Although many environmental justice (EJ) populations such as individuals from low-income families, at-risk/homeless individuals, and older adults face mobility challenges, at-risk/homeless individuals face some of the most significant transportation barriers. Yet, measuring or quantifying the barriers associated with transportation among this EJ population poses a significant challenge. Consequently, linking the role of transportation barriers with mental health cost and outcomes among at-risk/homeless population remains difficult. This research program is made up of two phases. In phase 1, we will i) conduct a scoping review on measures of transportation barriers in relation to EJ populations, and ii) use a triangulation of qualitative methods to develop questions that will guide the development of a Transportation Barrier Scale (TBS) for at-risk/homeless populations. In phase 2, we will i) use the TBS to examine the effect of transportation barriers on mental health and outline social and psychological pathways through which transportation barriers impact the health outcomes and well-being of at-risk/homeless individuals, and ii) quantify the economic cost to society in treating mental health impacts and negative outcomes that result from transportation barriers. The research design uses deductive and inductive approaches and will employ qualitative and quantitative methods. Drawing from the empirical literature, focus group discussions (FGDs), and surveys, we will draw information from at-risk/homeless youth and emerging adults aged 14-25 years from the Dallas-Fort Worth metroplex. This will produce a novel set of questions to be tested in phase 2 of this research program. Faculty members from The University of Texas at Arlington School of Social Work, College of Nursing and Health Innovation, and Civil Engineering will partner with the Transition Resource Action Center (TRAC) to undertake this research. TRAC serves youth and emerging adults aging out of foster care in the cities of Dallas and Fort Worth. Both cities have public transit, but services are limited and disconnected, despite the fact that many individuals have to travel across the metroplex for employment purposes. This research program will develop a novel set of scale questions which will facilitate the measurement of transportation barriers among at-risk/homeless youth and emerging adults, examine the sequelae of transportation barriers, and develop a roadmap for future research that can transform transportation planning practice to better serve the needs of EJ populations.