And some, such as Santa Monica, use both, as you'll note in the photo of that city's Second Street below:
This is part of a burgeoning of new bicycle facilities in Santa Monica, which does see a great deal of bicycle traffic and obviously anticipates (and hopes for) more. Here it looks as though the inside car lane becomes an overflow lane for cyclists; it also firmly states that cyclists are not required to stay within the (rather narrow) bike lane.
For a while it seemed that all of Santa Monica's new bike amenities were going in on Main Street, but now the city has turned its attention back to its downtown.
This was photographed looking north from Second and Colorado, near the city's new (privately-operated) "Bike Center," set to open in a couple of weeks.