Robert Cervero

TRAVEL DEMAND AND THE 3DS: DENSITY, DIVERSITY, AND DESIGN

Abstract The built environment is thought to influence travel demand along three principal dimensions —density, diversity, and design. This paper tests this proposition by examining how the ‘3Ds’ affect trip rates and mode choice of residents in the San Francisco Bay Area. Using 1990 travel diary data and land-use records obtained from the U.S. census, regional inventories, and field surveys, models are estimated that relate features of the built environment to variations in vehicle miles traveled per household and mode choice, mainly for non-work trips. Factor analysis is used to linearly combine variables into the density and design dimensions of the built environment. The research finds that density, land-use diversity, and pedestrian-oriented designs generally reduce trip rates and encourage non-auto travel in statistically significant ways, though their influences appear to be fairly marginal. Elasticities between variables and factors that capture the 3Ds and various measures of travel demand are generally in the 0.06 to 0.18 range, expressed in absolute terms. Compact development was found to exert the strongest influence on personal business trips. Within-neighborhood retail shops, on the other hand, were most strongly associated with mode choice for work trips. And while a factor capturing ‘walking quality’ was only moderately related to mode choice for non-work trips, those living in neighborhoods with grid-iron street designs and restricted commercial parking were nonetheless found to average significantly less vehicle miles of travel and rely less on single-occupant vehicles for non-work trips. Overall, this research shows that the elasticities between each dimension of the built environment and travel demand are modest to moderate, though certainly not inconsequential. Thus it supports the contention of new urbanists and others that creating more compact, diverse, and pedestrian-orientated neighborhoods, in combination, can meaningfully influence how Americans travel.

Travel and the Built Environment

Problem: Localities and states are turning to land planning and urban design for help in reducing automobile use and related social and environmental costs. The effects of such strategies on travel demand have not been generalized in recent years from the multitude of available studies. Purpose: We conducted a meta-analysis of the built environment-travel literature existing at the end of 2009 in order to draw generalizable conclusions for practice. We aimed to quantify effect sizes, update earlier work, include additional outcome measures, and address the methodological issue of self-selection. Methods: We computed elasticities for individual studies and pooled them to produce weighted averages. Results and conclusions: Travel variables are generally inelastic with respect to change in measures of the built environment. Of the environmental variables considered here, none has a weighted average travel elasticity of absolute magnitude greater than 0.39, and most are much less. Still, the combined effect of several such variables on travel could be quite large. Consistent with prior work, we find that vehicle miles traveled (VMT) is most strongly related to measures of accessibility to destinations and secondarily to street network design variables. Walking is most strongly related to measures of land use diversity, intersection density, and the number of destinations within walking distance. Bus and train use are equally related to proximity to transit and street network design variables, with land use diversity a secondary factor. Surprisingly, we find population and job densities to be only weakly associated with travel behavior once these other variables are controlled. Takeaway for practice: The elasticities we derived in this meta-analysis may be used to adjust outputs of travel or activity models that are otherwise insensitive to variation in the built environment, or be used in sketch planning applications ranging from climate action plans to health impact assessments. However, because sample sizes are small, and very few studies control for residential preferences and attitudes, we cannot say that planners should generalize broadly from our results. While these elasticities are as accurate as currently possible, they should be understood to contain unknown error and have unknown confidence intervals. They provide a base, and as more built-environment/travel studies appear in the planning literature, these elasticities should be updated and refined. Research support: U.S. Environmental Protection Agency.

