Hazardous Materials Safety Improvement Act of 2019 This bill requires the Pipeline and Hazardous Materials Safety Administration (PHMSA) of the Department of Transportation (DOT) to respond to petitions for regulatory action within 18 months, including petitions for reconsideration of a rule and petitions from groups with delegated authority. Any action taken by DOT or the PHMSA in response to a petition shall supersede any decision concerning the issue reached by a group with delegated authority.