During times of economic hardship, people turn to the road to see if they can make their luck somewhere else. As such, back in the days of the Great Depression, the U.S. saw an increase in the hobo population. Walking along long roads or hitching rides on trains, these hobos would travel about, looking for a place where they could get lucky and find a better home. Of course, such a life of wanderlust was difficult, especially since one has to travel without knowing anything of the landscape or local populace.
To combat this ignorance, the hobos came up with an ingenious sign language to communicate to each other along the way. This is not like the sign language that hearing-impaired people use to communicate; rather, it was markings and drawings that hobos would leave along the road for their fellow travelers. Whether a sign told others of locations of important places in town, the attitudes of the locals to tramps, or the best places to beg, the hobo sign language helped many get by in hard times.
The variety of messages passed between hobos are incredible. There are some basic traveling symbols such as "go this way," "don't go that way," or "get out fast." Then there's praises and warnings of the locals - "doctor, no charge," "police officer lives here, not kind to tramps," "dangerous neighborhood," "you may sleep in barn." Some of my favorites messages I've heard of are "good lady lives here, tell a hard luck story," "fake illness here," "road spoiled, full of other hobos."
Hobo signs were typically drawn onto utility poles using charcoal or some other type of temporary writing material that would wash out in time with the weather. Sometimes they would write on railroad trestle abutments, outcropping rocks, or even on houses when referring to those who lived inside. Billboards, when they first appeared, were also prime places for signs. When more automobiles, and consequentially more roads, were built, hobos created their own extensive system for charting routes for those who would travel the highway.
The hobo sign language was hardly a formal system, constantly in flux. The signs had to keep up with new ways of life (such as the addition of roads), and like most languages it had its own dialects in different parts of the country. Also, the signs were often changed when it became evident that locals were writing hobo signs for their own amusement. One had to keep meeting up at hobo gathering spots to stay on top of the current system.
Much of the hobo sign language has been lost with time, due to its temporary nature. The need for the language has decreased as well; there are many fewer hobos now than there were in the past, and the progress of communicative technology has made the use of signs somewhat outdated. Still, it is nice to know that people will leave along hints for how to get by for fellow knights of the road.