DOT Guidance on Induced Demand Effects of various Roadway Investments

Focus Area

Climate Change


Air Quality






1-2 years

Research Idea Scope

Operational improvements such as auxiliary lanes and intelligent transportation system treatments are designed to reduce congestion. Even these minor congestion improvement strategies and investments intended to optimize existing roadway system assets are increasingly facing opposition in the name of “induced demand”, often pointing to potentially disparate impacts on Environmental Justice (EJ) communities. The public raises concerns about increased greenhouse gases (GHG) and air quality impacts of such projects and lack trust that the traffic analyses are accurately accounting for induced demand. The concern has been used as a cudgel to suggest that even historically understood congestion improvement activities may actually be hurting, rather than helping, the air quality of the commuting and near-road communities. DOTs need assistance in understanding the best investments in lieu of induced demand to reduce roadway congestion.
Recognizing even operational and safety improvements may have some level of induced demand, and induced demand makes it more difficult to build our way out of congestion, DOTs are planning such investments and need to better understand their potential unintended consequences. Some data have shown substantial reductions in congestion with auxiliary lanes by reducing conflicts in complex weaving areas, brings these segments back up to their theoretical capacity. However, results are complicated by changes in travel patterns (re-routing, time of day), measuring reductions in incidents, and capturing safety impacts on parallel local roads and alternative modes. Even auxiliary lanes less than one mile to remove bottlenecks, which EPA has explicitly identified as exempt from conformity in their last PM Hot-Spot FAQ, now have had public concerns raised regarding their potential to induce demand. These challenges call for consistent analysis and transparency.
Most induced demand research is tied to significant new lane-mile capacity changes and has often been theoretical. Limited research has evaluated the real-world effects of these smaller roadway treatments, leaving key questions unanswered:
• Under what conditions do major and minor induced demand issues occur?
• Does the induced demand counteract the congestion improvement entirely or even substantially?
• Can these effects be mitigated (e.g., through tolling or land use controls, etc…)?
• How can roadway improvements be a responsible component of a transportation system with regards to tacking air quality and GHG emissions?
A definitive study of these and other minor capacity enhancements on both freeways and arterials, as well as mitigation options is needed to improve DOT investment decisions for the long term and enable more productive conversations with the public.
To provide confidence to both the travelling public and near-road communities (including EJ communities) that DOTs are taking the induced demand concern seriously and to implement roadway enhancements that are truly beneficial to them by:
• Perform a literature review, that informs the Assessment Framework, isolated new analysis, and outreach materials:
• Create an Assessment Framework that would:
o Definitions. Defines induced demand in a consistent way, i.e. between build and no-build future scenarios, and covers the spectrum of projects, i.e. large capacity enhancements to smaller operational improvements.
o Evaluation. Enable a structure way to evaluate the relative potential for generating typical induced demand “effects” resulting from various “project types”; and how those effects impact agency “outcome areas” such as air quality and climate, teasing out significant differences in effects across project types. Definitions and evaluation framework should be reviewed with selected outside reviewers prior to finalization.
o Factors & Mitigation. Identify contextual factors that alter potential latent and induced demand (i.e., base travel cost, land use, tolling, and improved local street connectivity). These factors should be supported with examples in practice. The GEMS tool may be
o Analysis Best Practices. Identify Best Practices in modeling & analysis to capture the effects defined above consistently.
• Create and implement isolated new research (analysis and/or modeling) to fill Assessment Framework gaps. Preference is for observed over modeled theory; anticipate gaps in smaller ITS, auxiliary lanes, operational projects. If modeling is used, air quality models may be unnecessary as improvements can be assumed if congestion improvements are demonstrated. However, evaluation of network models may be important to evaluate overall VMT and associated congestion changes.
• Create and pilot outreach materials, build on latest behavioral science on changing attitudes, including:
o Guidance. Develop materials to clearly define the various induced and latent demand causes and effects, typical project induced effects and mitigation options using the assessment framework. Case study examples will provide narratives to communicate complexities.
o Lookup Tool. Provide a lookup tool by which DOTs can match combinations of project types, treatments, and mitigation actions that will best meet various congestion circumstances to lead to desired public goals (i.e., congestion, air quality improvement)
o Pilot Implementation. Coordinate with a few DOTs to apply the outreach materials process in a couple locations, (urban/rural project applications by different DOTs), to assess how it affects relationship with the public on the project.

Urgency and Payoff

The public and interested groups are using induced demand arguments to oppose new roadway investments and small operational and safety roadway projects by DOTs across the country. In order to invest responsibly – balancing safety, economic, equity, and climate concerns – DOTs need to better understand the actual effects of these projects, especially of smaller roadway treatments and other mitigation options. Additionally, with tough tradeoffs, DOTs would engender more public buy-in through more nuanced discussions grounding in understandings of comparisons with other areas of the country. Ultimately this study would provide a broader understanding of effective roadway congestion strategies, and would help DOTs avoid investing tax dollars in projects/treatments that if left unmitigated are unlikely to achieve their intended goals.

Suggested By

Natalie Liljenwall

[email protected]