6 Transportation, Accessibility, and the Built Environment

Since the late 1950s, the hierarchy of mobility and speed has been central to transportation engineering and planning. This focus emerged in response to the rapid post-WWII growth in automobile ownership, the need to address congestion, and the goal of ensuring the swift movement of people and goods. Alongside Level of Service (LOS) performance standards, which define road speed performance based on functional classification, the four-step modeling process (discussed in Chapter Two) institutionalized a planning and decision-making framework centered on reducing congestion. This approach aimed to maintain adequate speed LOS through continuous road capacity expansion. As a result, for more than 50 years, the planning, design, and maintenance of the road network—often considered the connective tissue of the built environment—have prioritized fast, motorized mobility over accessibility and slower modes of transportation. This has contributed to the development of expansive, automobile-oriented development patterns and urban forms that favor regional over local accessibility.

Accessibility: the link between transportation and the built environment

Accessibility or access, first introduced in Chapter One and further explored in Chapter Four within the context of transportation equity, serves as the conceptual link between transportation and land use within the built environment. It is commonly defined as the ease with which opportunities—such as jobs, services, entertainment, or any activity—can be reached. This ease of access depends on various factors, including the mode of travel, the available transportation networks (ranging from trails and sidewalks to highways, rail and air and water systems), the spatial distribution of activities, and increasingly, virtual access. Accessibility can be considered at both local and regional levels.

Local accessibility, or local access, refers to how easily people can reach nearby destinations, such as stores, pharmacies, schools, and restaurants, within their neighborhood. The closer these destinations are to residents, the more accessible they are. Local accessibility is influenced by factors like proximity, a mix of land uses, street network connectivity, and the availability of pedestrian and bike infrastructure. The concept is not new; it has been at the core of the neighborhood unit movement of the 1950s, revived by the New Urbanist movement of the 1990s, and recently repackaged in the idea of a 15-minute city (Moreno et al., 2021), drawing controversy (Mouratidis, 2024). This idea is also reflected in many older, dense, “walkable” American neighborhoods, where most daily destinations, including internet access, are reachable within a 15- or 20-minute walk (Figures 6.3 and 6.4).

Table 6.2 summarizes the percentage change in network accessibility before and after a road improvement, as shown in the last column on the right. Since accessibility is measured in travel time, lower numbers indicate greater accessibility, assuming all other factors are equal. Each row’s sum represents the network’s accessibility for a specific node, calculated by adding the travel times from that node to all other nodes in the network.

The road improvement, which reduces travel time between nodes B and C, enhances accessibility across the entire network. This accessibility improvement is reflected in the “percent change” column, which shows the difference in row sums before and after the network change. A negative sign indicates a reduction in travel time, with the largest accessibility gains occurring at nodes B and C, followed by nodes A and E. Conversely, factors like traffic delays, road construction, or frequent road accidents between two nodes would increase travel time, thereby reducing accessibility for those nodes and the overall network.

Accessibility, travel time savings, and the built environment

Since spatial interaction tends to increase with improved accessibility, land use activity is likely to grow at nodes that benefit the most from these gains. These locations often attract development and businesses, leading to economic growth (assuming all other factors are equal). However, this pattern doesn’t always hold true for every city or region, as it depends on specific conditions within the broader regional economy. While transportation improvements that enhance accessibility are one of several factors influencing land use and the location of firms and households, they alone do not guarantee widespread economic development or significant changes in the built environment. Such improvements are typically a necessary condition for development—capitalized in higher land values and real estate markets—but they are not sufficient on their own. In some cases, particularly depending on regional economic conditions, enhanced accessibility through fast mobility in certain parts of the transportation network may simply lead to a redistribution of economic activities toward the urban fringe, where land and development costs are lower (Giuliano 2004).

Since the 1960s, this trend has been evident in the U.S. and later in many developed countries, where rising car ownership and the expansion of extensive highway and road networks have made regional accessibility gains commonplace. These changes have drawn activities and city life away from older areas that once thrived on high local pedestrian accessibility (Silva and Altieri, 2022). Gains in accessibility from faster travel and time savings have enabled households and businesses to reach evermore distant locations easily, furthering decentralization (suburbanization) of urban activities.

