- Southern Terminus:
- Pennsylvania-New York state line
- Northern Terminus:
- Southern Tier Expressway (I-86/NY 17) in Painted Post
- 12.89 miles
I-99 is a north/south highway that, when completed, will extend from the Pennsylvania Turnpike (I-70/I-76) in Bedford, Pennsylvania, to the Southern Tier Expressway (I-86/NY 17) in Painted Post, New York. The portion in New York has been fully completed, while the section in Pennsylvania has only been upgraded to interstate standards and assigned the I-99 designation south of I-80. The portions of Future I-99 north of I-80 are designated US 220 and US 15. In New York, the US 15 designation continues and is multiplexed with I-99 for its entire length in the state.
Unlike most other interstates, I-99 had its route number written into federal law by Congress. It is a few hundred miles out of place under the numbering scheme where interstates increase from west to east. According to an article in The New York Times, the number was selected by Senator Bud Shuster of Pennsylvania based on a street car that once existed in the region.
I-99 was the first highway in New York to have its exit numbers converted to use a milepost system. It has unique "Formerly Exit X" tabs on its signs that are different from the "Old Exit X" tabs used on other highways that were converted to milepost exit numbers at later dates.