Travel and the Built Environment: A Synthesis

The potential to moderate travel demand through changes in the built environment is the subject of more than 50 recent empirical studies. The majority of recent studies are summarized. Elasticities of travel demand with respect to density, diversity, design, and regional accessibility are then derived from selected studies. These elasticity values may be useful in travel forecasting and sketch planning and have already been incorporated into one sketch planning tool, the Environmental Protection Agency’s Smart Growth Index model. In weighing the evidence, what can be said, with a degree of certainty, about the effects of built environments on key transportation “outcome” variables: trip frequency, trip length, mode choice, and composite measures of travel demand, vehicle miles traveled (VMT) and vehicle hours traveled (VHT)? Trip frequencies have attracted considerable academic interest of late. They appear to be primarily a function of socioeconomic characteristics of travelers and secondarily a function of the built environment. Trip lengths have received relatively little attention, which may account for the various degrees of importance attributed to the built environment in recent studies. Trip lengths are primarily a function of the built environment and secondarily a function of socioeconomic characteristics. Mode choices have received the most intensive study over the decades. Mode choices depend on both the built environment and socioeconomics (although they probably depend more on the latter). Studies of overall VMT or VHT find the built environment to be much more significant, a product of the differential trip lengths that factor into calculations of VMT and VHT.

Walking, Bicycling, and Urban Landscapes: Evidence From the San Francisco Bay Area

Some claim that car-dependent cities contribute to obesity by discouraging walking and bicycling. In this article, we use household activity data from the San Francisco region to study the links between urban environments and nonmotorized travel. We used factor analysis to represent the urban design and land-use diversity dimensions of built environments. Combining factor scores with control variables, like steep terrain, that gauge impediments to walking and bicycling, we estimated discrete-choice models. Built-environment factors exerted far weaker, although not inconsequential, influences on walking and bicycling than control variables. Stronger evidence on the importance of urban landscapes in shaping foot and bicycle travel is needed if the urban planning and public health professions are to forge an effective alliance against car-dependent sprawl.

BUILT ENVIRONMENTS AND MODE CHOICE: TOWARD A NORMATIVE FRAMEWORK

Abstract Compact, mixed-use, and walk-friendly urban development, many contend, can significantly influence the modes people choose to travel. Despite a voluminous empirical literature, most past studies have failed to adequately specify relationships for purposes of drawing inferences about the importance of built-environment factors in shaping mode choice. This paper frames the study of mode choice in Montgomery County, Maryland around a normative model that weighs the influences of not only three core dimensions of built environments – density, diversity, and design – but factors related to generalized cost and socio-economic attributes of travelers as well. The marginal contributions of built-environment factors to a traditionally specified utility-based model of mode choice are measured. The analysis reveals intensities and mixtures of land use significantly influence decisions to drive-alone, share a ride, or patronize transit, while the influences of urban design tend to be more modest. Elasticities that summarize relationships are also presented, and recommendations are offered on how outputs from conventional mode-choice models might be “post-processed” to better account for the impacts of built environments when testing land-use scenarios.

Mixed land-uses and commuting : Evidence from the American housing survey

Past research suggests that mixed land-uses encourage non-auto commuting; however, the evidence remains sketchy. This paper explores this question by investigating how the presence of retail activities in neighborhoods influences the commuting choices of residents using data from the 1985 American Housing Survey. Having grocery stores and other consumer services within 300 feet of ones residence is found to encourage commuting by mass transit, walking and bicycling, controlling for such factors as residential densities and vehicle ownership levels. When retail shops are beyond 300 feet yet within 1 mile of residences, however, they tend to encourage auto-commuting, ostensibly because of the ability to efficiently link work and shop trips by car. The presence of nearby commercial land-uses is also associated with relatively low vehicle ownership rates and short commuting distances among residents of a mixed-use neighborhood. Overall, residential densities exerted a stronger influence on commuting mode choices than levels of land-use mixture, except for walking and bicycle commutes. For non-motorized commuting, the presence or absence of neighborhood shops is a better predictor of mode choice than residential densities.