As a result, megastores, outlet malls, car dealerships, food and hotel franchises, and large power retail centers (ranging from 200,000 to 800,000 square feet) have proliferated along beltways, interstates, and state highways at the urban fringe of most American cities. This type of freeway- or highway-oriented development, especially by big-box retailers like Walmart (Figure 6.9, 6.10), Target, and Home Depot, capitalizes on their close proximity to highways, which allows for lower shipping costs and better supply chain integration with their distribution centers (Ostrander, 2011). Besides enabling a culture of fast driving and road rage (Audirac 2008), the trade areas of these centers draw customers from 10 to 20 miles at the peri-urban fringe and five to 10 miles in the suburbs. Designed for cars, power centers have higher parking ratios than suburban malls, and “window shopping” involves driving from store to store. Many tenants offer both online and offline shopping options, with curbside pickup providing a complete drive-in experience (Christmann, 2024). Although more regulated and less extensive in Europe, hypermarkets—the equivalent of supercenters—have also proliferated at the edges of cities, with some established within city limits.

Land use planning policies, such as urban growth boundaries or green belts, have been effective in slowing the rate of suburbanization. Conversely, the construction of outer ring roads (beltways) to relieve congestion has led to further decentralization and longer driving distances. Key findings from their review of research and modeling include:

• Reducing Car Use: Policies aimed at reducing car modal share and decreasing travel time and distance can only succeed if they make driving more expensive and less attractive.

• Urban Density: Land use policies that aim to increase urban density or mixed-use development will have little impact unless driving becomes more expensive or slower. People will continue to drive longer distances to maximize opportunities within their travel cost and time budgets.

• Transport Policies: Policies that make driving less attractive (more expensive and slower) are the most effective in reducing travel distances and car usage, particularly in areas where settlement patterns are not too dispersed.

• Large Retail and Leisure Facilities: These developments increase driving distances and car usage. Policies that prevent such developments are more effective at influencing travel behavior than those promoting high-density, mixed-use development.

• City Centers: Policies that restrict car use in city centers have not been shown to harm the economic vitality of those centers, except where massive retail development has been permitted on the outskirts.

• Public Transit: Policies that make public transit more attractive have not significantly reduced car travel. While they have spurred some development near transit stations, they have also contributed to further suburbanization.

• Effectiveness of Policies: Overall, transport policies are far more direct and effective in influencing urban travel behavior than land use policies. (Wegener and Fürst,1999, 2004, p.83-84).

Transportation and built environment research

The relationship between the built environment (BE) and transportation (T) in the U.S. has been researched in two primary ways: the impact of transportation on the built environment (T → BE) and the influence of the built environment (land use) on travel behavior (BE → T).

While both research traditions (T → BE and BE → T) complement each other, they have different histories, planning contexts, and scales of analysis. T → BE research originated in the 1950s and 1960s, focusing on predicting changes in transportation demand to help transportation engineers and policymakers decide where to build new roads and rail lines to address congestion and improve regional accessibility. In contrast, BE → T research gained prominence in the 1990s, driven by the New Urbanism and Smart Growth movements. These movements emphasized land-use policy as tools for enhancing local accessibility through compact, mixed-use developments, aiming to reduce driving and increase transit use, particularly through transit-oriented development (TOD), as discussed in Chapter Five, as well as promoting active transportation like walking and cycling. This shift toward local accessibility also led to a move away from traditional census tracks and TAZ-aggregated modeling and research, favoring disaggregated models that focus on individuals’ travel behavior.

This section will first highlight key findings from T→ BE research before exploring insights from BE→ T research, which will serve as a segue into the final section on the accessibility paradigm in transportation.

Transportation effects on the built environment:  T → BE

As discussed in Chapter Two, trip-based modeling (four-step model) was developed to forecast travel demand and traffic flows during the construction of the Interstate Highway System (IHS). By the late 1960s, rising opposition to highway construction, as mentioned in Chapter Four, along with growing environmental concerns, led to increased federal investment in public transit as an alternative to driving. Consequently, cities like San Francisco, CA (BART), Atlanta, GA (MARTA), and Washington, DC (METRO) began developing heavy rail transit systems, while light rail projects were initiated in cities such as San Diego, CA, Portland, OR, and Buffalo, NY.

