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Most of I-40 between Barstow and Oklahoma City follows the course of Old Route 66.
I-40 has been proposed by many to extend westward from Barstow to at least Bakersfield. It could also go even further, to I-5 or all the way to US 101.
A Business Loop of I-40, in Winston-Salem, NC, is actually a freeway for its entire length. That's because I-40 was originally routed onto the freeway through downtown, and it continued to follow that route until a new bypass was built. When the bypass opened, I-40 was rerouted onto the new freeway. Other Business Loop freeways include I-85 in Greensboro, NC and Spartanburg, SC, and I-80 in Sacramento, CA.
I-540 in Raleigh, NC is part of the proposed outer beltway called I-640. The existing beltway around the city is signed as I-440.
I-840 around Greensboro is under construction. It is part of a new beltway that will also carry the new I-85 bypass, and part of I-73.
Future I-840 around Nashville is a complete southern loop around the city. The northern half of the proposed outer beltway has not yet been built, mainly because of legal issues. Currently, the highway is signed as TN 840.