I have the same problem on one of my regular rides, except it is a left turn into street that widens into a parking area for a community pool, and the lot is pretty gravelly too.  The road is so steep there is no way I can signal, and I definitely want both hands on the bars to be prepared for the gravel.  And then I need to be pedalling again very soon because once I make the turn there is a short steep rise to get over.  It is very hair-raising! 
 
What I try to do is take the full lane on the curve before the turn, and move to the left of the lane and broadcast my intentions by tipping my head that way if there are any cars in sight.
				
			 
			
		 
			
				
			
			
				Keep calm and carry on...