The National Academies Press

Currently Skimming:

Pages 19-70

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.

From page 19... ... 17 2.1 DEVELOPMENT PROCESS Modeling managers must consider numerous factors when developing an activity-based model system. They must evaluate how model designs and specifi cations can address policies and projects being considered and make tradeoffs between model capabilities and development costs and schedule. Read the entire page →
From page 20... ... 18 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER model system designed specifically to address the concerns and analytic needs of the particular region. Advantages of this approach are that the entire activity-based model system is designed and implemented as a single effort; the coherence of the overall model system design is enhanced and the development can include features important to the agency; and the overall amount of time required to have an operational, calibrated mode may be reduced. Read the entire page →
From page 21... ... 19 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM and inputs and outputs and by demonstrating the usefulness of the tool for analyzing real projects and policies. 2.1.2 Design Activity-based model design starts with an assess ment of an agency's analysis needs in relation to the policies and projects that are expected to be considered by the agency. Read the entire page →
From page 22... ... 20 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER additional data such as household survey or traffic counts by detailed time-of-day may be required, and assembling this information takes time. Funding also influences schedules. Read the entire page →
From page 23... ... 21 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM although in some cases GPS data have been collected for the entire household sample. GPS data can be used to understand underreporting of stops and tours, as well as the misreporting of activity locations and travel times. Read the entire page →
From page 24... ... 22 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER • Household size; • Household composition and life cycle (e.g., age of householder by presence of own children) ; • Number of workers per household; • Household income category; • Age and gender of each person; and • Employment and student status of each person. Read the entire page →
From page 25... ... 23 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM TAZ–TAZ skims are likely to be as accurate as, or more accurate than, all-streets networkbased skims, especially since those skims also include the effects of traffic congestion. Another recent variation, used in models in San Diego and other regions, is to use separate zone systems for the automobile and transit networks. Read the entire page →
From page 26... ... 24 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER information required by decision makers, it is necessary for an agency to carefully consider the critical questions and analysis needs over a time horizon of at least the coming 10 years. Agencies should consider whether there are policies or investments of interest to decision makers that are better evaluated using activitybased modeling approaches. Read the entire page →
From page 27... ... 25 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM segments and choices and involve the use of model estimation software. This software is used to describe the relative importance of variables factors affecting travel-related decision making. Read the entire page →
From page 28... ... 26 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER of effort to calibrate an activity-based model system may not be significantly greater than to calibrate a trip-based model system because all the choice components in the activity-based model system are more closely linked. 2.1.4.7 Auxiliary Demand Like trip-based models, activity-based models represent the trips made by residents of the modeled area when these residents are traveling entirely within the modeled area. Read the entire page →
From page 29... ... 27 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM activity-based models typically are used to provide more detailed information, such as network assignment model results by detailed time of day, and can also provide estimates of start and stop emissions by time of day. Because of their disaggregate nature these models can provide measures such as emissions by household, and, due to their flexible structure, activity-based models can easily accommodate new models, such as automobile ownership, that include information on vehicle type choice. Read the entire page →
From page 30... ... 28 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER better able to represent true modal availability and constraints. • Equity. Read the entire page →
From page 31... ... 29 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM zone-based model. However, it has become more common to include more detail to represent key spatial data such as employment and school enrollment, distance to transit stops, urban design, and local street infrastructure. Read the entire page →
From page 32... ... 30 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER projects provided a proof of concept for integrating demand-and-supply models with high levels of temporal (and spatial) detail, although significant methodological and practical issues associated with these advanced models remain. Read the entire page →
From page 33... ... 31 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM model. A minimal number of modes to include in an activity-based model would be automobile, transit, and nonmotorized. Read the entire page →
From page 34... ... 32 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER TABLE 2.2. COMPONENTS INCLUDED IN VARIOUS TYPES OF TOUR-BASED AND ACTIVITY-BASED MODELS Simple TourBased Advanced Tour-Based Day-PatternBased Day Pattern with LongerTerm Choices Day Pattern with LongerTerm and Mobility Choices With Explicit IntraHousehold Interactions Population segmentation Population synthesis Population synthesis Population synthesis Usual work (and school) Read the entire page →
From page 35... ... 33 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM to a micro simulation approach required the introduction of population synthesis in order to provide records for individual households and persons in a proto typical, representative population. The first practical use in the United States of what we now recognize as activity-based models came with the introduction of the day-pattern approach (Bowman 1995) Read the entire page →
From page 36... ... 34 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER region is one aspect that should be considered when selecting which control variables to use in population synthesis. An alternative approach to synthesizing a population separately for each forecast year is to synthesize the base-year population and then use a population evolution model to evolve that population over time to represent phenomena such as aging, births and deaths, marriage and divorce, and immigration and emigration. Read the entire page →
