That has created a lucrative industry. Smart Start, based in Texas, estimated its interlock revenue last year at $150 million — nearly double its revenue from four years ago — according to its chief executive. Dräger, a German company that is among the leading manufacturers of breathalyzer machines, now makes twice as much money from interlocks in the United States as it does from its traditional breath-test business. Private equity firms looking to cash in have bought several interlock makers, including Smart Start, and are circling others.

When regulatory warnings about rolling tests have come up, Interlock companies have pushed back.

In 2006, the National Highway Traffic Safety Administration, the federal regulator in charge of setting vehicle safety equipment standards, began revising its 14-year-old guidelines for how interlock devices should work. A 2010 draft of the document said the agency “does not intend” that users perform rolling retests and said they should be performed while stopped on the side of the road.

The interlock industry and others objected, arguing that rolling retests were safe and that, in any case, it was impractical to expect drivers to pull over.

“All interlock vendors advise the client/user to pull off the road in a ‘safe’ place to take the retest,” the president of LifeSafer, a leading vendor, wrote in a letter to NHTSA. He wrote that the “practical reality” of interlocks was that “99 percent” of retests happened while the vehicle was moving.

Another company, National Interlock Systems, wrote, “While it is possible that there have been traffic accidents while the test is being taken, we are not aware of any.”

And state officials from Colorado said rolling retests would be safer in many situations, like tunnels and “congested environments with tight lanes and limited shoulders.”

The regulator backed down. In its final guidance, published in 2013, it wrote that it was “very concerned about distracted driving” but would not specify how retests should be conducted. The agency also said that was “more appropriately a function for states and local jurisdictions.”