Police officers in Fort Worth, Texas, set up a roadblock on a busy city street last week, and directed motorists into a parking lot, where they were asked to submit samples of their breath, saliva and blood.

Asked, not required. That was an important distinction, yet one that was lost on at least one bewildered driver snagged in the roadblock, according to NBC in Dallas-Fort Worth.

The roadblock was part of a three-year study conducted by a private contractor on behalf the National Highway Traffic Safety Administration that, using a random sampling, aims to determine how many motorists are under the influence of alcohol or drugs. It was unclear from reports whether or not active-duty or off-duty police were conducting the tests, but they were involved in the road block that stopped drivers and diverted them to a parking lot where the tests were conducted.

Participation in the study was supposedly voluntary, although at least one Fort Worth driver felt coerced into giving samples.

"I gestured to the guy in front that I wanted to go straight, but he wouldn't let me, and forced me into a parking spot," motorist Kim Cope tells NBC in Dallas-Fort Worth. "I finally did the Breathalyzer test just because I thought that would be the easiest way to leave."

A police spokesperson issued an apology, saying it was sorry, "if any of our drivers and citizens were offended or inconvenienced."

Friday's roadblock in Fort Worth was part of NHTSA's National Roadside Survey Of Alcohol and Drug Use By Drivers. The survey involved randomly stopping drivers at 300 locations across the country, according to the agency's website.

Drivers are supposed to be paid $10 to $50 for their participation, although Cope said she did not receive any payment.

"It just doesn't seem right that you can be forced off the road when you're not doing anything wrong," she told NBC.

In a written response Wednesday, a NHTSA spokesperson told AOL Autos that no officials were available to address the survey methods today. Bernard Murphy, the chief executive officer of the Pacific Institute for Research and Evaluation, the contractor that helped conduct the roadblock with the help of off-duty police officers, did not return an email requesting comment.

AOL Autos dug up information from Pacific Institute for Research and Evaluation's earlier attempts of the same survey conducted for for NHTSA in 2007. Documents related to that survey described the company's methodology.

If drivers initially refused to participate, the document said, PIRE attempted to "convert" them with a financial incentive. Of 444 drivers who initially refused to participate in 2007, PIRE said in documents it converted 50 percent of them with financial incentives, that can range from payments up to $100.

If a driver still refused to participate they were counted as a refusal and the interviewer asked them for a breath sample before they left the site.

Law enforcement officers in Texas have been involved in multiple incidents in recent years where civil-liberties violations have occurred.

In May 2012, a state trooper performed illegal body cavity searches of two women on the side of the road during a routine traffic stop outside of Houston. A dashcam in a police cruiser captured the incident.

In July 2012, another trooper performed roadside cavity searches on two women during a traffic stop near Dallas that started because one of them had allegedly thrown a cigarette butt from her car window. No drugs or other contraband were found in either case, and the motorists were allowed to proceed. Texas officials later acknowledged both searches were unconstitutional, according to MSNBC.

NHTSA later released a written statement that said, "as a data-driven agency, NHTSA is constantly conducting research to better understand vehicle and behavior issues that impact safety on U.S. roadways. The National Roadside Survey on Alcohol and Drug Use by Drivers is one such research effort."

Pete Bigelow is an associate editor at AOL Autos. He can be reached via email at peter.bigelow@teamaol.com and followed on Twitter @PeterCBigelow.

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