What does a diamond shape painted on the road within a lane indicate?

A diamond shape painted within a lane marks that lane as restricted-use. Most commonly it indicates an HOV (high-occupancy vehicle, or 'carpool') lane that is restricted to vehicles with two or more occupants during certain hours. Diamond markings can also indicate bus lanes or bicycle lanes depending on local convention; usually a sign accompanies the marking to specify the rule. HOV lanes are typically the leftmost lane on a highway and are intended to reduce congestion by rewarding carpooling. Violating HOV restrictions carries fines, and many HOV lanes are now monitored by automated enforcement. Outside posted hours, some HOV lanes open to all traffic; check signage carefully.