This dude sez: "Wide. Real wide, at least 15 feet." http://tinyurl.com/b4wh8a
This fella likes 'em narra': 10 or 11 feet. http://tinyurl.com/bw7okkRemember not to confuse a narrow shoulder with a BIKE LANE, which must be at least 4 feet wide (five if there is a curb, gutter, or gaurdrail). Note that if a bike lane is present, and it is the outside lane, it would make for a really narrow outside lane. What might not be clear is that 13 foot-wide lanes are no good. This is because 13 is, you know, bad luck. So: "Hey, traffic engineers, no more outside lanes 12 to 14 feet wide! Thanks." Actually, there is another reason, but you'll have to work that out for yourself.