In Pixar's Cars, what highway does Lightning McQueen accidentally leave?

Lightning McQueen accidentally leaves Interstate 40 in Pixar's Cars.

While being transported to California for the tiebreaker race, Lightning escapes from his trailer during the night. He follows a truck he mistakes for Mack, becomes separated from the race team, and ends up on the road network near Radiator Springs.

The film connects this journey with the history of Route 66. Interstate 40 replaced much of Route 66 as a major cross-country route, leaving many older roadside communities bypassed. Radiator Springs is portrayed as one such town, whose fortunes decline after the highway is rerouted away from it.

The question concerns the interstate Lightning leaves, not the town where he is later found or the historic highway associated with the town. Those nearby facts are often blended together because the plot uses all three elements to explain how Lightning reaches Radiator Springs.