On the west side of Connecticut Avenue, near Thornapple Street in Chevy Chase, there's a small green sign that says "TEST DECK 3." Right under it, on the same post, is "SITE 1." As one drives toward the District, one finds other signs indicating Sites 2 through 5. Just before Chevy Chase Circle, there is a final sign that says "END OF TEST." What is a test deck? Where are test decks 1 and 2? It's a puzzlement.

Alexander Fraser, Kensington

Answer Man remembers when he was just a little Answer Boy. One of his clearest memories is of learning to read. Well, not so much the process of learning to read as the ability finally to read. And what he recalls most vividly is the day when all of the lines and squiggles he saw from the back of his parents' car started talking to him.

Shouting, actually. "GAS!" they screamed. "STORE! EXIT! ONE WAY! MOTEL!"

It was wonderful -- letters made words and words had meaning -- but it was also unsettling. Those signs had always been there. Answer Boy had seen them a thousand times. It was only when he was able to read that their secrets were revealed. What other messages were hidden in plain sight, kept obscure by his inability to crack some inscrutable code?

And so it is with the secret language of "TEST DECK 3."

The thing is, that sign isn't meant for you. It's meant for highway contractors and highway engineers from the state of Maryland.

A "test deck" sign signals that some material or method is being evaluated. In the case of that stretch of Connecticut Avenue, what was under investigation was a new type of reflective stripe.

Road stripes are put down a couple of different ways. Typically, a striping machine lays a coat of paint while from another spout comes an outpouring of extremely tiny, light-reflecting glass beads that stick in the paint. A stripe's ability to bounce the illumination from your headlights back at you is known as its "retroreflectivity."

Most highway departments across the country use a uniform size glass bead. But someone in Maryland decided to experiment with mixing different sizes. Would that improve a stripe's retroreflectivity?

Within the past five years (the State Highway Administration is a bit sketchy on the details), this mixed-size melange was tested at various locations across the state, including in Chevy Chase.

"The [test deck] signs were for the contractor to know where to do the striping," said SHA's Chuck Gischlar. They also were to keep other contractors from painting over the stripes. And they let engineers from the highway department -- who drove out on dark and stormy nights to check on the stripes' performance -- know that they were looking in the right place.

And what of those glass balls, some small, some large?

"You wouldn't think they were large if you looked at them," said Mildred Wade of Weissker Manufacturing, the Carbondale, Pa., firm that sells the beads. "They're large compared to standard beads."

The largest of the large beads are 1,400 microns in diameter, while the largest of the small beads are 212 microns. (A micron is one-millionth of a meter; a human hair is about 70 microns in diameter.) The beads -- virgin glass for the large ones, said Mildred, recycled glass for the small ones -- are imported from Russia and Germany.

You'll be glad to hear that the test was a success.

"It's definitely more visible," said Chuck of the 50-50 mix, which is now known far and wide as the "Maryland blend."

"We're the only ones in the country using it," he said. "The factory makes the blend just for us because we're so happy with it."

Drivers may stumble across other tests in the area. Chuck said there's a curious sign on I-70 near the Baltimore Beltway that announces distances to Columbus, St. Louis, Denver and Cove Fort, Utah. (Why Cove Fort? It's where I-70 ends, 2,200 miles away.) The sign is in something called the Clearview font, a relatively new typeface developed by the Pennsylvania Transportation Institute that researchers say is easier to read than what road geeks know as the Standard Highway Signs Alphabet.

"They were just checking the visibility of that new font," said Chuck.

As for those test deck signs on Connecticut Avenue, they had been forgotten until Answer Man's inquiries brought them to the attention of the sign executioner.

"They're probably going to come down in the next week," Chuck said.

Are you stumped by something you've seen in the Washington area? There's no guarantee that Answer Man can help, but, hey, you never know. Write answerman@washpost.com.

Signs on Connecticut Avenue in Chevy Chase signal a reflectivity experiment on the road.