Possible Clues:

• Road tripe quorum
• Road trip quorum
• Six in a sedan, say
• Vehicle's capacity
• Six people in a sedan, e.g
• Mom, dad and three kids, e.g
• Maybe packed saloon bars the centre of attention!
• Lots of travellers being prudent, avoiding the very interior
• Vehicle's worth
• Mom, dad, three kids and luggage, e.g.