PROJECT SUMMARY/ABSTRACT Safe Routes to School (SRTS) programming or infrastructure changes can impact total physical activity levels in children through increased active commuting to school (ACS). Previous evaluations of SRTS initiatives have shown increases in physical activity and in the proportion of students who engage in ACS, but no study has evaluated the effects of the implementation of a large-scale municipal SRTS infrastructure program on objectively measured physical activity and city-wide ACS using a controlled, prospective study design. In November 2016, $720 million in Mobility Bond funds were approved in Austin, TX, for transportation and mobility projects, including $27.5 million for SRTS infrastructure. Of the SRTS funding, $22.5 million was appropriated for larger-cost, transformative infrastructure projects such as protected bike lanes, pedestrian islands, shared use paths, and pedestrian hybrid beacons. This project provides a unique and time sensitive opportunity to conduct a rigorous evaluation of large-scale SRTS built environment improvements on the physical activity levels of a student school population. The overarching goal is to assess the effects of the SRTS infrastructure initiative on child physical activity (individual level changes), and to determine if the overall changes in built environment improve the proportion of students who engage in ACS in Austin relative to a comparison city (San Antonio) over the study period (population level changes). Three studies will be conducted: (a) Study 1, a quasi-experimental cohort study in Austin SRTS schools (n = 30) and San Antonio control schools (n = 15), to evaluate the effect of large-scale, transformative engineering infrastructure projects on student physical activity; (b) Study 2, twice yearly collection of cross-sectional data on the proportion of children engaging in ACS across ~70 Austin SRTS schools and 30 San Antonio comparison schools; and (3) Study 3, cost effectiveness of the infrastructure improvements in terms of increasing child physical activity will be determined. For Study 1, physical activity will be measured using accelerometers augmented by GPS; child and parent questionnaires will be used to collect self-report data on physical activity and domain-specific psychosocial factors; and the neighborhood environment will be assessed via environmental audits and Geographic Information Systems (GIS). Data for Study 2 will be collected using standard SRTS student travel tallies. Data from this proposed project will be useful in understanding the role that SRTS built environment changes have in increasing child physical activity and ACS.