。
She was late because of a traffic jam.
Literal
She [topic-は] traffic-congestion 's [for the sake of-ために] was-late.
~のために ('because of, due to') here marks cause rather than purpose — same surface form as the purposive 'for the sake of,' but parsed by context. 交通渋滞 ('traffic jam, traffic congestion') is a fixed Sino-Japanese compound: 交通 ('traffic') + 渋滞 ('congestion, stagnation'). Tokyo's chronic traffic congestion is something of an institution — surface roads, commute hours, holiday weekends all carry their own predictable jams. The ために frame keeps the cause formal-ish; in casual speech you'd more often hear ~せいで ('because of') for negative causes.