Carpool lanes are for carpools, right? In California, single drivers of hybrid vehicles could drive in carpool lanes right up until July of last year. After the state put the brakes on the program, transportation engineers shared bad news. Michael Cassidy, professor of civil and environmental engineering, and his graduate student Kitae Jang found that, with hybrids back in regular lanes, traffic slowed on Bay Area freeways. In a twist, though, the researchers discovered that carpool lanes slowed as well. Crawling traffic in regular lanes slowed vehicles entering and exiting carpool lanes, and, once traffic in carpool lanes was cruising, the team theorized, drivers may have felt nervous pushing 70 mph while cars next to them puttered along at 20 mph.