Multiple waves of T→ BE studies have examined the impacts of new highways and transit systems on the built environment, often using changes in land values as a proxy for land-use change. For highways, the first generation of studies occurred in the 1950s and 1960s, followed by a second wave in the 1970s and 1980s. For rail transit, the initial wave of heavy rail construction took place in the 1960s and 1970s, with light rail development in the 1970s and 1980s, leading to rail studies spanning the 1970s through the 1990s (Giuliano, 1989; Ryan, 1999). A third wave of highway and public transit studies emerged in the 1990s and early 2000s, driven by sustainability concerns related to highway-land use impacts on urban sprawl, increased driving due to improved fuel efficiency, and the associated rise in greenhouse gas emissions linked to vehicle miles traveled (VMTs).

Early studies on the impact of highway construction on land use showed significant effects, but these findings were primarily limited to the first wave of highway development. In contrast, the effects of rail transit have generally been minimal or confined to areas within a 1/2 to 1.5-mile radius around stations. The initial construction of the Interstate Highway System (IHS) greatly enhanced accessibility in metropolitan areas, spurring nationwide suburbanization of both residential and industrial areas in the post-WWII era. However, as motorized accessibility became more widespread, the relative impact of new highway and rail infrastructure on accessibility diminished (Giuliano, 1989; Giuliano, 2004).

Later research has produced mixed and sometimes contradictory results regarding the effects of transportation infrastructure on land use. A review of a selected group of intraregional (within a single metropolitan area) and regional (encompassing a state or several metropolitan areas) empirical studies—chosen for their robust methodology, high-quality data, and innovative research approaches—also found varied outcomes (Giuliano and Agarwal, 2017). Regional studies indicated that while highway investments did have an impact on land use, broader economic factors played a more significant role. Intraregional studies suggested that the significant impacts of highway investments on land use varied greatly depending on the local context of cities within the metropolitan region (Giuliano and Agarwal, 2017).

The effects of rail transit on land use were also found to be highly variable, depending on the specific city and its social and economic context. For example, the Bay Area Rapid Transit (BART) system had a notable impact on real estate values in San Francisco’s Central Business District (CBD) during its early years (1975-1992), contributing to an already thriving economy (Cervero and Landis, 1997). However, Oakland’s CBD, despite being the most accessible node in the BART network, did not experience the same level of impact due to its different economic and social makeup. Light rail systems primarily affected land values within the immediate vicinity of stations, and some of these increases occurred in anticipation of the rail line’s construction (Giuliano 2004). Table 6.3 provides a summary of these findings on the effects of transportation infrastructure on the built environment.

THE ACCESSIBILITY PARADIGM

While the 5-Ds framework has become a widely accepted approach for understanding the relationship between land use and travel, it has limitations, particularly in addressing accessibility. Handy’s (2018) critical commentary on “Ds” research (Box 5.# in Chapter Five) highlights its shortcomings, noting that it lacks a behavioral theory to explain travel behavior and overlooks the interdependent nature of the “D” variables through accessibility. By treating the “D” variables as independent, “Ds” research neglects “place accessibility,” which involves the distribution of destinations, the ease of reaching them by various modes, and the nature and intensity of activities at those destinations (p. 2). Simply put, accessibility—defined as the range of travel options the built environment offers individuals, considering the cost of overcoming distance—is viewed by some transportation researchers (Handy, 1994, 2018, 2020; Cervero, 1995, 2005; Litman, 2024) as an appropriate focus for BE→T research and central to shifting transportation thinking and practice from mobility to accessibility.