From page 37... ... 35 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM idence location based on where their workplace is l ocated, rather than vice versa. This is particularly relevant for multiworker households. Read the entire page →
From page 38... ... 36 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER In reality, some intermediate stops are planned in advance, and some are made on the spur of the moment for convenience sake, so there is no completely correct way to structure such models. Read the entire page →
From page 39... ... 37 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM members can adjust their joint travel patterns if conditions change enough. This issue brings up the importance of including travel accessibility effects at all levels of the model system, as the convenience of traveling together and the options for traveling by alternative modes can influence whether or not household members choose to travel together. Read the entire page →
From page 40... ... 38 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER stated preference research or letting the estimation data decide which nesting structure performs best. The nesting structure may vary depending on activity purpose, available modes, or other local characteristics. Read the entire page →
From page 41... ... 39 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM ward toward the tour destination, or outward toward the tour origin, which is the home location for home-based tours. Just as at the tour level, the relative ordering of the trip mode and departure time models can vary from one model design to the other. Read the entire page →
From page 42... ... 40 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER hierarchy makes it impossible to use a direct measure because it depends on a yet- unmodeled outcome. This would be the case, for example, for travel time in a destination-choice model that is higher in the hierarchy than mode and/ or time-of-day choice, since in order to measure travel time directly it is necessary to know the mode and time of day. Read the entire page →
From page 43... ... 41 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM 2.2.3 Other Model Component Design Considerations The preceding sections have described the issues that need to be considered when designing the core components that form the activity-based model system and configuring the interactions and exchanges of information among these components. However, the overall activity-based model system design must also consider the interactions with other tools that, in conjunction with the core activity-based model, make up the entire model system. Read the entire page →
From page 44... ... 42 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER to the issue of maintaining consistency with other model system components. The spatial, temporal, and typological resolution of the activity-based model must inform and align with these same dimensions in the network assign ment model. Read the entire page →
From page 45... ... 43 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM incapable of generating reasonable measures of link volumes and network performance indicators for small time periods less than 1 hour in duration. Typological (Including Auxiliary) Read the entire page →
From page 46... ... 44 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER interaction of individual vehicles or packets of vehicles being simulated or calculated using extremely fine-grained temporal resolution, such as seconds or fractions of seconds. DTA models are sensitive to operational attributes and are founded on traffic flow theory, but their wide adoption has been hindered by long run times and by their inherent stochasticity (Chiu et al. Read the entire page →
From page 47... ... 45 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM capabilities vary widely by land use model. Some models may represent the actions of disaggregate households, individuals, firms, and developers; may incorporate more complex interactions within a regional economy; or may address long-term demographic changes. Read the entire page →
From page 48... ... 46 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER agencies developing integrated models must also specifically consider how these different model components are configured to interact to achieve an overall solution. Some components may be interacted to equilibrate to a convergent solution or at least a stable solution, while other components may be interacted in a more path-dependent manner. Read the entire page →
From page 49... ... 47 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM there is typically a two-way exchange of information between these model system components. The land use model often provides information on location of housing units and employment (typically with some detail by industrial sector) Read the entire page →
From page 50... ... 48 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER 2.2.5.2 Network Assignment Model When linked with an activity-based model, static network assignment models can produce all the metrics that are typically produced when such network assignment models are linked with traditional trip-based models, such as link volumes and congested link travel times. Because activity-based models include explicit time-of-day choice models, the static network assignment models that are linked with activity-based travel models often include more time periods than those linked with trip-based models. Read the entire page →
From page 51... ... 49 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM 2.3 DATA DEVELOPMENT Activity-based model system development, like trip-based model development, requires assembling a diverse set of data. These data reflect travel behavior, regional demographics, land use, network configuration, and network performance, and many of the required data items must be available for all base-year, future-year, or alternative scenarios. Read the entire page →
From page 52... ... 50 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER 2.3.1.3 Synthetic Populations and Demographic A key input to most activity-based models is a synthetic population that is used as the basis for forecasting the behavior of the households and persons in the modeled area. The specific data required to generate a synthetic population are influenced by the design of the synthetic population, which is itself reflective of the specification of the activity-based model system components. Read the entire page →
