This is a problem I have noticed specifically in the Boston area. Oftentimes when there is construction, a race, or another reason for having a road blocked, the Boston Police Department will put up a "detour" sign. Usually, that is the extent of it. You are forced to turn right, instead of going straight, and then there are no additional signs instructing you how to get back to your route. If you're lucky, the cars in front of you will know where they are going and you can follow them, or maybe you had your GPS on and it quickly re-routes you. In most cases, however, you are totally screwed.
Boston is not Manhattan; it's not a convenient grid, where 3 rights makes a left. With the exception of a tiny area between Hynes Convention Center and the Public Gardens, Boston is a mishmosh of streets.
I understand when there is an accident blocking the road, the police officers did not have the time to set up an appropriate detour, but if there is construction or a road race, chances are that was planned and there was sufficient time to figure out how to best redirect traffic.
When I first moved to Boston, there was a detour downtown that forced me to make a left I had not planned on. I spent the next 20 minutes just trying to figure out how the heck to get back to where I needed to be to get onto 93N. If you live here, you know what I'm talking about. I cursed a lot that day. Last week it happened again, in Brookline with construction blocking an intersection. Luckily I know the area well from being there for 4 years, but I felt badly for anyone who was from out of town or a new resident. I don't understand why it is so difficult to put up a few extra signs. Is there a shortage of "Detour" signs? If so, I'll help you make some. With glitter!