"If the lane is too narrow to share" can legally be invoked in most cases from what I understand, because most lanes cyclists ride in are narrow enough.
https://www.bikeleague.org/content/traffic-laws
> Bikes can share the same lane with other drivers. If a lane is wide enough to share with another vehicle (about 14 feet), ride three feet to the right of traffic. If the lane is not wide enough to share, “take the lane” by riding in the middle.
In Texas, 14 feet is explicitly written into the law. As a cyclist I frequently point this out to aggressive drivers. From what I understand there's often case law establishing this in states without an explicit law, but as I said, I live in Texas, so I'm not entirely familiar with other states' laws.
Quotes from Texas' law:
http://www.biketexas.org/en/infrastructure/texas-bicycle-law...
> (a) Except as provided by Subsection (b), a person operating a bicycle on a roadway who is moving slower than the other traffic on the roadway shall ride as near as practicable to the right curb or edge of the roadway, unless:
> [...]
> (4) the person is operating a bicycle in an outside lane that is:
> (A) less than 14 feet in width and does not have a designated bicycle lane adjacent to that lane; or
Also, in the US, my understanding is that cyclists can ride whereever they want if they are at or near the speed limit. Still doesn't stop some drivers from almost killing me because they think 20 mph in a 15 mph zone is too slow.