I remember when I was a child in the 1970s, there were playsets available that included various traffic control signs. One of them was a “GO” sign, with GO displayed in a green circle. Even today, I’ll occasionally see a blister pack of road signs in the toy section of a store, almost always with a GO sign among them.
GO signs aren’t in the Manual of Uniform Traffic Control Devices (US federal guidelines for road sign design), but was there a time in the distany past when they were ever in use?
Well, there are GO signs at train stations all over Toronto, but that’s probably not quite what you meant. 
I suppose you could argue that traffic signals are STOP and GO signs, but they function differently than a metal STOP sign. At a red traffic signal, you must stop until the red signal goes away and is replaced by a green signal. At a metal STOP sign, you must stop, but you are allowed to go again under specific conditions even while the metal STOP sign remains.
There is a comparable situation with NO EXIT signs. Everyone has seen the NO EXIT sign, a red circle with a horizontal white bar that doesn’t touch the side of the circle. But there’s a corresponding EXIT ALLOWED sigh. It’s green, with a vertical white bar that breaks the side of the circle, dividing it into two parts. I’ve only seen it in use in the Montreal airport.
Here’s a manually operated traffic signal from 1922. Note the ‘GO’ sign.
Here’s a modern one. No info with the photo on where it is used.
http://www.freefoto.com/preview/21-55-52?ffid=21-55-52
Here’s a hand held go sign from the Andy Griffith show:
http://www.jodysgarage.com/images/funny/griffith/ernest_go_sign.jpg
Didn’t some of the early traffic signals use “go” and “stop” signs that swung out on mechanical arms?
I always thought they should have go signs at all intersections where they have stops signs for cross traffic. People sometimes steal signs you see. I’ve seen a few intersections with a bare metal pole where there should be a stop sign.
If you come to an intersection with no sign then you know to slow down and see what the across traffic is doing. In case the stop sign was stolen.
I recall seeing references to those, but most of my early 20th century history comes from Warner Bros. cartoons, so it may not be accurate.
I have no cite handy, but I’ve recently seen one in a safety equipment catalog at my work, except it was octagonal like a stop sign. I assume these would be used on private property.
There is a Keep Moving, but even then a remarkable number of people think it means ‘Stop’.
What do they do when they’re doing roadworks? That would be where I’d expect to see a “Go” sign - handheld by road crew, with a “Stop” sign on the back.
These days round here they mostly have “Stop” on one side and “Slow” on the other, but I wouldn’t be all that surprised to see a “Stop/Go” pair.
A Stop/Go rotating sign is exactly what is used here (South Africa) for when there are roadworks with alternating traffic. It’s an ordinary stop sign on one side, and the other side says “GO” on a yellow background. It’s sign “R1.5” on this page.
Stop signs and Go signs are used in the UK exclusively for temporary roadworks, as far as I know.
True - they come in three forms:
-Temporary traffic lights (with red/amber/green signals on a tripod instead of the normal ground fixed pole)
-Circular boards with ‘stop’ on one side and ‘go’ on the other, on some kind of motorised timer - for long sections of road works, there will be one at either end, synchronised to face the same way (with an interval to allow traffic to flow through).
-As above, but manually-operated. That’s right - for some temporary roadworks, it is the job of a person (two people, in the case of long stretches) to stand there and periodically rotate a sign.
I have some recollection of this also but no idea now where I saw them.
That was my first summer job: flagman at a gas company construction site. I almost caused an accident. 
The sign said STOP on one side and SLOW on the other.
The ‘priority road’ sign, used in many countries, would do the job - there’s an explanation here.
A moving obstruction can necessitate this - large hedge-trimming machinery on rural roads is an example I’ve come across more than once.
The Acme Semaphore Signal was popular in the 1920s and 1930s. The Smithsonian has one. Besides the familiar red and green lights, it had “STOP” and “GO” signs on arms that would swing out from the unit. More here.
“Acme”???
So Fear Itself was right!
I thought I remembered them as a child. What do you know, they are antiques! As I am now.
As an aside:
Several years ago, in my area, the typical red lights were replaced by ones that had an interior bright strobe that blinked intermittently. The idea was that the strobe would alert a driver that the light was red. After a few months the strobes were removed and the old red lights were reinstalled. I was talking to a local cop and asked him why they were removed. He said that some people interpreted the blinking strobe light as a blinking red; they would come to a stop, and then proceed. Accidents occurred with cars that had the green light.
Unexpected consequences.
