Heading west from New Orleans, you enter bayou country, where alligators and herons splash in the swamps and Spanish moss hangs heavy overhead. Sunlight filters through, and for a glorious moment, you feel like the only driver on this scenic road. While Louisiana’s Highway 31 is finest in spring and fall, when the weather is pleasantly cool, it’s summer that heralds peak road-trip season for much of the U.S.