The House highways and tax committees are holding a joint hearing next week on a proposal to raise the tax on gas and diesel by 4 cents. The hearing will be held Tuesday. The Senate-passed bill would provide more money over the next two years for highway improvements, then take some of the tax proceeds to pay off $200 million in borrowing toward completion of the I-93 project. Once the debt is paid off in roughly 20 years, the tax hike would expire. The bill would also eliminate the Exit 12 ramp toll booths in Merrimack. The House passed a 10-year highway plan that outlines a way to help pay the remaining $50 million needed to finish the I-93 project. The plan also proposes borrowing money to replace the Sarah Mildred Long Bridge.