At first, David Collins thought it was his imagination.

The restroom on the Boeing ­737-900 flying from Los Angeles to Atlanta seemed smaller than usual. “I had to take a deep breath to access the slot for the used paper towels,” he recalls.

Then he turned his professional eye on the lavatory. Collins is an expert witness for amusement park lawsuits, and he estimates that the restroom was only about 28 to 32 inches wide — roughly 20 percent smaller than the standard aircraft bathroom.

He was right. This summer, Boeing reportedly figured out a way to add as many as 14 extra seats to some of its aircraft by shrinking the size of its lavatories. But that is by no means the only unwelcome restroom change for the traveling public. With one or two exceptions, WCs are fast becoming an amenity-free wasteland for many travelers.

Vee Vik, a frequent flier and the founder of an e-commerce site that sells bathroom appliances, says the closet-size restrooms are part of a larger trend. Modern bathrooms aren’t just getting cramped. “They also rely on self-cleaning technology and don’t have to be maintained as frequently as the older bathrooms,” he says. And customers seem to tolerate it, believing — perhaps inaccurately — that toilets aren’t as dirty or hazardous as once thought.

Any discussion of restrooms must start with the users, and there, standards seem to be slipping, says San Francisco-based market researcher Lee Caraher. “People don’t clean up after themselves as much as they used to,” she says. “I definitely notice it more today than I did a few years ago.”

A recent survey by Clorox of cleaning professionals across a variety of industries showed they were often unaware of germ hot spots. For example, most cleaners incorrectly think that restroom handles harbor the most illness-causing germs and bacteria, particularly restroom door handles (65 percent), faucet handles (38 percent), and toilet or urinal handles (36 percent). Actually, trash cans have the highest concentrations of germs, according to the research.

Travelers are not happy. Dusty Prentiss, a retired engineer from San Francisco, sent me a complaint about a flight from San Francisco to Philadelphia during which the first-class restroom was closed because it was splattered with human waste and the flight attendants refused to clean up the mess. “I would expect after paying a substantial premium for a first-class ticket that the facilities would be clean and orderly,” he added.

Declaring an airplane restroom “out of order” isn’t that unusual and may not have anything to do with cleanliness. Polly Barks, a writer who travels frequently to Russia, says she’s been on flights with locked restrooms. “The flight attendants report that it’s too expensive to dump in the U.S.,” she says. Rather than do that, they close the restrooms and the plane flies with fewer working lavatories.

Heather Zorzini, a veteran flight attendant, isn’t surprised that complaints about airline bathrooms are proliferating. Even if most airplane restrooms remain the standard size, airlines are still trying to squeeze more passengers onto planes. “The increased number of passengers crammed on board and fewer flight attendants certainly makes [bathrooms] dirtier,” she says.

Not every part of the travel industry is letting its restrooms waste away or shrink, of course. Luxury hotels and cruise lines are upping their game, adding luxury amenities and keeping their restrooms squeaky clean. John Drabkowski, a travel agent who specializes in cruises, recently sailed on the Carnival Ecstasy and was impressed by the restrooms on board.

“The bathrooms were absolutely spotless and well maintained,” he recalls. “They were fully stocked with towels and toiletries, water pressure was adequate and consistent, and the rooms were fully lit.”

Drabkowski says Carnival is bucking the trend because it relies on repeat customers, who simply would not put up with dirty restrooms. It’s also worth noting that cruise lines are particularly obsessed with cleanliness, fearing they could be struck with the next big norovirus outbreak.

There are long- and short-term solutions to the problem. The latter involves traveling with antiseptic wipes, a recommendation made by several experts. If you suspect a toilet isn’t well maintained, find another one or clean the one you need to use. On planes, a broken lavatory may render the “first-class restrooms are for first-class passengers only” rule void, in the interest of preventing accidents. (But do consult a flight attendant before pulling the curtain back and making a beeline for the restroom up front.)

When Collins, the expert witness, is in a tiny lavatory on a regional jet, he opens the door when he washes his hands, which gives him a little extra space.

More substantive change requires pushback. So if you encounter a restroom that’s undersized or dirty or has run out of essential items, say something. In the long term, restrooms won’t get better until travelers make a stink about them.

Elliott is a consumer advocate, journalist and co-founder of the advocacy group Travelers United. Email him at chris@elliott.org.

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