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The Transportation Security Index

Reliable access to transportation is essential to holding a job, grocery shopping, and getting to school, child care, social services, and other activities. Transportation insecurity — the experience of being unable to move from place to place in a safe or timely manner — has important consequences for people’s ability to connect to opportunity and flourish.

University of Michigan researchers have developed the first validated measure of transportation security that offers insights into who experiences transportation insecurity and enables researchers and practitioners to determine which interventions can improve this condition. 

 

1 in 4 adults in the U.S. experience transportation insecurity. More than half of adults in the U.S. who are experiencing poverty are also experiencing transportation insecurity. Adults in the U.S. experiencing transportation insecurity: 19% white and non-Hispanic adults, 29% of Hispanic adults, and 33% of Black non-Hispanic Adults. 39% of urban residents, 22% of suburban residents, and 13% of rural residents experience transportation insecurity.

Measuring Transportation Security

Modeled after the Food Security Index, the Transportation Security Index is a 16-question survey composed of items that focus on the symptoms of transportation insecurity (for example, taking a long time to plan out everyday trips and rescheduling appointments). By focusing on symptoms of transportation insecurity, the measure spares researchers from attempting the impossible task of cataloging every possible input — from bus schedules to gas prices — that influences transportation insecurity.

The Transportation Security Index is informed by extensive qualitative work — including ethnographic observations and 187 interviews — capturing the lived experiences of people experiencing transportation insecurity, and it has been validated using a nationally representative survey. The index can be used to assign respondents a continuous score representing their level of transportation insecurity or to categorize respondents as experiencing no insecurity or insecurity that is marginal, low, moderate, or high.

The original Transportation Security Index (TSI-16) is composed of 16 items. An abbreviated 6-item TSI (TSI-6) has been developed, validated, and shown to perform equally as well as the original score.

Both the 16-item and 6-item TSI are available under the CC-BY-NC-ND license. Please include the following attribution on all copies of the TSI: “Copyright (c) 2018 The Regents of the University of Michigan,” the citations below, and a link to the CC-BY-NC-ND license on all copies of the TSI. Any noncommercial use of unmodified TSI is permitted. If you are interested in commercial use or in building upon or modifying the TSI, please contact TransportationSecurityIndex@umich.edu.

Citations

Gould-Werth, Alix, Jamie Griffin, and Alexandra K. Murphy. 2018. “Defining a New Measure of Transportation Insecurity: An Exploratory Factor Analysis.” Survey Practice, 11:2, pp. 1-36.

Murphy, Alexandra K, Alix Gould-Werth, and Jamie Griffin. 2021. “Validating the Sixteen Item Transportation Security Index in a Nationally Representative Sample: A Confirmatory Factor Analysis” Survey Practice. 

MacDonald-Lopez, Karina, Alexandra K. Murphy, Jamie Griffin, Michael Bader, Alix Gould-Werth, and Nicole Kovski. 2023. “A Driver in Health Outcomes: Developing Discrete Categories of Transportation Insecurity.” American Journal of Epidemiology.

Murphy, Alexandra K., Alix Gould-Werth, & Jamie Griffin. 2024. “Using a Split-Ballot Design to Validate an Abbreviated Categorical Measurement Scale: An Illustration Using the Transportation Security Index.” Survey Practice, 1-17.

Administer the 16-item Transportation Security Index
Administer the 6-item Transportation Security Index
Potential Use Cases

The Transportation Security Index’s simple-to-administer questions allow policymakers, planners, practitioners, and researchers to use the index for a wide variety of purposes that help us better understand and address transportation insecurity. Examples of potential use cases include:  

