There’s no such thing as free parking. Just ask RTD. It costs the agency millions of dollars a year to give away spaces at its expansive park-and-ride lots, according to a study it released last month [PDF].

Across metro Denver, RTD could raise tens of millions of dollars annually if it charged for parking. That’s money which could be spent on more frequent service or more affordable fares, instead of subsidizing driving and parking.

Subsidized parking is the norm at RTD stations because that’s how the state legislature wanted it. Yes, it’s illegal for RTD to charge people who live within its sprawling tax district to park (at least for the first 24 hours, after which it costs all of $2 per day).

RTD is not blameless. It has a penchant for building drive-to transit stations, not transit anchoring walkable neighborhoods. The agency owns about 34,000 parking spaces — a number that could grow to 52,000 as more stations open in the years ahead, according to Brian Welch, the study’s project manager.

This study, initiated by the RTD board of directors, does not address the possibility of shifting away from the park-and-ride station model, but it does illustrate why legislators should loosen the restrictions on charging for parking.

The study shows that “implementation of a widespread pay-for-parking program would in fact be worth it,” said Welch, “from a financial point of view.”

So — what if RTD charged drivers a little bit to store their private vehicles?

The study tested three scenarios. In the lowest price scenario, in-district drivers would pay $2 per day. In the highest price scenario, they would pay $5 per day, or $7 at high-demand stations. Whether any scenario would be a net revenue generator depends on the effect on ridership.

All three scenarios would generate a surplus, the study authors found, of at least $10 million and as much as $22 million annually. The latter number equals 5 percent of RTD’s total budget this year.