1 Introduction to Transportation Land-Use Modeling

Chapter Overview

This chapter introduces a land use and transportation modeling framework and the need for such planning approaches in major metropolitan areas. It then proceeds to describe the basic components of transportation land-use integrated models and the fundamental concepts connecting transportation and land use, such as mobility and accessibility. Next, it elaborates on important transportation policy and planning concerns including externalities and transportation equity along with macro factors that can intermittently or continually influence transportation and land-use patterns. Finally, it closes with an example of an integrated land-use transportation model developed and implemented at the state level.

A brief introduction to land-use transportation modeling

A model serves as a simplified representation of reality. It is a necessary tool because planning agencies cannot rely solely on trial and error to evaluate the effectiveness and consequences of their programs and policies in the real world. Therefore, models are utilized to simulate and forecast the reciprocal impact that changes in land use have on transportation facilities and the effects that changes in transportation facilities exert on land use and development patterns (Heyns & Van Jaarsveld, 2017).
The control and planning of land use lie at the core of planning practices in many countries. This control significantly affects the built environment and the demand for urban services, including transportation. Land use and transportation models focus on understanding the numerous relationships between land use change and transportation. Changes in land use directly influence the location choices of households, businesses, and economic activities of firms, thereby affecting the number of trips, choice of destinations, and mode of transport (Waddell, 2011). On a regional and spatial scale, land use change can impact the fairness of transportation infrastructure and the distribution of benefits and disadvantages of such services (Litman, 2017; Sharifiasl et al., 2023). Figure 1.1 presents a simplified depiction of the interrelationships between land use and transportation.

In Box 1.1, modelers Kazim Oryani and Britton Harris explain that early models operated under the assumption that land uses (residential, commercial, industrial, etc.) were fixed. These 1950s models projected transportation service demand, such as identifying congested roads and determining the need for new lanes, based on this assumption. However, over the following two decades, it became evident that changes in transportation facilities by affecting accessibility also influence land use changes. For instance, developers might purchase land in anticipation of a new road extension, or the closure of a road might prompt businesses to relocate, thus altering traffic patterns. This realization underscored the importance for transportation and land use modelers to consider the dynamic feedback loop between transportation facilities, land use, and travel behavior. Consequently, this led to the development of transportation-land use interaction models.

Core concepts in transportation land-use modeling

Locational accessibility, which refers to the ease of reaching destinations, serves as a vital link between land use and transportation. Figure 1.2 provides a comprehensive overview of the elements involved in modeling land use and transportation within a city or metropolitan area.

On the left side of the diagram, non-transportation factors like socio-demographic characteristics and employment trends of the population influence the demand for floor space in diverse types of land use, such as residential, commercial, office, or industrial spaces. These demands, shaped by public planning and local land-use regulations, affect the availability of land floor space supply and the physical arrangement of the built environment. Ultimately, this determines where residential areas or workplaces are situated, significantly impacting travel demand, like daily commuting, and the need for transportation infrastructure.

Meanwhile, on the right side of the diagram, travel demands originating from households and businesses create trips and traffic flows, which are assessed in terms of speed, volume, and cost. These demands are influenced by multiple factors such as vehicle ownership rates, road network capacities, and fuel costs. The dynamic interplay between the demand for trips and the available supply is typically captured by suitable models simulating road traffic, incorporating various trip purposes and destinations.

The model depicted in Figure 1.2, developed by Southworth (1995), primarily aimed at environmental analysis, particularly assessing policies aimed at reducing fuel consumption and emissions. However, the foundational structure of this model, which is further elaborated in Chapter 6, originated from Ira Lowry’s Model of Metropolis (1964). Lowry’s model operated under the assumption that the place of employment determined the place of residence. It focused on predicting and simulating the locational choices made by residents and workers in the industry and service sectors, integrating these decisions into a gravity framework inspired in physics for transportation and land-use modeling. Although Lowry’s model greatly simplified metropolitan dynamics, its logical simplicity and computational ease empowered practitioners and influenced subsequent generations of models.

