Transportation planning plays a critical role in shaping economic health and quality of life. Transportation planning not only provides insight into the mobility of people and goods but also influences patterns of growth and economic activity. Metropolitan transportation planning is a challenging topic because of the lack of future traffic information, such as evaluations of capacity sufficiency on large metropolitan road networks with signalized intersections. The Highway Capacity Manual provides methods for the analysis of signalized intersections and urban streets for planning; however, these methods need detailed traffic volume inputs and lane configurations at signalized intersections, which are not normally available in metropolitan transportation planning. The conventional four-step planning process is widely used to forecast directional traffic volumes on road segments, but the projected volumes are not enough for the analysis of the capacity sufficiency of future road networks through existing methods. This paper discusses a proposed method developed through the integration of available transportation planning data and the characteristics of signalized intersections. The paper links traffic assignment results from conventional transportation planning procedures to the estimation of capacity sufficiency. With the proposed method, transportation planners can estimate the capacity sufficiency of future metropolitan road networks through the use of data that are readily available in transportation planning. This method will dramatically decrease the effort required for the capacity evaluation of large signalized metropolitan road networks.