Maybe I got it wrong when I concluded that car rental companies are just trying to help customers by offering “optional” toll payment services with their cars.

Maybe it’s a widespread conspiracy that involves not only the auto rental companies and third parties that handle electronic billing, but also local toll road authorities -— all of which would profit from a tourist driving down a poorly-marked turnpike.

At least that’s what Shira Newman, a property manager from Portland, Ore., suggested after I mentioned my own toll road trouble, which had cost me $27.10 in charges for a short drive to the airport.

“I had a similar thing happen to me about a year ago when I took my family to Denver,” she told me. “Apparently, they have toll roads there but do not disclose them — there is basically one way to go out of the Denver airport, but no one really says anything about it.”

Newman is referring to E-470, which is a convenient way to get from Denver International Airport to the suburbs, but not the only way. And hers isn’t the first complaint I’ve received about its lack of disclosure. (Remember Dave Medin, the reader from Cedar Rapids, Iowa, in my last report — he was caught by E-470, too.)

“So we paid the toll out of the airport and back to the airport — we were there for six days, but used it twice, to and from the airport,” says Newman. “But I, like you, was apparently charged their toll charge for each day that I rented the car.”

“It is a scam, I believe,” she adds.

Yes, but is it a conspiracy? You would have to prove that the practice went far beyond one or two locations adding pricey transponders with little or no disclosure. You’d need pictures of car rental officials meeting with toll road bigwigs in a Chinese restaurant on the wrong side of the tracks (deals like that are always done in a Chinese restaurant, for some reason).

And you’d need to show me the checks or transaction records proving the car rental managers paid officials to remove the E-470s roadsigns.

As much as I love investigative journalism, I don’t think I’m going to find that smoking gun. (Besides, I’m a consumer advocate. Investigative reporters get bigger paychecks and they have big employers that protect them from litigious readers.)

But still, it’s worth noting that many others feel the same way Newman does. I think they’re onto something.

Why? First, there’s absolutely no denying that these transponder or plate-registration services are a big, highly profitable business. Car rental companies don’t have to charge you for every day you “use” these services. They could bill you for the tolls and add a modest surcharge to cover their expenses.

Instead, they start the meter when you blow through the first toll booth, and they keep it running for the rest of your rental. Some might say that’s unethical.

Second, having spent the better part of the last year driving around the country, I can confirm that some toll roads are not clearly marked, and that the poor signage only helps the people who operate the roads.

And finally, I wouldn’t have as much of a problem with this if it was confined to Denver, but toll roads are popping up everywhere. In some cases, the toll roads were once free, which is to say they were funded by your tax dollars. I find that highly annoying. (Don’t look now, but there’s a good chance there’s a toll road or two coming soon to your neighborhood.)

Put it all together, and you have what may be the single biggest ancillary revenue opportunity for the car rental industry in a generation.

Oh, and did I mention there’s no way to appeal these charges? Not for me, and not for Newman.

“About a month after getting back home, there was this charge on my card, and I tried to dispute it,” she said. “But American Express wouldn’t let me.”

I’m not a big-government kind of guy, but if this doesn’t make you wish for some kind of federal oversight, I don’t know what will. Question is: Who needs to be reined in — the toll authorities, third-party companies providing these lucrative services, or the car rental companies?