The accessibility paradigm represents a shift from focusing on mobility as the primary framework and performance measure of transportation systems to emphasizing accessibility. While the concept of accessibility has existed since the 1920s, its modern understanding emerged in the 1950s with Hansen’s (1959) mathematical formulation and gained further traction in the 1970s as a tool for evaluating transportation equity (Wachs and Kugmani,1973) and as a substitute for mobility and speed measures (Morris et al.,1979). These early works argued against using mobility and speed as central transportation measures, as they overlook the fundamental purpose of travel: reaching destinations and participating in activities—reflecting the derived nature of travel demand (Levine, 2020). By the 1990s, accessibility re-emerged, emphasizing local accessibility (proximity) as the rationale behind compact, mixed-use New Urbanism and transit-oriented development (TOD). These approaches aligned with sustainability goals aimed at limiting urban sprawl, reducing vehicle miles traveled (VMT), and promoting health and social equity, forming what Levine (2020) calls the smart growth and urbanist policy agenda. The shift from mobility to accessibility has also been driven by the growing recognition that congestion mitigation through road expansion—summed up in the phrase “we can’t build our way out of congestion”—is both ineffective, as it induces more travel (EPA, 2002), and is increasingly unaffordable (Handy, 2020). Despite California’s recent efforts to replace level of service (LOS) performance measures with an accessibility-based approach (Handy, 2020), the adoption of such practices remains slow across the U.S.

Barriers to accessibility-based transportation planning

At least three views explain this delay. The first view emphasizes three reasons: the presence of competing transportation concepts, such as “universal accessibility” related to transit services for persons with disabilities; the conflation of “mobility” and “accessibility” in policy documents, from legislative acts like TEA-21 to the FAST Act, and in metropolitan planning organizations’ long-range plans (see Proffitt et al., 2019); and accessibility measures being more complex to calculate and communicate to non-experts, in addition to their lack of standardized guidelines, making them more difficult to adopt when compared to mobility measures (Handy, 2020). The second viewpoints to the entrenched institutional focus on mobility measures, which has resisted the viability of accessibility-based concepts. This view also points to the “urbanist” proximity-based policy agenda that prioritizes local accessibility (walking, cycling, and transit) over regional accessibility (faster motorized modes), thereby hindering changes in how transportation success is evaluated (Levine, 2020, p. 4).

The third perspective highlights the widespread appeal of automobility among the public and policymakers, and the enduring influence of the Motor Age, including its extension into the current and future Smart City concept (as discussed in Chapter Eight). This view points out how this appeal often leads to an underestimation of the negative impacts of car dependency, such as public health issues, sustainability challenges, social equity concerns, and extensive urban decentralization (Filippi, 2024).

A proposed accessibility-based model for cities

Filippi (2024) compares three evolving transportation paradigms observed in cities worldwide: North American automobility, European multimodality, and the emerging accessibility paradigm (Figure 6.11). While Western European cities are often viewed as models for American cities aiming to balance local and regional accessibility through extensive transit services and real-time intermodal integration, car travel still holds a high modal share in these cities, albeit with differences among the 13 EU countries shown in Table 6.4. For example, car travel represents the largest share of total distance traveled per person per day in Italy (74%) and Slovenia (81%), while Poland (58.8%) and the Netherlands (62.2%) have the lowest car shares. As a result, most European cities have experienced rising greenhouse gas emissions and worsening congestion (Filippi, 2024).

According to Filippi, the accessibility vision for EU cities, drawing ideas from European and American proposals, and inspired by Paris’ 15-minute cities and Barcelona’s superblocks, involves a triple-access system that integrates three different types of accessibility: digital connectivity, active transportation through local accessibility (spatial proximity), and regional accessibility via multimodal motorized transportation, see Table 6.5.

• Digital Connectivity: Encompasses remote work, online education, shopping, banking, telemedicine, etc., and digital intermodal integration, as discussed in Chapters Eight and Five, respectively.

• Active Transportation System: Utilizes spatial proximity concepts, like the 15-minute city or 20-minute neighborhood, to enable access to local amenities within walking or biking distance. Through mobility hubs, this system serves as a feeder to public transit networks, making cars an option rather than a necessity.

• Multimodal Transportation System: Focuses on quality transit networks and shared mobility services to provide access to regional destinations, such as city centers (CBDs), subcenters, and other neighborhoods.

Table 6.5 Accessibility systems.

