When your kids are little, a morning spent in warm sunshine and cool breezes, followed by an hour plus car ride, guaranteed they slept on the drive home. 

When they are driving age, you put them behind the wheel and take a nap yourself.

(Note To Concerned Readers:  It was my husband who left the driver seat and traded places with our son.  I remained in the shotgun seat, trying to avoid taking a nap myself. Not an easy task on undulating, country roads.)