Mobility and accessibility

Transportation modeling serves as a complex digital representation of the intricate connections between urban transportation and land-use systems. Transportation modeling captures the demand for travel and the intricate movement patterns resulting from people’s travel choices to access various destinations. Consequently, two core concepts, mobility, and accessibility, underpin transportation models. Accessibility, as defined by Cervero et al. (1999), refers to the ease of reaching potential destinations, which depends on the available opportunities—like employment or grocery stores—within a specific travel time or distance, such as the commute to work.

Accessibility measures quantify the relationship between land use and transportation indicating location choices. For instance, firms tend to favor locations with improved access to labor markets, while households prioritize proximity to employment opportunities and amenities like grocery stores and markets (Iacono, 2007). Hansen (1959) applied the gravitational concept from physics to develop the gravity model of accessibility, quantifying the attractiveness of a location. This model assumes that employment significantly shapes where households choose to live, defining accessibility based on the spatial interaction between employment centers and residential areas (Briassoulis, 2020).

Mobility, defined as the distance traveled within a specific time period, differs from accessibility in that it solely focuses on the total distance covered, without considering specific opportunities or points of access. Figure 1.3 illustrates how accessibility and mobility interact within the hierarchy of the American roadway system. For instance, local roads and collectors are highlighted in purple for their higher accessibility, providing numerous opportunities to reach various destinations, including multiple road intersections, driveways, parking lots, and residential areas, albeit at lower speeds. In contrast, arterials, freeways, and limited access highways (like interstate roads), highlighted in yellow, offer better mobility due to their higher speed limits. However, these routes have fewer access points and connections to destinations of interest, such as employment centers and shopping locations.

Equity

Equity, a fundamental principle in transportation planning, refers to the fair and impartial distribution of transportation benefits, costs, and services across all segments of society. It revolves around the public policy goal of ensuring equal and unbiased access to transportation infrastructure, facilities, and services, irrespective of factors such as income, race, gender, age, or ability.

Some essential inquiries pertaining to transportation equity and planning include:

• Who reaps the advantages of enhanced accessibility within the transportation system?
• Who bears the negative impacts resulting from the design of transportation policies or systems?
• To what degree are people’s travel patterns influenced by their choices or limitations?
• How are the advantages and disadvantages of transportation systems distributed among different segments of the population, considering factors like race, income, and gender?

Types of Equity

Horizontal Equity, also known as fairness or egalitarianism, aims for an equal and impartial distribution of impacts among various social groups. It stands for policies where benefits and costs are fairly shared among these groups, avoiding favoritism towards any group.

Vertical Equity on the other hand, focuses on distributing benefits and consequences based on the differing socioeconomic and demographic characteristics of groups, such as income or race. For instance, in transportation, a policy is deemed vertically equitable if it offers more support to disadvantaged groups. This might involve providing discounts or specialized services for economically or socially marginalized communities. In terms of mobility needs, vertical equity ensures that impacts are fairly distributed among groups with varying mobility requirements, accommodating diverse abilities. For example, implementing wheelchair-accessible facilities in all public transit areas aligns with this equity approach.

Attaining equity in transportation planning involves balancing both horizontal and vertical equities. Horizontal equity emphasizes equal transportation cost-sharing for all users, irrespective of their income levels. Conversely, vertical equity necessitates providing support or subsidies to disadvantaged groups. Striking a balance between these equities requires careful consideration of trade-offs and ensuring that the distribution of costs and benefits is fair across all user groups.

Externalities

Externalities, initially a concept from economics, refer to costs or benefits affecting an individual or group that did not actively choose to bear those costs or enjoy those benefits. Also called spillover or third-party effects, these impacts are not accounted for in the direct exchange of money, like in markets, and can be either positive or negative. For instance, when a car owner drives, pedestrians might experience negative effects such as noise pollution, air pollution, or accidents, despite having no role in the car owner’s decision.

Transportation projects can lead to various types of externalities. Positive externalities may involve:

• Increased accessibility and mobility due to cars using urban arterial roads.
• Enhanced health and safety resulting from pedestrian or bicycle paths.
• Reduced travel times in areas well-served by bus rapid transit or rail lines running on dedicated rights of way.

Conversely, negative externalities might include:

• Air pollution caused by emissions from automobiles and trucks.
• Noise pollution stemming from a neighborhood’s proximity to a major highway.
• Increased travel durations caused by congestion.

