Transmission planning should seek to maintain or im- prove system security over time and facilitate robust wholesale power markets by improving transmission ca- pacity for bulk transfers across wide regions. Uncertainty about the development of future generation resources and regulatory policies complicates the process for long- range transmission planning. Furthermore, the evolution toward regional transmission planning requires that planners analyze and suggest improvements over signifi- cantly larger networks than have been studied in the past. Methods are required to screen numerous alternatives to determine those that offer the most cost effective means of security enhancement. This paper presents a methodology for using linear sensitivity analysis to evaluate potential transmission connections and aid selection of those that provide the most cost-effective improvements to overall system secu- rity. The methodology is applied to the planning of an extra high-voltage (EHV) overlay for the Southwest Power Pool (SPP) regional transmission system.