K. M. Kockelman. Travel Demand and the 3 Ds: Densty, Dversty, and Desgn. Transportaton Research D, Vol. 2, No. 3, 1997, pp Holtzclaw, J. W. Usng Resdental Patterns and Transt to Decrease Auto Dependence and Costs. Natural Resources Defense Councl, San Francsco, Calf., Boarnet, M. G., and S. Sarmento. Can Land Use Polcy Really Affect Travel Behavor? A Study of the Lnk Between Nonwork Travel and Land Use Characterstcs. Urban Studes, Vol. 35, No. 7, 1998, pp Boarnet, M. G., and M. J. Greenwald. Land Use, Urban Desgn, and Nonwork Travel: Reproducng Other Urban Areas Emprcal Test Results n Portland, Oregon. In Transportaton Research Record: Journal of the Transportaton Research Board, No. 1722, TRB, Natonal Research Councl, Washngton, D.C., 2000, pp Crane, R. On Form Versus Functon: Wll the New Urbansm Reduce Traffc or Increase It? Journal of Plannng Educaton and Research, Vol. 15, No. 2, 1996, pp Cambrdge Systematcs. The Effects of Land Use and Travel Demand Management Strateges on Commutng Behavor: Fnal Report. U.S. Department of Transportaton, Washngton, D.C., Rosenbloom, S. Travel by Women. In Demographc Specal Reports: 1990 NPTS Report Seres. FHWA, U.S. Department of Transportaton, 1995, pp Lave, C. Trends n Our Tmes: An Occasonal Access Almanac. Access, No. 3, 1993, p Psarsk, A. E. Commutng n Amerca II. Eno Transportaton Foundaton, Lansdowne, Va., Orsk, C. K. Why Do Commuters Drve Alone? In Resource Paper for the 1994 ITE Internatonal Conference. Insttute of Transportaton Engneers, Washngton, D.C., 1994, pp Pucher, J. R., T. Evans, and J. Wegner. Socoeconomcs and Urban Travel: Evdence from the 1995 NPTS. Transportaton Quarterly, Vol. 52, No. 3, 1998, pp Myers, D. Changes over Tme n Transportaton Mode for the Journey to Work: Agng and Immgraton Effects. Conference on Decennal Census Data for Transportaton Plannng, Irvne, Calf., Cohen, J., and P. Cohen. Appled Multple Regresson/Correlaton Analyss for the Behavoral Scences, 2nd ed. Lawrence Erlbaum Assocates, Hllsdale, N.J., Johnston-Anumonwo, I. Dstance, Race, and Labor Force Partcpaton: Implcatons for Women of Color. In Women s Travel Issues: Proc., 2nd Natonal Conference. FHWA-PL FHWA, U.S. Department of Transportaton, 1998, pp Taylor, B. D., and P. M. Ong. Spatal Msmatch or Automoble Msmatch? An Examnaton of Race, Resdence and Commutng n U.S. Metropoltan Areas. Urban Studes, Vol. 32, No. 9, 1995, pp Tran, K. Qualtatve Choce Analyss: Theory, Econometrcs, and an Applcaton to Travel Demand. MIT Press, Cambrdge, Mass., Gulano, G., and C. Lave. The Hgh Cost of a Bargan: Wnnng the Rght to Use Part-Tme Transt Drvers. Transportaton Research A, Vol. 23, No. 2, 1989, pp Maddala, G. S. Lmted-Dependent and Qualtatve Varables n Econometrcs. Cambrdge Unversty Press, Cambrdge, Unted Kngdom, Stata Corporaton. Stata Reference Manual, Release 5.0, Vol. 1 (A F). Stata Press, College Staton, Tex., Dunphy, R. T., and K. Fsher. Transportaton, Congeston, and Densty: New Insghts. In Transportaton Research Record 1552, TRB, Natonal Research Councl, Washngton, D.C., 1996, pp Shoup, D. C. Evaluatng the Effects of Cashng Out Employer-Pad Parkng: Eght Case Studes. Transport Polcy, Vol. 4, No. 4, 1997, pp Allson, P. D. Mssng Data. Sage Publcatons, Thousand Oaks, Calf., Rubn, D. B. Multple Imputaton for Nonresponse n Surveys. Wley, New York, Publcaton of ths paper sponsored by Commttee on Transportaton and Land Development.