Some states issued plates made from soybean-based fibreboard, much to the delight of the goats who thought they were tasty.

While some collectors entered the hobby out of a love of all things automotive, for others, plates incite the kind of passion typically reserved for vintage Bugattis.

In the US, the most dedicated collectors belong to the Automobile License Plate Collectors Association, or ALPCA – a tightly knit cadre of 3,500 members with interests as diverse as the plates they collect.

So why amass license plates? As a few ALPCA members say, it's simple: One collects what one likes.

"Most of us collect license plates like people collect stamps," Marco Tramelli, a co-founder of ALPCA's New Jersey chapter, said at a recent meet-up. "People will collect runs – every year in New Jersey since 1908, for example. Or police plates. Everyone collects police plates."

As much as any car enthusiast, ALPCA members are gluttons for trivia. These are the people you contact when you want to know when Massachusetts put a codfish on its plate (1928 for cars, 1929 for trucks), or which US state was the only one to have issued plates made from copper (Arizona).

The club's website is chock-full of obscure tidbits. Tennessee, for instance, issued orange and white plates in 1951 to commemorate the University of Tennessee's victory over the University of Texas in the Cotton Bowl that year. Between 1912 and 1918, front plates in Illinois were designed to allow air to pass through so a car's radiator could cool off adequately. During metal-scarce World War II, some states issued plates made from soybean-based fibreboard, much to the delight of the goats who thought they were tasty.

On a cold, rainy early spring day, the Garden State chapter met at the Resorts Casino Hotel in Atlantic City, New Jersey. Outside, the sea had been whipped into an angry froth. A handful of surfers bobbed forlornly in the fog. Inside the casino was the typical collection of pensioners staring fixedly at jingling slot machines and straight-faced card dealers.

But further inside the labyrinth, past the smoke-filled gaming floor and buffets, the dozen or so ALPCA members who had braved the weather had set up tables at the centre of a cavernous banquet hall. Though the group seemed minuscule amid a room built to accommodate several hundred people, the banter was outsize, the spirit of the gathering like a small rummage sale. Several members explained that they sold and traded plates at such events to acquire specific plates they're after.

John Anshant – ALPCA member No 908, who joined in 1968 as a 12-year-old – had a table filled with New Jersey state police plates. Anshant, an auctioneer who buys, sells and trades plates strictly as a hobby, remortgaged his house to come up with the $35,000 needed to purchase the collection, even though the batch contained only a few plates that he truly wanted.

"We're all collectors first and dealers second," Anshant said. "Most of the stuff I'm selling here is leftovers from that batch."

Though he has about 5,000 plates in his personal collection, Anshant and his collecting partner, John Willard – ALPCA member No 909, who joined when he was 20, just after Anshant signed up – have about 40,000 plates between them, which they sell and trade at meets and on eBay.

"I wish eBay had never happened. It's taken a lot of the fun out of the chase and it has influenced prices," Anshant lamented. "It was fun to get up at three in the morning and come back home with a carload of plates. Having said that, there are a few plates in my collection that I wouldn't have been able to get without eBay."

Members have different motives for being in the hobby. Chuck Bahor, from Indianapolis, Indiana, started collecting plates from the various states he visited travelling back and forth between his son's university in California and his home. Then he expanded into the more valuable antique porcelain plates.

Roy Rich, a math professor in the New York suburbs, began collecting as a child.

"When I was a very small kid, my job was to go out and burn the garbage," he said. "One day, I had to burn a 1960 New York plate, but I thought, hey, someone should save this for posterity. So I kept saving them. Now, I do pretty good business trading matched pairs."

Jeff Francis, known among the ALPCA crowd as one of its top collectors, said he used to ride his bicycle around St Petersburg, Florida, as a child, picking up old license plates that had fallen off of cars. Later, he began to ask older residents in his neighbourhood for their expired plates. He now commands a master collection of about 30,000 plates, he says, mostly from the US, but with some from Central and South America.

"I travel about 250,000 miles per year looking for this stuff," he said when I interviewed him at an estate auction near Hartford, Connecticut, which included hundreds, if not thousands, of old license plates. During the auction, his hand was one of the few that continued to pop up when bidding crossed the $1,000 threshold on a few lots.

Dick Yourga, a retired police officer from Amherst, Massachusetts, said he started collecting ostensibly for the same reasons that other members cited: the love of history, the thrill of the chase. But as other ALPCA members noted, something else kept him coming back.

"After all these years, it's the people I've made friends with that's the best part of all this," he said. "I've met people from all over the world doing this thing."

Tramelli, ALPCA’s Garden State chapter co-founder, echoed that sentiment. "Everybody knows everybody. You get to know people pretty quick," he said. "I could be travelling in Nebraska or something and look at the ALPCA roster and there'll be a member there. If I call that person up and say, 'Hey, you wanna hang out,' they'll say, 'Yeah!'"