When hybrids were first available, California handed out yellow bumper stickers to encourage sales of cleaner, high-mileage cars. Lone drivers with no passengers got access to carpool lanes, and solo drivers in regular lanes learned to hate the Prius. The program worked, and, with more and more hybrids, the carpool lanes began slowing down. So, in July, the yellow sticker privileges ended. But did traffic in the carpool lanes begin speeding up? It did not, according to Michael Cassidy, Professor of Civil and Environmental Engineering at the University of California Berkeley.