I've been through some pedestrian tunnels that run under the 101 freeway in the San Fernando Valley. They can be convenient, but late at night it pays to be a little cautious because some tend to be homeless hangouts. There are tunnels near Fallbrook, Winnetka, and White Oak.. those are the only ones I'm aware of. A co-worker of mine says that there are many in India except they call them "subways" there, and that not many take them past 7pm for fear of crime. I hope they stay open because this town deserves to have passageways that aren't just dedicated to cars. Plus, it can be exciting to explore.
The tunnel near Fallbrook ^