M: How long will it take us to drive to your brother’s house?
W: ______ we’re out of the city, it’s about 120 kilometers.
1 Answer
In many cases, "once" and "when" used in this way do not significantly alter the meaning of the sentence. For example, in the two following sentences, the opening adverb (once or when) is immaterial to the overall meaning of the sentence:
When we leave the city, it's about 120 kilometers. Once we leave the city, it's about 120 kilometers.
Both sentences are clear that the journey includes distance inside the city. But these two sentences are not the same:
When we're out of the city, it's about 120 kilometers. Once we're out of the city, it's about 120 kilometers.
The slight change in phrase ("we leave the city" vs "we're out of the city") makes the difference. In "we're out of the city," it's not clear whether the speaker is referring to 1) the journey from inside the city, exiting the city limits, and then going another 120 kilometers, or 2) a reference to a previous time in which the speaker was somewhere outside the city limits and found that it was 120 km from that point to the brother's house. This is a little nitpicky, and almost all native English speakers would infer meaning #1 (which makes both phrases equivalent in meaning). However, the more clear of the two--and I would argue, the more correct of the two) is choosing "once" instead of "when" in this context.
