Remember those yellow diamond-shaped "Baby on Board" signs that seemed to be in half the cars on the highway a few years back? Now, you seldom see them or the parodies they inspired, like "Mother-in-law in Trunk."

Since this sign fad involved kids, cars, a simple warning and humorous responses, it had potential for spawning urban legends. But the only story I heard was invented by an individual and never caught on.In 1987 a man in Englewood, Colo., a suburb of Denver, sent me the following "new urban legend" which, he stressed, "is absolutely untrue, since I made it up as I was driving down the freeway today." Here's the story:

"There is an underground kidnapping ring operating in Southern California. These people steal babies to sell on the black market.

"Their method of locating children is to look for the little yellow 'Baby on Board' signs in the back windows of cars. They follow the mothers to the supermarket, or tail them to their homes, and kidnap their babies."

My Colorado correspondent vowed to tell this story to 25 people in the Denver area. Also, his son would tell the story to his 6th grade classmates, and his wife would repeat it to her church group.

He concluded, "We wonder how long it will take to reach your ears or to come up in a newspaper column or a talk show."

Apparently, his experiment in creating a legend didn't work. At least, I've never heard the story told anywhere else, and this is the first mention that I'm aware of in a newspaper column.

Maybe the story wasn't believable enough, or perhaps it lacked the bizarre twist urban legends require. Possibly the fad for "Baby on Board" signs peaked before the legend could take hold.

Recently, "Baby on Board" signs came up for discussion on the urban folklore computer newsgroup that I read regularly. Someone posted this story:

"The teller of this story swore it was true. She heard from an ambulance attendant about an accident in which the car caught fire. A rescue worker helped haul out the adults from the front seats, then saw the sign in the back window and thought there must be a baby in the back seat as well.

"He went back in to rescue the baby and was burnt to death, but there was no baby in the car, just the sign."

Sure enough, someone else knew a variation. She posted a terse reply, summarizing the story a friend had told her:

"Couple rescued from car accident in winter. Knocked unconscious, but the next morning the wife wakes up and asks husband `What about the baby?'

"Rescue workers return to scene of accident and find baby wedged under the seats. It had survived the accident but died of exposure overnight.

"This led (friend assured me) to baby-on-board signs."

A third newsgroup member soon added, "I saw these signs in Germany months before they appeared in this country. I think they have a European origin."

And a fourth person wrote, "I haven't heard these stories, but there was a move afoot to make these signs illegal in New York. Supposedly, they obscured the view through the back window."

In these four comments, I detect the hint of a legend tradition in stories purporting to explain the signs' origin and possible hazards. Each story alludes to a fatal accident involving a baby in a car.

It's likely that some such event did give someone the idea for the signs. But did these stories preserve the truth? I dunno.

Several important details are left dangling, such as what the signs were supposed to tell us (slow down? drive carefully? smile? wave?), whether people actually posted the signs in their cars only when a baby was indeed on board, and where all the signs disappeared to.

While I'm on the subject, a current fad needs an explanation: Why are yellow ribbons, of all things, displayed to show concern for hostages and overseas servicemen?

I don't know of any traditional folk beliefs or customs about yellow ribbons, but only the pop song, "Tie a Yellow Ribbon 'Round the Old Oak Tree."

Perhaps it's time for someone out there to try their hand at trying to make up another new urban legend.

- "Curses! Broiled Again," Jan Harold Brunvand's fourth collection of urban legends, is now available from Norton. Send your questions and urban legends to Prof. Brunvand in care of this newspaper.