  • Evaluating transportation interventions (e.g. providing free bus passes or ride sharing vouchers) pre- and post-intervention to determine whether such interventions are moving people from transportation insecurity to transportation security or, conversely, whether they are exacerbating existing disparities. Such evaluations can help determine which interventions perform best across different geographies and help planners and policymakers understand which interventions yield the most return on investment.  
  • Documenting the prevalence of transportation insecurity and tracking it over time on reoccurring, nationally representative surveys to see whether disparities are widening or closing, how disparities change with shifts in the geography of poverty, and how new transportation technologies (e.g. autonomous vehicles) impact rates of transportation insecurity. This could also be done at the regional level.
  • Identifying  geographic “hotspots” where people experiencing transportation insecurity spatially cluster to decide how (and where) to allocate resources, including mobility interventions (e.g. new transit routes and stops, transit vouchers) and social services.
  • Screening patients or clients during intake for social services to identify whether they are experiencing transportation insecurity and in need of transportation-related assistance.
Transportation Security Index In Action
 Researchers, planners, practitioners, and government officials have begun to use the TSI for their own purposes. Some examples include:

Research Team

Contact the research team at TransportationSecurityIndex@umich.edu.

Learn More

Frequently Asked Questions

What motivated you to create the Transportation Insecurity Index?

Several of our team members spent years interviewing and spending time with individuals with low incomes who faced significant transportation-related challenges. As we looked at available tools to capture the experiences of these individuals, we found many measures were proxies, falling short in capturing what we saw on the ground. For example, survey researchers sometimes look at whether people have cars to assess whether they can get around adequately. But we found there are many people who have cars who cannot get around when they cannot pay for gas, insurance, or repairs. Proximity to bus lines also cannot tell you much about transportation insecurity: our interviews revealed that some people live close to public transit but do not take it because they worry about their safety or have small children and find public transportation too cumbersome. Our qualitative research also revealed that social networks — friends, family, neighbors, co-workers — are a key way that people get around. No existing measures captured this important, relational dimension of mobility. Because many existing measures are based on travel behavior (e.g. commute time), they miss people who barely get around at all (“unmet demand”). Finally, many researchers, planners, and practitioners use different measurements and language to talk about and measure transportation insecurity. We thought providing a single, consistent measure and concept would provide a shared language for the community and enable better comparison of findings across populations and outcomes, thereby allowing us to better understand the causes and consequences of transportation insecurity. For all of these reasons, we determined a new measure explicitly designed to directly capture transportation insecurity itself was warranted. 

In designing the Transportation Security Index, why did you use the Food Security Index as your model?

The Food Insecurity Index, developed by the U.S. Department of Agriculture, is a well-respected, rigorous tool used to assess food insecurity in the U.S. One of its great strengths is that it draws on qualitative research to directly capture the experience of food insecurity. That is, rather than asking questions about “inputs,” like calories consumed, it asks questions that revolve around the symptoms of food insecurity (i.e. “In the last 30 days, did you ever cut the size of your meals or skip meals because there wasn’t enough money for food?”). Given the many different modes of transportation people use, we thought a Transportation Security Index based on symptoms rather than inputs was ideal. Since existing measures missed the qualitative experiences of transportation insecurity we saw on the ground, we also thought it best to develop new, original questions designed to actually capture the phenomenon of transportation insecurity. 

What are some of the strengths of the Transportation Security Index?

Because the Transportation Security Index was designed to capture transportation insecurity at the individual level, it is the only existing measurement best suited to do this. The index is “mode agnostic,” meaning it does not ask about people’s mode of travel specifically, so it is impervious to changes in the landscape of transportation and is thus well positioned to understand how the introduction of autonomous vehicles, for example, will impact transportation insecurity. The index is the only one of its kind to capture the relational dimensions of transportation insecurity. Finally, one of the most challenging issues for transportation researchers is capturing trips not taken (i.e. “unmet demand”). By creating questions tailored to people who face the most difficulties getting around, the Transportation Security Index is uniquely able to capture people unable to leave the house and skipping trips because of problems with transportation. 

Who can use the Transportation Security Index?