To address congestion’s negative externalities, policies like congestion pricing or highway tolling are implemented. These policies aim to offset the adverse impacts on drivers’ mobility on highly trafficked highways due to additional vehicles entering the network, which slows down or halts traffic flow.

Equity and externalities

Transportation externalities tend to be unequally distributed among different segments of the population, especially minorities in the US. Negative externalities often result from specific groups’ actions, such as drivers affecting pedestrian or cyclist safety or environmental pollution from fuel emissions affecting respiratory health. However, a larger portion of the population, including those who do not use automobiles, ends up experiencing the negative effects or incurring costs to address them. Pinpointing the sources of environmental externalities and understanding the intricate relationship between the transportation sector and the environment is a complex task. These relationships are often indirect, cumulative, and influenced by various interconnected factors. Nonetheless, planning strategies addressing equity concerns and externalities in transportation aim to help policymakers create policies for a more inclusive, sustainable, and fair transportation system.

Measuring the negative environmental impacts, which include economic, social, and environmental costs, is a crucial task in policymaking. Specific policies aim to correct and reduce the adverse effects caused by the transportation sector. Typically, these policies revolve around the idea that groups responsible for these impacts (such as solo drivers) should bear the consequences of their actions However, because often impacts are indirect, cumulative, and diffused, polluters rarely bear the costs or consequences of their impacts (Rodrigue, 2020; Tillema et al., 2012). Figure 1.4 shows eight categories of (negative) economic, ecological, environmental, and human health (social) transportation-related externalities stemming from vehicles’ emissions and particulates, noise and vibrations, traffic accidents and deaths, pollutants and chemicals from infrastructure construction, maintenance, and runoff seeping into waterways as well as the ecological destruction of fauna and flora caused by transportation projects.

Land use and development patterns can lead to adverse environmental effects within a community (Grengs et al., 2013; Litman, 2014). Poorly planned or unplanned land development, especially without adequate transportation infrastructure, can trigger severe traffic congestion in urban areas, resulting in heightened air pollution levels and potential health risks.

To address these negative externalities, policymakers aiming to shape urban form and land use have advocated Smart Growth policies (Smart Growth America). These initiatives, premised on people’s willingness to shift transportation mode from driving to riding transit, aim to mitigate several adverse impacts by implementing policies such as increasing housing density zoning and promoting multi-modal transportation (Cervero, 2006). Reductions in total vehicle miles traveled (VMT), increases in public transit usage, and average daily commuting time are some of the indicators used to assess the impacts of these initiatives (Rodrigue, 2013).

Macro Trends and Forces in Transportation Planning

The relationship between components of urban spatial structure and activity formation also responds to several ongoing influential macro forces. Ongoing, influential macro forces, such as those listed below, continually drive transformations in urban areas and their transportation systems. These changes, whether gradual or rapid, visibly impact the structure of urban spaces, their layout, and how land is utilized. They often follow a particular trajectory, a concept explained by Rodrigue as “path dependence” (Rodrigue, 2013, p. #) This concept suggests that once a specific development path or course is chosen, subsequent developments tend to align with that path, making it challenging to deviate from it. For instance, the dominance of automobiles in the US transportation system since the 1950s illustrates this path. In contrast, cities like Shanghai in China relied on bicycles for mobility and commuting until the 1980s when the city’s trajectory shifted towards car-centric transportation.

These macro forces encompass:

• Residential patterns,
• Employment patterns,
• Global environmental challenges (such as health pandemics and climate change),
• Economic crises,
• Energy prices, and
• Technological advancements

As introduced earlier in this chapter and depicted in Figure 1.4, land use and transportation modeling considers the dynamic spatial interplay between the location of economic activities reflected in land-use patterns and the demand for movement from various transportation mode users to access destinations through infrastructure like roads, rail, and sidewalks, all collectively imprinting the urban form of the city. Modeling this spatial interaction between land use and transportation is like building a digital representation of the city or metropolitan area’s urban spatial structure. Figure 1.5 shows how transportation can influence the urban spatial structure and land use patterns. Transportation on one hand provides means and infrastructure for mobility while land use activities (routine activities, institutional activities , or production) influence the anticipated or generated trips by workers, customers, suppliers, etc. In this regard, information on population and employment is critical for any land use-transportation model as we will explore through different modeling frameworks in this book.

