(FORTUNE Small Business) – Allan Odell was tired. He'd been on the road for days, doing door-to-door sales for his father's business. But as he drove across rural Illinois in 1925, something caught his eye: a series of roadside signs, each touting a nearby gas station. One sign said, "Gas," then "Oil," then "Restrooms," and so on, culminating in a sign pointing toward the gas station itself. It gave Odell an idea that would lead to one of the 20th century's most storied ad campaigns.

Odell was selling Burma-Shave, a shaving cream produced by his father, Clinton. The elder Odell had done poorly a few years earlier with a liniment called Burma-Vita--so named because its essential oils came from Burma--but he had high hopes for Burma-Shave. And now Clinton's son would hand his father a truly new sales tool. Returning home to Minnesota, Allan told his father about the signs he'd seen and suggested they try something similar. Billboards per se didn't exist in 1925, and Clinton Odell's advertising friends were skeptical. But Allan persisted, convincing his father to spend $200 on the idea. That autumn, two sets of wooden signs, 100 feet apart, appeared in southern Minnesota. They said: "Shave the modern way/Fine for the skin/Druggists have it/Burma-Shave." Not the most scintillating verse, perhaps, but the results were immediate: By year's end, druggists were placing their first repeat orders, and sales jumped to $68,000.

The Odells soon demonstrated the playful sense of humor that became Burma-Shave's trademark. Some signs promoted the product. ("Stores are full/Of shaving aids/But all you need/Is this/And blades.") Others gently touted its sex appeal ("The answer to/A maiden's prayer/Is not a chin/Of stubby hair") or gave advice ("Don't take/A curve/At 60 per/We'd hate to lose/A customer").

Details of the Burma-Shave story differ in the many accounts since written. But its genius and exquisite timing are plain. The campaign blossomed along with Americans' love for the open road. At the ads' peak in the early '50s, there were 7,000 sets of signs (600 verses) in 45 states, and Burma-Shave was lathered onto millions of American chins every morning.

The Odells did note a few changes in the business: When electric razors hit, a set of signs warned, "A silky cheek/Shaved smooth/And clean/Is not obtained/With a mowing machine." But Burma-Shave failed to advertise on radio or TV. And the rise of the Interstate Highway System was fatal, siphoning motorists away from back roads and allowing them to exceed the 35-mph speed limit for which the Odells had calibrated their signs. In 1963, with the brand in decline, Burma-Shave was purchased by what is now Philip Morris, and the sign campaign was abandoned--a relic of a bygone age, much like the "Chew Mail Pouch Tobacco" barn ads of the same period.

Burma-Shave today is a marginal brand owned by American Safety Razor of Verona, Va. The signs hang in museums or show up on eBay. But their most fitting epitaph appeared in a book about Burma-Shave, called The Verse by the Side of the Road, with an illustration of a car cruising past the following signs: "Farewell, o verse/Along the road/How sad to/Know you're/Out of mode."