Passing lane
The meat:
"A passing lane is often colloquially referred to as a fast lane because it is often used for extended periods of time for thru traffic or fast traffic. In theory, a passing lane should only be used for passing, thus allowing, even on a road with only two lanes in each direction, motorists to travel at their own pace. Good driving practice is to slip out of the passing lane once slower cars have been passed."
Also, to save some of you some time:
col·lo·qui·al
–adjective
1. characteristic of or appropriate to ordinary or familiar conversation rather than formal speech or writing; informal.
2. involving or using conversation.
—Related forms
col·lo·qui·al·ly, adverb
col·lo·qui·al·ness, col·lo·qui·al·i·ty, noun
—Synonyms 1, 2. Colloquial, conversational, informal refer to types of speech or to usages not on a formal level. Colloquial is often mistakenly used with a connotation of disapproval, as if it meant “vulgar” or “bad” or “incorrect” usage, whereas it is merely a familiar style used in speaking and writing. Conversational refers to a style used in the oral exchange of ideas, opinions, etc.: an easy conversational style. Informal means without formality, without strict attention to set forms, unceremonious: an informal manner of speaking; it describes the ordinary, everyday language of cultivated speakers.
—Antonyms 1. formal.