Note: Adapted by authors from Filippi (2024)
Accessibility System	Connectivity Infrastructure	Destination Interactions	Attractors
ICT /Digitized	Digital, IP, IOTs, intelligent transportation, multimodal integration	Online, real-time information	Online work, education, shopping, banking, telehealth, e-commerce, etc.
Active transport (local)	Micromobility, walking, cycling, local streets, sidewalks.	Spatial proximity (15-20 minute), local accessibility,	Neighborhood retail, services, and amenities
Motorized transport (regional)	Bus, shared demand response, streets, mobility hubs, network stops	Sector of a city, 15 to 30 min.	Centers, corridors
Metro, LRT, BRT, TOD, Rail and dedicated line networks.	City, metro area < 60 min	CBD, airport, transit stations, tourist sites, universities, shopping centers.
Commuter train, TOD Rail network, and stations	Region > 60 min	CBDs, subcenters, suburbs

Figure 6.13 above contrasts the current mobility-based four-step model, discussed in Chapter Two, with a proposed accessibility-based model, as outlined by Filippi (2024). Instead of forecasting travel demand, this new model begins by formulating a desired future scenario, developed with input from the public and stakeholders. It then backcasts to the present, identifying the policies and projects needed to achieve that scenario. The transportation plan is evaluated based on accessibility.

In this model, the zones from the four-step model no longer function as trip attractors or generators. Instead, they represent the first level of access to services and activities, classified into three levels: single main centers (within a 60-minute motorized travel shed), multiple subcenters (within 30-minute motorized travel sheds), and neighborhood centers (15-minute walking sheds). The 15-minute neighborhood zones serve as the model’s basic unit, prioritizing local accessibility at walking and cycling speeds and favoring public transit over automobiles for regional travel beyond the local neighborhoods. A gap analysis comparing current and desired accessibility levels to key destinations guides the development of strategies aimed at maximizing accessibility while minimizing the need for mobility.

Filippi (2024) acknowledges that this paradigm is still in its infancy, requiring significant coordination between transportation and land use planning institutions to overcome the entrenched mobility-based paradigm. However, the transition should proceed through pilot studies, trials, and experimentation to avoid overwhelming stakeholders. Establishing new accessibility standards and indicators to replace traditional tools like the four-step model should be a priority in advancing this new paradigm.

Transformation or reform in accessibility shift proposals?

Recently, there has been a surge in the development of accessibility metrics and tools. Litman (2024, pp. 46-51) provides extensive information and links to websites dedicated to this purpose. And while there is broad international agreement among academics that a shift to an accessibility-based paradigm is necessary, there is still no consensus on a framework and on how it would be done. Despite several initiatives to develop practitioner-friendly frameworks and metrics—such as the  Accessibility Observatory at the University of Minnesota, the publication of accessibility guides like the  NCHRP Report 1000 Accessibility Measures in Practice: A Guide for Transportation Agencies, SSTI’s Measuring Accessibility, and the University of Sydney’s Committee of the Transport Access Manual —the adoption of accessibility measures by practitioners remains limited.

This limited uptake suggests a need for simpler accessibility indicators, such as the number of stores within a half mile of home, that directly relate to built-environment qualities and the ease of reaching destinations. According to Handy (2020), combining these straightforward indicators with a robust accessibility framework would help facilitate the shift and promote the diffusion of the accessibility paradigm.

However, proponents of the shift towards accessibility differ in goals, priorities, and degree of integration with the current mobility system. Martens (2020) classifies proposals into radical, limited, and moderate, summarized by the authors in Table 6.6.

Table 6.6 Three accessibility paradigm shift approaches.

	Radical Shift (Handy, Cervero, Litman, Filippi)	Limited shift (Handy)	Moderate shift (Levine, Grengs, and Merlin)
Mobility system’s problem	Congestion mitigation via capacity expansion induces more travel, reinforces automobility.	Level of services, traffic speeds, travel time savings and performance metrics prioritize speed and automobility.	Fails to recognize mobility (travel) as derived demand. People travel to reach destinations, not for the sake of movement.
Accessibility system’s goals/purpose	Instrumental for overcoming mobility system’s environmental and social externalities (traffic safety, GHG pollution, urban sprawl, obesity, equity,) building livable communities expanding modal choice.	Reforming mobility system’s metrics and performance indicators toward enhancing accessibility likely to overcome mobility system’s externalities.	Increasing people’s capacity to reach destinations. Accessibility: an inherent system’s goal and benefit based on derived demand. Accessibility a goal among many (education, employment, housing)
Accessibility system’s         priorities	Reduce need to travel; more locally accessible land use development; improved ICT integration. 15-minute city; non-motorized transport and public transit.	Accessibility performance indicators and less road capacity expansion; more public transit enhancement, travel demand management, and road and parking pricing	Mobility along with proximity and connectivity (virtual or physical) are means for accessibility. But mobility and proximity interaction can have (+ or -) effects on accessibility. No particular urban form favored, nor mode.
Accessibility planning approach	Comprehensive planning balancing trade-offs between land use and transportation policies with focus on level of service of the whole metropolitan area, rather than just of the transportation system.	Existing mobility system’s planning incorporates accessibility performance indicators primarily in the evaluation phase.	Accessibility evaluative framework and metrics integrated at all phases of transportation and land use planning. Public input for evaluating projects’ outcomes: do they increase or degrade accessibility? Do they advantage mobility by some modes versus others? Do they render its distribution more or less equitable?”
Note: Table by authors based on Martens (2020).

