Appendix

Research to Address Key Knowledge Gaps and Contested Topics and Metrics

KEY KNOWLEDGE GAPS AND CONTESTED TOPICS

For readers interested in areas for further research and policy innovation, summarized below are high-level research questions to address the key knowledge gaps and contested topics that are stated or implied in Critical Issues in Transportation for 2024 and Beyond. Pursuing these high-level questions would require applying the disciplines, methods, and data appropriate for each question and may require broad programs of effort rather than individual projects. The gaps and topics are organized by the main headings in the text, although some apply to more than one heading. Note that this list is not comprehensive because the Executive Committee focused on selected critical issues in this publication. Research that incorporates the many interdependencies among the issues and topics listed below is more likely to help public and private decision makers steer the transportation sector toward serving a thriving society than research on individual topics in isolation.

Movement of People and Goods

• Can mode choice be shifted at significant scale to affect 2050 climate goals if policy makers do not adopt steep carbon or fuel taxes, and, if not, how could this be done in the most cost-effective and politically acceptable ways?
• What research methods and data would be required to gain better understanding and measures of why people travel, are inhibited in doing so, how this affects their personal growth and happiness, and how these individual aspirations can be aggregated to represent society as a whole?

Building and Sustaining a Strong, Competitive Economy

• What are the causes of declining truck and rail freight total factor productivity (TFP) growth and static transit TFP growth, potential remedies, and trade-offs associated with implementing these remedies?

Mitigating and Responding to Climate Change

• What are the most effective, politically achievable, and cost-effective strategies for accelerating the transition to electric drive and reducing vehicle miles traveled by fossil fuel–powered vehicles and what are the trade-offs in implementing them?

• What are the technical and cost barriers to shifting electric vehicle battery designs and chemistries to be less reliant on critical and environmentally costly minerals?
• What are the best options (such as electrifying vehicles and developing net-zero carbon fuels) considering cost, achievability, life-cycle greenhouse gas (GHG) emissions, and trade-offs for decarbonizing heavy-duty trucks, rail, and large ships?
• What is the most cost-effective way to produce net-zero carbon fuels at scale for aviation while accounting for life-cycle emissions, including impact on existing natural carbon sinks?
• Are probabilistic design standards for resilience that account for climate uncertainty feasible and achievable? If so, what is required to accelerate their development?
• What is needed to improve and implement measures of infrastructure vulnerability and resilience to climate change at the local and national scales?

Increasing Road Safety

• What are the causes of rising highway fatality and injury rates, including for pedestrians and cyclists, and how can they be effectively mitigated or reversed?
• What is needed to develop scientific, data-driven standards for Vision Zero, Complete Streets, and Safe System Design that are appropriate for retrofitting existing infrastructure?
• What evidence and methods are needed for building public and political support for the use of effective behavioral safety enforcement measures?
• What are the causes of, and remedies for, the disproportionate deaths and injuries among Native American/Alaska Native and Black individuals?

Advancing Public Health

• What are the scale and significance of PM_2.5 emissions from brake linings and tires and the alternative materials to address them?
• What are cost-effective mitigations for the harmful health effects of bio-based low-carbon fuels?
• What transportation options can improve access to health care by low-income populations in the most cost-effective ways?
• What are the locations and conditions best suited for adding or retrofitting infrastructure that would significantly shift demand toward safe active travel?

Promoting Equity and Inclusion

• What are the most effective and appropriate measures of transportation-related inequities, options for their mitigation, and the economic costs and benefits of doing so?
• What is required, including theory, methods, data, and specific research, to develop a better linkage between accessibility (by mode) and measurable outcomes such as employment, earnings, and health?²¹²
• What would be needed to further develop the concept of a sufficient level of accessibility needed for households to thrive?
• What are improved governance models for transportation infrastructure and operations that would better incorporate meaningful input from residents of low-income and discriminated against communities?
• If remote working remains at current levels, what are the implications and appropriate and achievable options for transit, center cities, and transit-dependent populations?

Infrastructure Systems

• What is needed to develop improved infrastructure condition and performance measures across all modes that account for efficiency, safety, environmental, and other consequences that are appropriate at the national, state, and local levels?

Governance

• What are the methods and data needed to define best practices in managing governance and policy goal conflicts across jurisdictional borders and the public and private sectors to achieve climate, safety, equity, and economic goals?

Land Use

• What are best practices in coordinating land use and transportation infrastructure provisions and regulations to achieve climate, safety, equity, and economic goals that are transferable across jurisdictions?

Funding and Finance

• What are publicly and politically acceptable replacements for motor fuel taxes and the best means to mitigate the equity consequences of greater reliance on user fees?
• What are the most promising conditions, broadly defined, for reliance on private versus public provision of infrastructure?

Workforce

• What is required, in addition to documenting best practices, for attracting, training, and retaining a diverse, skilled transportation workforce, particularly in the public sector?

Innovation

• What are best practices in building and sustaining cultures of innovation in government transportation agencies? What is required to develop and implement them and how can this best be done?