From page 53... ... 51 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM while more complex accessibility indicators can include combination mode-choice and destination-choice logsums. 2.3.1.5 Calibration and Validation Calibration refers to the process of adjusting model parameters to better match some base case observed conditions, while model validation involves the application of the calibrated model and the comparison of the results to observed data that have not been used in the model estimation or calibration process. Read the entire page →
From page 54... ... 52 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER involves model estimation, it includes specifying the model utility functions and alternative availability constraints in a model estimation software package, and then carrying out the estimation in an iterative process. This process requires expert judgment to determine what variables to include and sometimes involves constraining some coefficients to typical values in cases where the data for estimation are inadequate. Read the entire page →
From page 55... ... 53 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM Many survey data sets already are structured into household-level, person-level, and trip-level records, so two of the main steps in the data processing are tour formation and day-pattern formation. Tour formation is the process of combining home-based and workbased trip chains into tours and writing records with the relevant tour attributes. Read the entire page →
From page 56... ... 54 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER 2.4.2.1 Components Not surprisingly, the components of the software system tend to mirror the components of the model system itself. Typically, a software platform uses objected oriented code (e.g., in C#, Java, or Python) Read the entire page →
From page 57... ... 55 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM a very substantial sample size. Otherwise, it is likely to be more accurate to use the coefficients from a model system estimated on a large survey in a comparable region elsewhere, and use the local survey data to simply recalibrate certain key model coefficients such as alternative specific constants. Read the entire page →
From page 58... ... 56 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER both controlled for and uncontrolled for attributes. In the United States, Census PUMS data are often used as the source for this sample, although it is also possible to use a regional household survey as the basis for the disaggregate sample. Read the entire page →
From page 59... ... 57 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM mand estimates generated by the core activitybased model components prior to the network assignment stage in the overall model system. 2.4.6.1 Internal-External and External-External Travel Internal-external and external-external demand represents travel in which one or both of the trip ends is outside the modeled area. Read the entire page →
From page 60... ... 58 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER 2.4.7 Calibration and Validation Calibration refers to the process of adjusting model parameters in order to better match some base case observed conditions. Model validation involves the application of the calibrated model and the comparison of the results to observed data that have not been used in the model estimation or calibration process. Read the entire page →
From page 61... ... 59 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM Automobile ownership model results are usually compared to household survey data or U.S. Census data. Read the entire page →
From page 62... ... 60 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER ment model, involving the adjustment of model parameters and input assumptions. However, calibration of a dynamic network assignment model is significantly more involved because of the additional details included in the model, such as explicit inclusion of traffic flow models and the vastly more detailed network input assumptions that affect results such as lane configurations and signal timings. Read the entire page →
From page 63... ... 61 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM mation from the commercial package to the activity-based model and exchange of demand information from the activity-based model to the commercial package. 2.4.8.1.1 Configuration Options Activity-based model systems should provide the ability to configure individual model components as well as the overall model system. Read the entire page →
From page 64... ... 62 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER ning and re- running of the model components and the overall model system. Model run times are typically reduced by providing additional computing resources and through software engineering and optimization. Read the entire page →
From page 65... ... 63 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM based on the random numbers used to simulate choices. In application, one important issue is controlling for how the random number seeds and sequences are used to choose outcomes. Read the entire page →
From page 66... ... 64 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER there are additional checks that are similar to those performed for trip-based models, such as evaluations of trip length frequency distributions, trip flows by geography, and trip mode choices. In addition, because, at present, activity-based models are typically linked to static network assignment models, many of the reasonable checks appropriate to trip-based static network assignment models are equally applicable to static network assignment models linked to activity-based models. Read the entire page →
From page 67... ... 65 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM seems very likely that in the near future many agencies will start to employ remote computing resources. Local computing resources are typically in-house servers or workstations. Read the entire page →
From page 68... ... 66 Part 1: ACTIVITY-BASED TRAVEL DEMAND MODELS: A PRIMER schedules as well. Agencies must carefully consider the schedule trade-offs between developing new capabilities or region-specific models, project or analysis schedule constraints, and agency staff and resource availability. Read the entire page →
From page 69... ... 67 Chapter 2: TECHNICAL ROAD MAP FOR DEVELOPING AN ACTIVITY-BASED MODEL SYSTEM and facilitates immediate testing and application by agency staff. 2.5.5.3 Application and Maintenance In contrast to activity-based model system development, agency staff have typically played a much more significant role in activity-based model system-level applications. Read the entire page →

From page 19...

... 17 2.1 DEVELOPMENT PROCESS Modeling managers must consider numerous factors when developing an activity-based model system. They must evaluate how model designs and specifi cations can address policies and projects being considered and make tradeoffs between model capabilities and development costs and schedule.