On May 23, the U.S. Department of Transportation (DOT) proposed a new rule meant to better protect air travelers from hidden fees. The proposed rule would require airlines and ticket agents to disclose any baggage or other ancillary fees along with ticket prices in order to help customers compare the real cost of flights. Additionally, the rule would also shed more light on the quality of airline service, such as on-time performance.

Traditionally, U.S. airlines included the cost of basic services such as checked baggage and assigned seating in the price of flight tickets. Since 2008, however, most carriers have “unbundled” their airfares. Airlines and ticket sellers now advertise a seemingly-low base fare and then charge extra for basic services after the initial point of sale.

To make matters more confusing, no standards exist across carriers for applying these charges. For instance, some airlines charge for checked baggage but permit carry-on bags with no fee, while other companies charge for both, and yet others charge for neither.  But as many travelers already know, finding out about such fees ahead of time can be difficult, especially when booking through a ticket agent.

The DOT believes that greater transparency will allow passengers to more effectively compare prices and make better purchase decisions. The proposed rule would require ticket sellers to disclose the fees for a carry-on bag, the first and second checked bag, and an advance seat assignment.

In addition, DOT proposes to expand the information about airlines’ service performance. Many airlines already report to DOT about the on-time performance of their passenger service, mishandled customer baggage, and overbooked passengers. DOT then publishes this information in its Air Travel Consumer Report to assist travelers in selecting a reliable airline.

But currently, not every airline has to report this information. Under rules developed in the 1980s, only airlines that account for at least 1 percent of the domestic passenger market have to report this service quality data. DOT is proposing to modernize the rules by expanding the requirement to airlines with at least 0.5 percent of the market. That would require additional carriers, like Spirit Airlines, to disclose their performance, making the information more complete and useful for customers.

Increased transparency will strengthen protections for customers and encourage airlines to treat passengers more fairly. The public can share their views by commenting on the proposed rule by August 21.

Lukas Autenried contributed to this article.