College students enjoy a number of perks and discounts thanks to their university affiliation, with one of the most common discounts, particularly for urban campuses being public transit.

Bus and train fares certainly add up, and for students who leverage public transit to get to and from campus everyday, a discount card is an invaluable service. But what does it cost to provide that service?

As reported by The Chicago Tribune, use of free and reduced-fare Ventra cards is being put under the microscope, as illegal card-sharing is costing the agency millions of dollars each year.

Now, new card enforcement measures are seeing CTA employees paying greater attention to photo IDs at turnstiles and conducting more consistent ID verification. And while Chicago Police are not yet issuing tickets to violators, CTA officials have not ruled out that option should fraudulent card use continue.

As with other major metropolitan areas, Chicago’s “entitlement” cards are issued for discounted and free rides for students as well as low-income senior citizens and individuals with disabilities. The revenue loss is not solely the result of student transit cards, though the transit authority does run a separate pass program for college students.

Authorities are monitoring the use of U-Pass transit cards issued to college students. The student entitlement card provides full-time college students with unlimited CTA rides during the academic year.

Just last month, increased card enforcement led to some 1,800 cards being confiscated for fraudulent use. The CTA estimates a loss of $3 million in revenue from those confiscated cards alone.