I'm ok with someone flashing their brights, when they are slowing to let me make a left turn from SW Walker Rd. The traffic is not so fast that it holds anyone up, but it is seemingly endless in both directions. When I'm standing out in the middle of the lane signaling to turn left, I'd like to get off Walker ASAP
The flashing of brights is useful, because then I know they've really SEEN me, as opposed to slowing to fiddle with the radio, talk on the their cell phone, interact with another passenger in the car, etc. If I don't know they are slowing for me, I won't go. For all I know, they'll realize they've slowed down and speed it up, just in time to hit me!
As wsbob says, it is hard to really see what the driver is doing/intending - glare off windshields, tinted windows, etc.
However, if there is a big hole behind them, I'd sooner they just go and I'll take my turn.
For the record, when I am DRIVING on Walker Rd, I have been known to slow and let cars turn left. The street backs up darn quick when someone is waiting to turn and can't get through.