The Boston Region Metropolitan Planning Organization (MPO) selected Route 138 in the Town of Milton as the subject of a priority corridor study in federal fiscal year 2018. The Route 138 Priority Corridor Study in Milton focuses on one of the locations identified in the regional needs assessment of the MPO’s Long-Range Transportation Plan, Charting Progress to 2040. This report presents details of the analyses of existing conditions, assesses safety and operational problems in the corridor, discusses options for roadway improvements, and makes recommendations for implementing improvements. The recommendations, if implemented, should result in Complete Streets benefits, such as enhanced pedestrian and bicyclist accommodations, increased traffic safety, more efficient traffic operations, reduced congestion, and promotion of multimodal transportation. The report contains technical appendices, which cite the methods used and data applied in the study, and includes detailed reports about safety and capacity analyses.