A car ritual is something young kids (and on occasion those who are just young at heart) do when they are trying to break the monotony of driving.

Some common rituals I know of include;
punch buggy
Lifting your feet while going over bridges or railroad tracks.
Ducking under overpasses.
Singing "Happy birthday dear car", or buying ice cream when the odometer hits large round numbers.
Yelling "Cow alert" to signal the other passengers to plug their noses when passing Cows. Regardless of whether you can smell them or not.

These are simply the really common ones. Hundreds of others must exist that are in practice by no more than a few families.

The exact details and the rituals practiced vary from family to family. If your really bored you can lift your feet going over every overpass, but this can get quite tiring in large cities.