Turn signals are a good safety item on your Model T and A Fords if you do a lot of touring, because most drivers don't know a hand signal when they see one.
The add on turn signal kits don't have automatic signal cancel after you make a turn. Far too many drivers forget that the signals are on and drive for miles with turn lights flashing.
In regard to leaving your turn signals on - on my Model Ts I put a diode from each of the front turn light wires to a buzzer inside the car (Radio Shack sells both 6 and 12 volt ones). It gives me a good warning reminder to turn the flashers off. I use the diode trick even when the turn light system has a warning light - I have found that terminal on the flasher to be erratic with some systems, and the diodes assure that the warning blinker and buzzer get a reliable signal. I have also found that I and a lot of other drivers just don't see that warning light blinking away.
I put a switch in series so that I can cut the buzzer off, such as when I need to drive a long distance with four way hazard warning flash going. I use the four way to warn cars that are overtaking me when I am going up a long slow grade. This helped a lot while pulling Vail Pass on Interstate 70 a couple of times during the 1996 MTFCA Western National Tour at Frisco Colorado.
I wire my turn lights so that the circuit is always hot in case of a break down where the car will be sitting by side of the road. This means that it is also easy to just forget to shut off the turn lights when I park; the buzzer trick also helps to remind me if the turn indicators are still on when I shut the T down.
Several people have contacted me in regard to where the basic turn signal units can be obtained. In my diagram of a turn signal setup for Model T/A's I suggest using control units 48022 or 48272 by GROTE. These are the numbers on control units purchased from J. C. Whitney some time back (phone number 1-312-431-6102). The difference in numbers was the 6 volt or 12 volt flasher in the kits. Whitneys current catalog No. 608C, on page 60, has turn signal flasher kits as catalog number 13VC8972X for 6 volt and 13VC8973N for 12 volt at $39.95 each. I put one of the Grote units in a 26 tudor and had to cut the end of the handle off to give more clearance from the window regulator handle. The open end of the cutoff handle was then filled with material, such as black RTV or black plastic bumper repair filler. I hope that this helps.