“There’s so many different arrangements that if we had waited to figure that out, we may have jeopardized the whole bill,” said Bobby Scott, the chairman of the House Education and Labor Committee, who pushed for the inclusion of the study commission in the bill. Mr. Scott, a Virginia Democrat, said he was optimistic that Congress would revisit the issue once more information was available. “Hopefully we’ll get that information soon, and it won’t be long until we can include ground ambulances.”

This week’s legislation also sets up a new arbitration system for handling payment disputes between insurers and medical providers. Adding ambulances to that system may prove easier than starting from scratch.

“The law really captures every single form of surprise billing that we think of in this context, except ground ambulances,” said Loren Adler, an associate director of the U.S.C.-Brookings Schaeffer Initiative for Health Policy, who recently began a research project on ambulance costs and billing practices. Mr. Adler said he thinks emerging research will bring ambulances into the foreground of the policy debate. “It does seem like the last frontier in the surprise-billing world.”

Most states that have passed laws banning surprise billing have also omitted ambulances from the rules so far. That includes Texas, which banned other surprise medical bills in 2019 but allowed ambulances to continue engaging in the practice.

“Now you do hear people complaining about ambulance bills, because they’re the only entities still sending surprise ones,” said Stacey Pogue, a senior health policy analyst at Every Texan, a consumer advocacy group.

This month, the Texas Department of Insurance published new research finding that 85 percent of ambulance rides in the state are out of network. In Houston, the state’s largest city, no city-run ambulance services participate in any private insurance plans, a recent investigation by The Houston Chronicle found.

Along with the new research, the Texas Department of Insurance recommended that the Texas legislature revisit its 2019 law, and begin regulating ambulance billing as well.