I have taken BART to Union Square many times, at all times of day, and I'm really surprised at the responses here. I have never considered the route suggested (Powell St. to O'Farrell) to be the least bit dangerous. The area is well travelled throughout the day and evening, both by tourists and locals alike.

It's hard to get turned around, because Powell St. is the endpoint for the cable car, so it's a matter of following the cable car line for two blocks and then hanging a left. What am I missing here?