Parking is the second-largest revenue driver for Denver International Airport, but according to City Auditor Timothy O’Brien, lax oversight with Lyft and Uber could be letting money slip through the cracks.

In a report released Thursday, O’Brien found the airport charges Lyft and Uber $2.60 for each pickup and drop off at DIA. But the report noted the monthly bill is calculated by data provided by the rideshare companies and isn’t independently verified by the airport.

With nearly $5 million in fees at stake, should DIA trust those numbers? Other airports don’t.

San Francisco International Airport began using an independent tracking system in 2015 and found rideshare companies under-reported in nine out of 24 months, according to O’Brien’s report. San Francisco was able to recoup over $150,000 in additional fees with the new system in place.

You can read the audit by clicking here.

There are several tracking systems available, but they all work by setting up digital boundaries, known as a geo-fence. When a driver with their Lyft or Uber app open crosses the boundary, the ride would be counted and billed.

Creating a geo-fence at Denver’s airport, which is one of the largest in the country, may not be as easy as in San Francisco. DIA covers over 33,000 acres, whereas the Bay Area’s airport covers just over 5,000 acres.

Not all of Denver’s acreage would need to be geo-fenced to capture rideshare drivers, but with over 300 lane miles of roads going into and out of the airport, it would be a bigger lift.

“One of those challenges is working with a vendor to find a system that works with our unique requirements as a facility.” Heath Montgomery, a spokesman for Denver International Airport said by phone Thursday. “We are in discussion with various companies that provide those systems.”

As those discussions get underway, cost will need to be taken into consideration. The three most popular tracking systems all take a portion of the revenue tracked. Denver’s airport has seen a sharp rise in rideshare revenue, going from $1.2 million in 2015 to $4.5 million in 2017 (and that was only data collected through September). San Francisco negotiated caps on how much they would give their tracking company, and may need to be considered here to help mitigate revenue loss.

“We are committed to working with various vendors to find software that works for us,” Montgomery said. “If there is a technology that’s proven to be accurate and appropriate, we will pursue that.”