mi opinion is, "likely" states it is one of the main reasons of why it happens, BUT there could be another reasons for it to happen (it= in this case, traffic congestion), it just simply states its perhaps the most common reason for it, another example... babies cry likely because they are hungry, BUT the can cry too because need a new diaper, or because they want to fall asleep... LIKELY just states a reason but not an absolute reason, perhaps only the most common reason...
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JigneshbharatiWhat is the grammatical function of "likely" and what does it modify here?It is a sentential adverb, that is, an adverb that modifies the whole sentence.