ActivitySim is an open-source, advanced travel demand modeling platform used by transportation planners and policymakers around the world to understand and forecast travel behaviors and patterns. Led by a consortium of public agencies and overseen by the Association of Metropolitan Planning Organizations (AMPO), ActivitySim delivers detailed insights that support informed decision-making in urban planning and transportation policy.

The value of ActivitySim is in its detailed simulation of the activities and travel decisions of individual agents. This allows for a more precise analysis of travel patterns, which is crucial for planning efficient and effective transportation systems. Furthermore, the platform's collaborative development process ensures that it remains cutting-edge and reflective of the needs of its growing community of global users, with continuous updates and enhancements supported by a diverse group of experts and firms, including RSG.

As part of ActivitySim’s continuous improvement, the platform will receive a significant update in June 2024. This article offers additional information on what’s new with ActivitySim and provides expert insights and perspectives on the platform’s future.

The Latest Version of ActivitySim Will Include Several Updates to Improve the Platform’s Functionality

The latest version of ActivitySim will combine consortium-led updates with changes developed and shared by ActivitySim’s consultant teams. These updates comprise both larger updates and smaller enhancements to the platform’s functionality.

Notably, one of the larger updates involves a comprehensive restructuring of how ActivitySim handles its data pipeline. This change streamlines code maintenance and simplifies continuous integration testing. Importantly, this update supports the stability and reliability of ActivitySim's future development cycles by ensuring that new developments do not disrupt existing functionalities.

Another notable update is the integration of Pydantic, a Python library used for data validation and settings management. The addition of Pydantic aims to streamline the configuration process within ActivitySim. Pydantic allows users to specify various settings and configurations for running ActivitySim, ensuring that all inputs are valid and correctly formatted. This enhancement provides users with clear documentation and automated checks, making ActivitySim easier to set up and run.

The latest version of ActivitySim also includes an input checker tool, which was an RSG contribution to the platform. This tool validates data input files, ensuring that all inputs used in ActivitySim are correct and consistent, including land-use data, the synthetic population, transport networks, and level-of-service skims. It verifies the accuracy of activity-related data and can also be configured to check network data. This feature is particularly useful for users running multiple scenarios, as it helps prevent errors that could lead to unexpected results or software crashes.

As a result of these changes, ActivitySim will continue to be highly customizable to different regions’ implementations. In fact, RSG has worked closely with the Southeast Michigan Council of Governments (SEMCOG) and other agencies to test their ActivitySim implementations and guide it to suit their needs while supporting the ongoing work of the consortium.

In addition to these larger updates, this update to ActivitySim includes numerous smaller enhancements and bug fixes. These improvements are emblematic of the consortium’s ongoing commitment to refine and optimize ActivitySim, ensuring it remains a powerful and versatile tool for the transportation modeling community. The collaborative nature of ActivitySim's development, involving multiple consultant teams and contributions from various stakeholders, continues to drive its evolution and effectiveness as the leading tool in activity-based travel demand modeling.

The ActivitySim Consortium Helps Oversee the ActivitySim Platform and Directs Future Enhancements Through Its Bench of Consultants

ActivitySim's development involves multiple consulting teams, including RSG. The consortium directs these firms and includes several metropolitan planning organizations (MPOs) as well as state departments of transportation (DOTs), and other transportation agencies. AMPO, the project’s administrator, represents the diverse needs and interests of MPOs, which coordinate regional transportation planning in more than 400 urban areas across the United States.

The consultant teams that help update ActivitySim are informally called “the bench” and help to facilitate a dynamic development process for ActivitySim and its users. Each team can leverage their specialized skills and insights to improve project outcomes. This collaborative effort also ensures that ActivitySim continues to evolve as a leading tool in travel demand modeling, benefiting from the collective expertise and innovative ideas of all stakeholders.

The update process for ActivitySim is highly collaborative and adaptable. It begins with the consortium identifying and funding specific updates, which range from new functionalities and models to performance enhancements. For instance, consortium-led updates have included enhanced accessibility measures, improved shadow pricing mechanisms, and explicit school escorting features. Additionally, updates often focus on usability improvements, such as input checking, and performance-related maintenance tasks.

Once the consortium agrees on the updates needed, consultants on the bench perform the development work. These consultants also provide input on which updates to prioritize. Although outside contributions from agencies or the public are occasionally incorporated, these are often minor compared to those initiated and funded by the consortium. The final step in the update process involves rigorous testing of the new features and fixes by the consultant team before they are officially released.

ActivitySim’s Focus on Continual Updates and Improvements Benefits Its Users and the Travel Demand Modeling Community

One of the key advantages of joining the ActivitySim consortium is the ease of adopting updates. This collaborative framework reduces the need for member agencies to budget for extensive consultant hours dedicated to maintaining and updating individual models. Instead, these agencies can focus on tailoring their implementation efforts to local travel behaviors and markets, using their expertise and the expertise of their consultants to develop new features and improve existing functionalities. This approach not only benefits the agencies by reducing the cost and complexity of maintaining their models but also allows consultants to contribute more effectively to the overall advancement of travel demand modeling.

