In Pixar's Cars, Interstate 40 is named as the highway that bypassed Radiator Springs.
The town once depended on traffic traveling along the older road now associated with historic Route 66. When Interstate 40 opened nearby, travelers stopped passing through Radiator Springs, causing businesses and visitor numbers to decline.
This detail gives the film's setting a recognizable historical background. Across the American Southwest, improved highways redirected traffic away from older roadside communities, leaving many towns dependent on tourism, preservation, and nostalgia.
Route 66 is the older road connected with Radiator Springs, not the answer to the question about the bypass. The film's fictional geography compresses and adapts real places, but Interstate 40 is the named modern highway in the story.