An example of an integrated land use and transportation model

In the 1990s, spurred by national policies such as the Clean Air Act Amendments and the Intermodal Surface Transportation Efficiency Act (ISTEA), which required Metropolitan Planning Organizations (MPOs) to integrate land-use and transportation planning, the state of Indiana faced the necessity to tackle escalating traffic congestion arising from changes in land use patterns. In response, Indiana integrated its operational statewide travel demand model (ISTDM) with its land-use planning, the Land Use Central Indiana (LUCI) model. This integration aimed to forecast land conversion for residential and economic development in the forthcoming decades. The integrated results would enable forecasting of land use alongside travel patterns for a designated horizon year, with intermediate reports provided for each five-year period in between.

Indiana’s 2008 integrated transportation land use demand model (INTRLUDE) linked the two previously separate models. The new travel demand model (ISTDM V.4) encompasses ninety-two counties and incorporated a series of models for household classification, trip generation, trip distribution, mode choice, truck model for freight, and trip assignment models. On the other hand, the LUCI2 model is a land use simulation model that forecasts changes in employment and the amount of land converted to residential and economic activities. Using a random utility model and aggregate logit models, it estimates vacant and rural land conversion to residential and employment-related purposes.

Accessibility to both population and employment plays a key role in INTRLUDE. This integrated model utilizes updated congested travel time data from the travel demand model to measure accessibility. A five-year simulation period was adopted to estimate changes resulting from land use and transportation affecting each other. As outlined in Figure 1.5, the lagged outputs, employment, and population (residential) growth from the land use model (LUCI2) serve as input to the transportation model (STDM v4) generating an integrated transportation-land use continuous five-year simulation (Jin and Fricker, 2008).

To achieve the target year projections, the two models interact, as depicted in figures 1.6 and 1.7. This integration involves feeding congested travel times (output of the transportation model) to the land use model to assess accessibility and estimate population and employment. Then, the new demographic and employment data are fed into the travel demand model to update travel time, which in turn updates the land use model until the target year is reached. This integration has been implemented using Caliper’s TransCAD,’s user interface shown in Figure 1.6, which allows the user to specify different scenarios for both the transportation and land use model.

CONCLUSION

This chapter presented a focused discussion on integrated land use and transportation models. The need for such integration arises from the dynamic relationship between changes in land use and the demand for urban services, including transportation. This necessitates an integrated approach to transportation and land use modeling that acknowledges their interdependency and feedback loops Without an integrated land use and transportation framework, transportation planning agencies are ill-equipped to address urban development issues and externalities, such as environmental and equity considerations.

Through integrated land use and transportation modeling efforts, various types of externalities can be measured and simulated under different scenarios, aiding planning agencies to anticipate the impacts of their decisions before implementation. In essence, integrated land use and transportation models serve as powerful and versatile toolkits available to planning agencies, enabling them to predict the future spatial distribution of various activities, including economic and residential activities, and facilitating informed decision-making.

Glossary

• Accessibility refers to the measurement of ease of reaching to various types of opportunities, activities and destination like work location
• urban structure refers to the pattern of land use within a city or urban areas
• mobility is the ability or potential for moving around using one or a combination of transportation modes
• gravity model is a type of accessibility measurement in which the employment in destination and population in the origin defines thee degree of accessibility between the two zones.
• Horizontal equity, referring to equal distribution of impacts between all groups regardless of ability or need. In this equity type, everyone is treated the same.
• Vertical Equity refers to the notion that disadvantaged groups based on different indicators such as income, age or race should receive a greater share of resources due to imposed limitations based on different factors such as race or gender
• Vertical Equity with respect to mobility needs a is a special type of vertical equity in which physical abilities of people for movement is the main focus for distribution of impacts
• Externalities is a side effect or consequence of an activity, policy, project or programs such as highway development.
• agglomeration benefits refers to the benefits that is generated when activities like industries locate near one another in urban areas or clusters
• Institutional activities are usually located at particular locations within the cities, and the flow of trips to these locations tends to be more irregular than routine activities. An example can be a college, university, or sports stadium.

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