In Pixar's Cars, Interstate 40 is the bypass that takes traffic away from Radiator Springs.
The film explains that Radiator Springs once prospered because travelers followed U.S. Route 66 through the town. When Interstate 40 was built nearby, drivers could avoid the older road and its businesses. The resulting loss of traffic leaves the town economically isolated.
This fictional history reflects the real effect of interstate construction on many Route 66 communities in the United States. Pixar uses that background to give Radiator Springs a recognizable cultural setting rather than making it an arbitrary desert town.
Route 66 is therefore the historic road associated with Radiator Springs, not the newer bypass. The distinction matters: McQueen is traveling along the old route when he enters the town, while Interstate 40 represents the modern road system that displaced it.