Hello how may I help you? What is the longest thing in the world? There are many types of things. Can you be more specific? How about the longest highway in the United States Highway 1 is the longest single highway in the US. anything else? How long is it? Highway 1 is 655 8 miles long. Would you prefer this measurement in metric? No thanks. So I assume it doesn't run coast to coast? Highway 1 runs from Maine to Florida. Ok. Thanks for the information Glad to help!