At the nonprofit Insurance Institute for Highway Safety, a spokesman, Russ Rader, acknowledged that the systems could offer useful information, but said that they had the potential to be a distraction as well.

“We just don’t know to what extent it could be a distraction,” he said, adding that the group had yet to study the systems. “But if they proliferate widely, the organization could analyze whether they make a difference in crash risk.”

Despite the legal limbo, the displays are already available in many cars, including those made by Lexus, BMW, Cadillac, Mercedes and Audi, among others. The equipment lives under the dashboard, and the feature arrives either as an option, costing $500 to $1,000, or comes bundled as standard equipment.

The technology can be disabled by the driver, usually with a button on the steering wheel or the center console.

Manufacturers say they are comfortable with the systems and that customer feedback has been positive, if limited.

“Driver distraction is a huge concern for us, and we really believe that H.U.D. is a great way to keep people looking in the right direction,” said Gary Robinson, manager of product planning for Honda’s Acura division. He said Honda was planning to add the displays to a broader range of models in the future.