Interregional Transmission Planning Improvement Act of 2019 This bill requires the Federal Energy Regulatory Commission to initiate a rulemaking to consider (1) the effectiveness of existing planning processes for identifying transmission projects across regions that provide economic, reliability, operational, and public policy benefits, taking into consideration the public interest, the integrity of markets, and the protection of consumers; (2) changes to such processes to ensure that efficient, cost-effective, and broadly beneficial transmission solutions are selected for construction; and (3) cost allocation methodologies that reflect the multiple benefits provided by interregional solutions.