A carpool turns separate commutes into an agreement that must survive ordinary delays.

Sharing a journey turns a private routine into a small contract. Alone, running ten minutes late costs you ten minutes; together it costs somebody else theirs, and the arrangement stands or falls on how those ordinary slips get handled. Most shared things break at that level rather than over anything dramatic. The question is never whether delays happen but what people do about them.

If you share anything regularly, agree in advance what happens when someone is late. Say the awkward part out loud early, while it is still hypothetical. Setting a rule is far easier than negotiating one in a cold car at seven in the morning.