Translation:Because of the traffic, we are late.
Yes they can. It is fine just as long as they have a main clause to support within the sentence.
Duo, please read I get that you CAN start a sentence with 'because' it just feels awkward. I would rather say 'we were late because of traffic.' I am a native speaker of English, but no master. I think a 'because/so' kind of sentence would have you choose one or the other as in 'there was traffic, so we were late' or 'we were late because there was traffic.' Again, it feels strange here.
因为...所以 is the Chinese way to describe cause and effect. In English, it is unnatural to use both "because" and "so" in a single sentence. You have to choose one or the other when translating into English.
"We were late because of the traffic."
"There was traffic so we were late."