Interstate 99

Interstate 99 (I-99) is an Interstate Highway in the United States with two segments: one located in central Pennsylvania and the other in northern Pennsylvania into southern New York along US 15. The southern terminus of the route is near exit 146 of the Pennsylvania Turnpike (I-70/I-76) north of Bedford, where the road continues south as U.S. Route 220 (US 220). The northern terminus of the Pennsylvania segment is at an at-grade intersection with Musser Lane near I-80 near Bellefonte. The northern segment is entirely concurrent with US 15, beginning at I-180 in Williamsport northward into New York to an interchange with I-86 in Corning. Within Pennsylvania, I-99 passes through Altoona and State College—the latter home to the Pennsylvania State University—and is entirely concurrent with US 220.

In November 2024, the northern segment was extended down US 15 to I-180 in Williamsport, entirely concurrent with US 15. Long-term plans call for the two segments of I-99 to be connected using portions of I-80 and US 220 through Pennsylvania. The only current progress on the connection is the direct access interchange at I-80.

Unlike most Interstate Highway numbers, which were assigned by the American Association of State Highway and Transportation Officials (AASHTO) to fit into a grid, I-99's number was written into Section 332 of the National Highway System Designation Act of 1995 by Bud Shuster, then-chair of the US House Committee on Transportation and Infrastructure, the bill's sponsor, and the representative of the district through which the highway runs. I-99 breaks the AASHTO numbering pattern associated with Interstate Highways, as it would be expected to lie east of I-97 but instead lies east of I-79 and west of I-81.