Formulated by Barney Stinson of "How I Met Your Mother", detailing how relationships are like freeways, and there are exits that can be taken to get out of them:
1. 6 hours (aka one night stand)
2. 4 days
3. 3 weeks
4. 7 months
5. 1.5 years/18 months
6. 18 years
As Robin would later put it, if one person says, "I love you", then the relationship moves into the carpool lane, where there is a big diamond.
Robin: I usually don't get this far in a relationship. I usually take the 3 week exit.
Lily: Don't tell me you're actually buying into Barney's "freeway theory"