Pennsylvania
Have I told you how much I hate this highway? An interstate so numbered should be located east of Interstate 95. It's not even located east of Interstate 81, for crying out loud!
I-99's designation was written into law by Pennsylvania Representative Bud Shuster, who wanted an interstate highway passing through his hometown of Altoona. Since then, I-86 in New York and I-66 in Kentucky have also had their designations written into law.
Previously, the highway was known as US 220.