Updating ActivitySim can follow different paths depending on the nature of the changes. For instance, when RSG worked with SEMCOG to deploy ActivitySim in their region, our team delivered several model enhancements that were specific to their needs. RSG made unique changes tailored to the University of Michigan and integrated them into SEMCOG's regional modeling system. These localized changes leverage ActivitySim's core functionalities but remain specific to the client agency, ensuring that the updates serve their unique requirements without necessarily being incorporated into the broader ActivitySim codebase.

RSG has worked with SEMCOG in Michigan on their ActivitySim deployment, which comprised two phases focused on model implementation and model enhancement.

On the other hand, there are updates arising from specific projects that have broader applicability. These types of updates are often “pulled” into the consortium's core codebase, making them available to all ActivitySim users. This dual-path approach allows ActivitySim to cater to both specific regional needs and the broader community of users.

Updating ActivitySim for agencies such as SEMCOG is often straightforward, though it does not occur automatically. SEMCOG, with their specific version of ActivitySim tailored to their needs, benefits from a system where they can integrate updates with minimal effort. This is possible because the ActivitySim testing framework includes examples using SEMCOG's actual configuration files and extensions. The goal is to ensure that new updates do not disrupt existing configurations, allowing SEMCOG to pull the latest version of ActivitySim and run it with its current, customized setup.

Before ActivitySim, regional approaches to travel demand modeling were isolated and fragmented. Different regions would develop their models independently of one another. This led to several approaches and solutions that did not necessarily benefit other regions. While these models’ codebases were in many cases shared, they were shared across a much smaller group of agencies and often maintained by only one consulting firm. With ActivitySim, however, there is a consistent and widely shared codebase that regions can adopt—and contribute to. This standardization means that improvements made in one region can benefit all users.

The process of updating ActivitySim described here highlights a significant, consortium-led paradigm shift in how agencies manage their modeling needs. By adopting a consistent and regularly updated platform, agencies can streamline their workflows, reduce the need for large-scale, time-consuming, and expensive updates, and instead focus on incremental improvements. This approach mirrors broader trends in technology and data collection, with frequent updates and improvements eclipsing infrequent, large-scale changes. Ultimately, this approach benefits both the agencies and the consultants, fostering a more efficient and innovative environment for travel demand modeling.

ActivitySim Brings Together the World’s Leading Players in Activity-Based Modeling

A collaborative undertaking to update the travel demand models used by the Metropolitan Transportation Commission (MTC) and the San Diego Association of Governments (SANDAG) exemplifies the collaborative nature of ActivitySim’s platform. This work has sought to transition the travel models for both SANDAG and MTC from their existing CT-RAMP models to ActivitySim. Since some specialized models in CT-RAMP did not have direct equivalents in ActivitySim, the consultant teams undertook a joint development effort to build these functionalities within ActivitySim, ensuring that the new models produced similar predictions to the original ones.

The consultant teams assisting MTC and SANDAG on their model development projects collaborated in moving from CT-RAMP to ActivitySim.

Specifically, the joint effort saw consultant teams dividing the task of porting models from CT-RAMP to ActivitySim. Both firms tackled different components, using the same coefficient files, model configurations, and parameters to ensure consistency. For example, the firms merged the joint tour frequency and composition model in ActivitySim to allow for more intricate interactions between coefficients that reflect how much joint travel is occurring and who is participating. The firms adapted another critical model, the coordinated daily activity pattern that determines whether people travel, to include a household-level decision component that considers the utility of joint travel. At the time of this writing, SANDAG has started using their ActivitySim/ABM3 model for their 2025 Regional Plan.

Unlike past model development projects that saw one firm assisting one client with their individual model, the level of cooperation for the MTC-SANDAG work highlights that ActivitySim is not the product of a single firm but a consortium-driven initiative. The collaboration between top firms in this space also ensures that the software benefits from diverse perspectives and rigorous peer review, fostering continuous innovation and improvement. Users of ActivitySim can take confidence in knowing that the tool is the result of coordinated efforts among industry leaders, each bringing their expertise to bear to enhance its functionality and reliability.

What’s Ahead for ActivitySim

The consortium and consultant teams leading ActivitySim's development share a strategic perspective on the platform and future enhancements. ActivitySim’s latest updates deliver a decrease in memory requirements and shorter computational times within the model flow. These enhancements will ensure the model continues to run efficiently and effectively, with significant progress made to date.

Each phase of ActivitySim’s development has involved consortium members proposing and selecting top priorities. This approach ensures that ActivitySim continues to evolve in response to both the consortium's strategic vision and the emerging needs of its users. As ActivitySim continues to improve, future updates will build on this collaborative foundation, further enhancing its capabilities and utility for consortium members. This approach ultimately benefits all stakeholders, providing a more efficient, reliable, and up-to-date modeling platform.

As one of ActivitySim’s original developers and a current bench member, RSG brings nearly a decade of experience developing and using ActivitySim to support our clients’ travel model development projects. RSG has worked closely with clients in Michigan, Minnesota, Washington, DC, Chicago, San Diego, British Columbia, and Seattle on their ActivitySim deployments, and our ActivitySim experts are available to answer any questions about the platform and explore potential use cases with agencies considering their own ActivitySim deployments.

If you have questions about the consortium’s work, or if you are interested in learning more about ActivitySim, you can contact AMPO directly at ActivitySim@ampo.org.