COMMUTING IN TRANSIT VERSUS AUTOMOBILE NEIGHBORHOODS

Abstract In recent years, there has been a chorus of calls to redesign Americas suburbs so that they are less dependent on automobile access and more conducive to transit riding, walking, and bicycling. This article compares commuting characteristics of transit-oriented and auto-oriented suburban neighborhoods, in the San Francisco Bay Area and in Southern California. Transit neighborhoods averaged higher densities and had more gridded street patterns compared to their nearby counterparts with auto-oriented physical designs. Neighborhoods were matched in terms of median incomes and, to the extent possible, transit service levels, to control for these effects. For both metropolitan areas, pedestrian modal shares and trip generation rates tended to be considerably higher in transit than in auto-oriented neighborhoods. Transit neighborhoods had decidedly higher rates of bus commuting only in the Bay Area. Islands of transit-oriented neighborhoods in a sea of freeway-oriented suburbs seem to have negligible ...

Travel choices in pedestrian versus automobile oriented neighborhoods

The New Urbanism movement calls for redesigning American neighborhoods so that they are less oriented toward automobile travel and more conducive to walking, bicycling, and transit riding, especially for non-work trips. New Urbanism calls for a return to compact neighborhoods with grid-like street patterns, mixed land uses, and pedestrian amenities. This paper investigates the effects of New Urbanism design principles on both non-work and commuting travel by comparing modal splits between two distinctly different neighborhoods in the San Francisco Bay Area. The neo-traditional neighborhood, Rockridge, and the nearby conventional suburban community, Lafayette, were chosen as case studies because they have similar income profiles, freeway and transit service levels, and geographical locations. Rockridge residents averaged around a 10 percent higher share of non-work trips by non-automobile modes than did residents of Lafayette, controlling for relevant factors like income and transit service levels. The greatest differences were for shop trips under one mile. Modal splits were more similar for work trips, confirming the proposition that neighborhood design practices exert their greatest influence on local shopping trips and other non-work purposes. For work trips, compact, mixed-use, and pedestrian-oriented development appears to have the strongest effect on access trips to rail stations, in particular inducing higher shares of access trips by foot and bicycle.

Influences of Built Environments on Walking and Cycling: Lessons from Bogota

ABSTRACT Bogotá, Colombia, is well known for its sustainable urban transport systems, including an extensive network of bike lanes and set-aside street space for recreational cyclists and pedestrians on Sundays and holidays, called Ciclovía (“cycleway”). This paper examines how such facilities along with other attributes of the built environment—urban densities, land-use mixes, accessibility, and proximity to transit—are associated with walking and cycling behavior as well as Ciclovía participation. We find that whereas road facility designs, like street density, connectivity, and proximity to Ciclovía lanes, are associated with physical activity, other attributes of the built environment, like density and land-use mixtures, are not. This is likely because most neighborhoods in built-up sections of Bogotá evolved during a time when non-automobile travel reigned supreme, meaning they are uniformly compact, mixed in their land-use composition, and have comparable levels of transport accessibility. Thus facility designs are what sway nonmotorized travel, not generic land-use attributes of neighborhoods.

Polycentrism, commuting, and residential location in the San Francisco Bay area

The San Francisco Bay Area has taken on a distinct polycentric metropolitan form, with three tiers of hierarchical employment centers encircling downtown San Francisco, the regions primary center. In this paper it is found that polycentric development is associated with differentials in suburban and urban commute trip times: commute trips made by employees of suburban centers are shorter in duration than commute trips made by their counterparts in larger and denser urban centers. Differentials were even greater, however, with respect to commuting modal splits. Lower density, outlying employment centers averaged far higher rates of drive-alone automobile commuting and insignificant levels of transit commuting. Smaller, outlying centers were also the least self-contained, with a large number averaging twenty or more times as many external as internal commutes. The effects of housing availability and prices on the residential locational choices of those working both in urban and in suburban employment centers are also investigated in this paper. Locational choices are stratified by occupational class and type of center. High housing prices in and around employment centers were found to displace workers to residences in other subregions, except in the case of professional workers in fast-growing, outlying centers. These workers were attracted to higher-priced nearby housing. In the empirical analysis, significant segmentation in housing choices among workers in fast-growing suburban centers was found. This could be partly due to selective land-use policies implemented by local governments in these areas.