Watch: Carpool Karaoke with Gwen Stefani, George Clooney, and Julia Roberts

James Corden's "Carpool Karaoke" just upped the ante. Although he was having a grand old time belting duets with Gwen Stefani and learning the lyrics to that one indecipherable moment in Sweet Escape, their trip was almost cut short when they approached the HOV lane which apparently requires four passengers.

Leave it to George Clooney and Julia Roberts to save the day. The Oscar winning pals hitched a ride and sang along to Holla Back Girl. And Gwen finally explains just what a Holla Back Girl is.

Watch "Carpool Karaoke" and sing along: