I can't leave early -- the other assistant in my office leaves at 4:30 pm every day to catch her train, so I have to stay until 5 pm. I probably would have enough time if it took as long as Google maps says (15-20 min from my house), but not if there were traffic? The reason I could do the Cambridge one is that it's actually between work and my house.

(My other data point is silks practice, which is just about the same distance from my house and starts at 7 pm and for which I am never on time :->)