Think about the big special interests in Salt Lake City — you know, the Zions Banks or Larry H. Miller Group, the labor unions or any of the prospective inland port developers.

None of them spends anywhere near the money or exerts the kind of influence on the city’s mayoral race as one that you might not expect: the big billboard companies and, more specifically, Reagan Outdoor Advertising.

So far this year, Reagan has reported contributing $162,452 of free billboard space to candidates, nearly half of that since the August primary election.

The real value of that space is likely much higher, since billboard companies can report a discounted rate for the billboards they donate — not the actual cost they would charge you or me if we wanted to rent space on the same board.

And they skirt both state and city campaign finance laws by not reporting who enjoys the benefits of their largesse — and somehow manage to get away with it.

No matter what you think about our campaign finance law, the one thing that pretty much everyone agrees on is that there needs to be transparency. Voters need to know where the money is coming from and where it is going when they decide whom they plan to vote for.

Because of Reagan’s opacity, we don’t know exactly who they’re comping billboards to in the Salt Lake City mayor’s race, but we do know that the leading recipient of the billboard company’s generosity has been Luz Escamilla.

It’s why you saw her face all over billboards leading up to the primary, and it’s why you’ll probably see a lot more in the four weeks before the November election. The billboard companies have placed their bets.

This kind of tactic works for them. Four years ago they put a target on Mayor Ralph Becker’s back, because Becker had been trying to put limits on billboards in the city, both during his time as mayor and, before that, when he was a state legislator.

In that election, they reported donating about $150,000 of billboard space. Becker told me on Wednesday that, beyond that, for the first time in city politics, they set up a dark money campaign where they could spend money to defeat him without having to report the funds.

“There’s no question in my mind that having that large corporate effort that went outside the boundaries of what is allowed under city ordinances was skirting policies and was an effort by a special interest … to do everything they could to influence the outcome of the election,” he said.

The reason they go to such lengths is obvious.

Cities aren’t big fans of billboards. They are ugly, tacky visual blight; neighborhoods don’t like having them towering over their homes; the presence of the billboards — and the company’s contracts and insistence they not be relocated — can prevent redevelopment of blighted areas (especially with a new law the Legislature passed last year).

But billboard advertising is a big-money business and the ordinances that cities could adopt can cut into that bottom line.

Their offensive in city elections is only part of the story. They follow the same playbook with the Legislature, comping billboards or giving discounted rates to just about any candidate who wants one — a significant benefit in small-budget legislative races.

Then, surprise, the billboard companies can go to the Legislature and get lawmakers to overturn local ordinances the companies don’t like or give them special protections against cities that want to use eminent domain to relocate billboards for things like economic development projects.

Last year, the Legislature passed a bill that would give companies greater leeway — and restrict cities — when it comes to relocating or replacing billboards. Escamilla voted for the bill.

“The Legislature gives the billboard companies more than any other business I know,” Becker said. “There’s very little local control left over something that impacts a city dramatically.”

Want to see what happens when billboard companies get free rein? Just get off the 600 South exit of Interstate 15 in downtown Salt Lake City and see what these policies yield.

Want to see how they get to that point? Just look at how they have flexed their political muscle — and how they’re flexing it again this year.

My colleague Taylor Stevens talked with this year’s mayoral candidates about where they stand on the billboard issue. Mendenhall said she will make a goal of reducing billboard clutter and would try to negotiate an end to the impasse with the billboard companies. Escamilla said if people want changes to billboards, she’s willing to have the discussion, but she wants “property owners” to feel heard.

And it’s fine for billboard companies to be heard. But city residents and cities generally should have a chance to be heard, as well. And as long as one special interest is willing to spend hundreds of thousands of dollars to amplify its voice and sway an election, we run the risk of it drowning out the voices of tens of thousands of city residents.