Though there is convergence in the critique of the current mobility system, there is also divergence among proposals for the new system. Those questioning “radical” models like Filippi’s proposal raise concerns about whether this change can achieve broader societal goals, such as sustainability and health improvements. They argue that prioritizing local accessibility and walkability in neighborhoods that are already car-dependent for regional travel will likely result in minimal reductions in driving and vehicle miles traveled (VMT), as people will continue to rely on cars to reach their destinations (Levine et al., 2019).

 

Rather than imposing broad societal goals on accessibility frameworks without certainty of outcomes, some suggest that these models should focus on the derived nature of travel and on accessibility as an inherent goal of transportation systems. This would shift the core question from “How fast can people travel?” to “How many places can people reach?” (Levine et al., 2019).

Others, like Martens (2020), argue that focusing solely on accessibility is insufficient because it overlooks the needs of people. Martens advocates for a principle of justice that emphasizes “sufficient accessibility for all” as the fundamental goal of transportation systems. He contends that it is the government’s responsibility to ensure that everyone has adequate access to essential services and opportunities. In this people-centered approach, transportation planning would be redefined as the “field of government intervention that seeks to guarantee sufficient accessibility for all” (Martens, 2020, p. 20). Success would be assessed by the system’s ability to provide sufficient accessibility to everyone, rather than by its ability to alleviate congestion, which is the current standard. This approach would focus on solutions for groups lacking sufficient accessibility.

 

Readers looking to deepen their understanding of how to quantify and assess accessibility, and how it applies to evaluating the performance of transportation and land use patterns, are encouraged to explore the “Concepts,” “Uses,” “Planning,” and “Measures” sections of the Transport access manual: A guide for measuring connection between people and places  by David Levinson and COTAM (2020).

CONCLUSION

This chapter examined key concepts linking transportation and the built environment, emphasizing the U.S. transportation system’s focus on motorized mobility and speed. This approach is reflected in the FHWA’s road network classification, Level of Service (LOS) standards, and four-step travel demand modeling. Accessibility emerged as a critical link between transportation and land use, differentiating local and regional accessibility and illustrating the feedback cycle between transportation, land use, and travel activity. A hypothetical road network example highlighted how travel-time savings drive urban decentralization and automobile-oriented growth at city edges, exemplified by big-box power centers along highways.

Research on transportation’s impact on the built environment (T→BE) confirmed that highway investments have fueled urban decentralization in the U.S., with economic activities shifting to city edges—a trend that began in the 1920s, accelerated in the 1950s and 1960s, and continues today. Similarly, increased driving in European cities has improved regional accessibility but diminished local accessibility, particularly in city centers. In contrast, built environment research (BE→T) has identified five factors—density, diversity, design, destination accessibility, and distance to transit (the “5 Ds”)—that promote active transportation and reduce driving. However, this framework has been criticized for neglecting accessibility as the core link connecting these factors and for shifting focus away from the needed transition to an accessibility-based approach.

While the accessibility paradigm seeks to address the failures of the current mobility-focused system, its adoption has been slow due to institutional inertia, conflicting proposals, and the challenges of reshaping car-dependent urban forms built over decades of investment in road infrastructure.

Readers are encouraged to follow the evolution of the emerging accessibility paradigm by exploring the links to new accessibility metrics and guidance provided in the chapter.

QUIZ