(https://lh5.googleusercontent.com/-whM27PxPCng/UawiEi7KgWI/AAAAAAAACEk/-NeJauG2OcQ/w130-h142-no/RTDCueSign.png)
(From Google Streetview)
RTD in Denver uses these types of signs to guide operators who may not be intimately familiar with a route, especially if the route deviates off a main road onto a smaller neighborhood street. Generally, the local and regional routes use green signs, while the express routes use red, but it isn't a hard rule (the regional and express colors correspond to the system map, but local routes are blue). RTD uses these signs fairly consistently throughout its service area, but I have never seen a sign like this elsewhere? Do other transit agencies use these types of signs, or is it a unique concept to just the Denver area?
I've seen temporary ones in Boston for bustitutions of rail service. In fact there's a couple that appear to linger permanently in Brookline and/or Newton near and along MA 9, leftover from busing the Green Line D Branch.
They're plastic sandwich boards on the side walk that read something to the effect of "shuttle bus turn ->". I've never thought to take a pic though.
I've never seen those before. But I've had bus drivers, both local buses and long-distance coaches, ask for directions before, so I can definitely see their usefulness. It was rather disconcerting the time my Greyhound driver asked the passengers how to get to Chicago's terminal... :-o
I think I've seen random tiny arrow signs in the Boston area that could be these, but my memory's a bit fuzzy.
Maybe the lack of these in Winston-Salem explains why no bus is ever on time.
Portland's TriMet uses a small red arrow, usually mounted within a bus stop sign but occassionally mounted on its own small sign. They are virtually worthless, unless you know exactly where to look (and in which case, you really don't need it).
Here's an example on S.W. Multnomah Boulevard. Look just above the green Tualatin River Watershed sign (http://maps.google.com/?ll=45.46748,-122.728643&spn=0.000003,0.001762&t=h&z=20&layer=c&cbll=45.46748,-122.728489&panoid=iAFBnNvooy3b4YtGrM5Geg&cbp=12,288.56,,1,6.58).
Here's an example of one mounted within a bus stop sign (http://maps.google.com/?ll=45.454953,-122.758741&spn=0.000015,0.014098&t=h&z=17&layer=c&cbll=45.454952,-122.758854&panoid=_r1ZGl7M0ztlyCGfkZU6AA&cbp=12,323.59,,1,9.92), in unincorporated Washington County (between Tigard and Portland).