A lot of interstates simply follow existing US Numbered Highways. I-40 parallels US 70. I-75 parallels US 41 for the southern half of its route. Just like with the US Routes system, even numbered routes go East to West, odd numbers North to South; however, whereas US 1 is on the East Coast, I-5 is on the West Coast. The numbers are switched to avoid confusion.

Also, similar to the US Routes system is the usage of three numbered routes. A highway with a 1,3, or 5 at the beginning signifies a spur that splits from the parent route but doesn't return. A highway with a 2,4, or 6 is a loop that splits from the parent route and returns to the parent route. Also, there are some alternative routes that take you from the same point A to the same point B but via different cities. This was all carried over from the US Routes system.

The main goal with the interstate system was quicker travel to meet the demands of post WWII America and assist with national defense if necessary. Eisenhower go the idea when he saw the Autobahn in Germany and remembered his week long trek across the country along the Abraham Lincoln Highway in 1919.

The US Routes system connected the right cities. It just wasn't as effective. Generally, the interste system merely supplanted the US Routes system. Even as early as the beginning of the interstate system, some US Routes had been decommissioned. If an interstte wasn't necessary, it wasn't built. For instance, US 66 from Chicago to LA was decommissioned, and no interest follows that route. It's simply superfluous.

In some areas, the US Routes were simply built over, like how US 41 has been upgraded to an interstate near Copper Harbor, Michigan, or how Kentucky's Parkway System was grandfathered into the interstate system even though they didn't totally meet the standards. In California, many US Routes were conveyed to CA State Routes because they didn't really serve a purpose as federal highways anymore.