Mapsonus is a new one to me. I've always used Mapquest.
NY-DC 232 miles
NY-Philly 94 miles
Trivia question: Which state (or states) have the most "Interstate" highways?
Rules: They just have to be called (named) Interstate highways, even if they don't cross a state border. Bypass highways don't count, just two- digit Interstates, even if the 3-digit one is longer than a two digit one that doesn't cross a border.
Try to guess before looking at an atlas. That will make for more fun. I've already cheated. Or, look at a US highway map, and guess based on a quick look, without counting.