Final answer:
Three-number highways with an odd first number are typically spur routes that lead into a city or urban area, as opposed to even first numbers, which indicate a circumferential route around a city.
Step-by-step explanation:
Three-number highways with an odd first number are typically spurs or bypasses that branch off from a main interstate highway and lead into a city or urban area. These routes are part of the Interstate Highway System in the United States, and the numbering convention is designed to help travellers navigate. When a three-digit interstate route has an odd first number, it means the route connects at one or both ends to the parent interstate highway, facilitating access into or around a city.
Conversely, three-digit interstate routes with an even first number usually indicate that the highway forms a beltway or loop around a city, returning to the main interstate.