Brown announced veto of the bill on Saturday.
"Carpool lanes are especially important in Los Angeles County to reduce pollution and maximize use of freeways," Brown wrote in a veto message. "We should retain the 24/7 carpool lane control."
Assembly Bill 405 had been approved by both houses of the state Legislature earlier this month. The Assembly approved the bill 69-1 and the Senate passed it unanimously.
"There is no reason for drivers to be stuck in traffic when a midday or late-night accident or mysterious slowing clogs the rightmost freeway lanes, while the carpool lane sits empty," said the bill's author Assemblyman Mike Gatto, D-Los Angeles, during the legislative process."AB 405 is a cost-effective and common-sense way to ease unnecessary congestion, while still promoting ride-sharing and carpooling during the busiest travel hours."