On Friday afternoon, Kim Cope was driving along Beach Street in North Fort Worth on her lunch break when she was stopped at a police roadblock. After being ushered into a nearby parking lot, she was given a menu of options.

"They were asking for cheek swabs," she told NBC 5. "They would give $10 for that. Also, if you let them take your blood, they would pay you $50 for that."

She opted for the $0 Breathalyzer, she told the station, "just because I thought that would be the easiest way to leave."

Sobriety checkpoints are an established law-enforcement tool that's been upheld by the U.S. Supreme Court, but they usually don't happen in the middle of the day, they're usually not quite so invasive and they typically are not a commercial transaction.

But Cobb wasn't stopped at a "sobriety checkpoint." She was merely asked if she cared to participate in the 2013 National Roadside Survey of Alcohol and Drugged Driving, an occasional study conducted by the National Highway Transportation Safety Administration. It's completely voluntary, though the blockage of a major street police officers (Fort Worth PD says they were off-duty) tends to give the opposite impression.

The NHTSA survey, previously conducted in 1973, 1986, 1996 and 2007, wasn't always so invasive. In the first three iterations, drivers' blood-alcohol content was measured through breath tests. In the latter study, after concluding that it was feasible to collect saliva and blood samples, researchers began to do so. As a result, they can now detect the presence of drugs other than alcohol.

In 2007, 86 percent of drivers provided a breath sample, 71 percent gave a saliva sample, and 39 percent gave a blood sample. Of the drivers who offered saliva or blood samples, 8.6 percent tested positive for marijuana, 3.9 percent for cocaine, and 1.3 percent for methamphetamine.

It's not clear what, if any, public outrage was ginned up during 2007, but the story has been somewhat different this year. In July, when researchers were collecting data in Alabama, the roadblocks prompted a modest outcry and an inquiry from the governor.

The Pacific Institute for Research and Evaluation, the federal contractor conducting the study, has downplayed concerns, describing its time in Alabama as a "wonderful experience" and that the "public was very receptive," and the NHTSA offered assurances that DNA was not collected.

It will be a couple of years yet before the results of the current survey are released. The 2007 study showed that the percentage of drunken drivers has dropped by two-thirds since 1973, from 7.5 percent than 2.2 percent. It also concluded that men and women with the same BAC have equal risk of being involved in a crash but that, because fewer women reach high BAC levels, they are involved in fewer fatal crashes.

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