Anyone! The index was designed to be used by multiple parties, for different purposes, across different sectors from researchers to policymakers to planners to practitioners. The index can easily be included on surveys, whether nationally representative or regional, to understand broad patterns and to deepen our understanding of the condition of transportation insecurity itself. The index is easy to administer and can be used by individual healthcare providers or social service providers as screening tools to determine which clients and patients are experiencing transportation insecurity. It can also be used as an evaluation tool to assess whether a transportation intervention (like providing free bus passes) is having its intended impact.

If you are interested in how you might use the Transportation Security Index, contact us at TransportationSecurityIndex@umich.edu. We are happy to explore how you might put the index to work. 

If you are using the TSI in your own work, please let us know! We would like to keep track of how the index is being used and also what people are learning in using it.

Can I use a subset of the questions to identify transportation insecurity?

Unfortunately, no. The Transportation Security Index was developed using complex, rigorous statistical methods to determine how questions are being endorsed and how they relate to one another in ways that constitute an index that measures the construct of interest (i.e. transportation insecurity). Accordingly, while you may use a subset of questions that comprise the index in your own work, it is important to recognize that no subset of questions can be considered to constitute an index in ways that will allow for comparisons with the data generated by the index itself. Nor will a subset of questions identify transportation insecurity itself. Importantly, the research team is hard at work developing a short-form index that will consist of a smaller set of items than the current index.

Can I change the wording of the TSI items and/or the ordering of the questions?

We strongly advise against this. In terms of changing item wording, the construction of each item was informed by extensive qualitative research and refined using cognitive interviews. Through such interviews, we know that respondents understand the question as it is phrased well and that their responses are capturing the kind of data that is intended. Altering question wording could comprise respondent burden, comprehension, and judgement. Furthermore, the questions as they exist are those that have been validated; altering their wording may change the results in ways that make comparisons across findings difficult. The same is true of changing the ordering of questions. The ordering as they are presented is the ordering that has been validated. Changing the ordering (or adding additional questions in between those items that comprise the index) may change the results in ways that compromise the quality of the data and the ability to make comparisons across findings.

Questions in the index tap into the material symptoms of transportation insecurity (rescheduling trips, being stuck at home, arriving late to destinations) and the relational symptoms (feeling bad, relationship effects, worrying about inconveniencing others). The relational symptoms seem less relevant to my interests. Can I use only the material symptom questions in my work?

If your goal is to measure transportation insecurity, all questions in the index must be used (see the previous question for more information). That being said, our team wishes to emphasize the importance of the relational questions in the index. Our extensive qualitative research uncovered that the relational symptoms of transportation insecurity were a key way transportation insecurity manifested in the everyday lives of people and could be just as troubling for people as the material symptoms. Our quantitative analysis has confirmed this. Transportation insecurity is a unidimensional condition experienced both materially and relationally.

I am interested in transportation insecurity as it relates to work or health. Why don’t questions in the index have any items that ask people about difficulties getting to work or the doctor?

Transportation insecurity, of course, has important implications for employment and health, among other things. However, in designing the Transportation Security Index, we were intentional in not including such destinations or outcomes of interest in the TSI questions. We did this so researchers can use the Transportation Security Index in a causal inference framework to look at the consequences of transportation insecurity on outcomes like employment, health, education, etc.

What are your next steps in developing the Transportation Security Index?

Measurement development is an ongoing process. Currently, we are working on developing an even shorter abbreviated score that will be comprised of a smaller set of items than the 6-item abbreviated score (ideally 2-3), with a similar set of psychometric properties, and thus will be equally as able to accurately capture individuals experiencing transportation insecurity.

Funders

This project has been supported by the National Science Foundation (NSF) (OIA09936884); Stanford University’s Center on Poverty and Inequality (through funding provided by grant number H79AE000101 from the U.S. Department of Health and Human Services); the University of Michigan’s Poverty Solutions, College of Literature, Arts, and Science, Office of Research, Department of Sociology, the Center for Public Policies in Diverse Societies, Mcity, and the Population Studies Center (through NICHD center grant P2CHD041028). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views or official policies of the NSF, NIH, or HHS.