Who would be at fault in this situation,or would both be?

[al78]

Driver A is driving along a main road and there are two side roads very close together coming up. Driver wants to turn left into the second side road. Driver B is waiting at the first side road to turn right. Driver A starts signalling left before reaching the first side road (where driver B is waiting) intending to turn into the second side road. Driver B interprets this as meaning driver A wants to turn into his side road and pulls out. Driver A crashes into the side of Driver B.

This happened to my father once (without the crash), but am curious if this is entirely Driver B's fault or whether Driver A takes some blame and should have started signalling after it was clear he didn't want to turn into the first side road.