METRICS

This edition of Critical Issues in Transportation introduces metrics for measuring the impact of transportation on achieving societal goals at the national scale. As desirable as such metrics are, important gaps would need to be filled in theory, measures, and data collection before they could be used to measure progress toward the societal goals emphasized in this publication. This appendix identifies challenges for expert practitioners and scholars regarding improved metrics for future use.

Transportation’s contributions to Building and Sustaining a Strong, Competitive Economy can be inferred from myriad economic statistics collected and developed by the Bureau of Economic Analysis and the Bureau of Labor Statistics. This edition of Critical Issues in Transportation focuses on transportation productivity, but this is only one possible measure. Other possible metrics might include (a) measures of competition in private aviation, ridehailing and taxis, and freight services, including competition across modes, because competition fosters efficiency and reduces the cost of transportation and (b) efficient access to employment through transportation, as well as barriers in gaining that access by individuals and groups, and the impacts on personal and societal prosperity.

In Mitigating and Responding to Climate Change, transportation’s total GHG emissions can be readily tracked through data that are carefully estimated and reported by the U.S. Environmental Protection Agency (EPA). The impact of recent federal legislation and transportation vehicle electrification can be monitored through electric vehicle sales and charging installations that are collected and reported by the U.S. Department of Energy. National goals have been set, against which progress can be measured, and credible forecasts are being produced from a variety of sources. Measures of the extent of infrastructure that is vulnerable to severe weather, fires, drought, and flooding due to climate change are available for a few jurisdictions, but not consistently or nationally,²¹³ nor do available measures take into account the speed of climate change and its spatial and temporal variability. The development of metrics in this area is still in its infancy and is an important area for further research.

The benefits of Advancing Public Health through reduced emissions, particularly PM_2.5, can be inferred from emissions data collected by EPA. Monitoring emission changes serves as a proxy for changes in mortality, which can be modeled periodically to calibrate appropriate estimates. If society were to commit to mitigating traffic noise near schools, residential areas, and other sensitive locations, techniques and measures would have to be developed analogous to those used for monitoring and tracking aircraft noise to track progress in these environments.

Considerable progress has been made over the years in collecting detailed data to measure progress in Increasing Road Safety, primarily through the U.S. Department of Transportation. Fatalities are well reported in all modes. Injuries, which are far more numerous than fatalities, are less well measured, but estimates have been developed using sampling and models. Measures of risk exposure, however, are lacking for active modes of travel. Anonymous cell phone data available from private vendors might be able to fill some gaps. Similar data are being explored by researchers and some jurisdictions, as is passive data collection from samples of data-collection sites using a variety of technologies.

Promoting Equity and Inclusion through transportation is possible, but progress is impeded by the varied, complex, and local nature of inequities, as well as a lack of consistent metrics for assessing their scale and impacts. No single federal agency has responsibility for the development and reporting of such metrics. Primary responsibility resides with EPA for measures of emissions, with the U.S. Department of Transportation for measures of accessibility to transit and personal vehicles, and with the U.S. Census Bureau for development of data that can be used for measures of residential segregation and income inequality.

Regarding travel, the U.S. Department of Transportation collects data on all modes that provide national estimates, but active travel has been measured infrequently and imperfectly. This creates gaps for both travel and safety metrics that researchers are exploring ways of filling.

Measures of condition and performance for Infrastructure Systems have been improving over time and should continue improving for highways specifically due to the performance measures that states are required to collect and report to the Federal Highway Administration. For private infrastructure, condition data can be derived from information gathered about safety, such as defects detected in inspections and reports of mechanical failures of safety devices, although many measures are not aggregated and reported at the national scale. For aviation system performance, on-time departure data are an available metric but are not particularly meaningful. The Federal Aviation Administration, as the manager of air traffic control, may have data with which to develop better

estimates of trends in airport-to-airport trip times while accounting for weather conditions, traffic volume, and runway capacity constraints. For marine infrastructure, little comparative information is available about the physical condition of port assets, but information about the performance in moving cargo in and between the largest U.S. ports is available from the Bureau of Transportation Statistics, which it has compiled from other agencies. Information about the age of locks and dams in inland waterways is available from the U.S. Army Corps of Engineers, which can be adjusted to account for major rehabilitations.²¹⁴ Improved performance data in the form of lockage delays are also available from the U.S. Army Corps of Engineers.²¹⁵

Funding and Financing information varies widely across modes, with much more information available for roads and highways than any other mode. Even within the public sphere, it is much harder to monitor trends in financial condition and revenue sources for airports, transit agencies, and ports and to track city and county roles in funding and financing transportation infrastructure. Much of the data are available when owners are public, but they are not aggregated or reported nationally. Funding and finance trends for private infrastructure presumably track those of the private sector more generally.

As important as they are, the influences of Governance, Land Use, and Innovation do not readily lend themselves to metrics at the national scale, although there is a large body of literature exploring correlations between measurable land use patterns and travel choices in specific jurisdictions.

Development of appropriate and meaningful metrics of transportation’s role in achieving all societal goals at the national, state, and local levels as well as defining the data collection required are important topics for future research and development.