READING, Pa. — Orange cones and flashing police lights confronted Ricardo Nieves as he rounded a bend on the way to his mother’s house. Before he knew what was going on, Nieves said, a man working for a government contractor stepped in front of his car and forced him to turn into a parking lot. There, a woman repeatedly tried to question him about his driving habits and asked for a mouth swab that would detect the presence of illegal or prescription drugs in his system.

Nieves refused. Then he sued, contending his rights were violated.

His Dec. 13 experience has been repeated thousands of times in cities around the country as the federal government tries to figure out how many of the nation’s motorists are driving while drunk or high.

U.S. transportation officials call the National Roadside Survey of Alcohol and Drugged Driving, which has been conducted five times since 1973, a vital tool for monitoring the safety of America’s roadways. But some motorists and civil liberties advocates contend the government’s methods are intrusive and even unconstitutional. Some police departments have refused to partner on the survey or regretted their decision to do so in the wake of public outcry, while in Tennessee, legislation that would ban law enforcement from helping out on the survey unanimously cleared the state Senate last month.

In the southeastern Pennsylvania city of Reading, Nieves is angered over what he views as an abuse of power.

“I didn’t even have a choice to make a decision” to stop for the survey or keep going, he said. “That choice was taken away the moment he stepped into my right of way.”

Conducted in 60 cities around the nation, the survey yields the government’s best estimate of the prevalence of impaired driving. It works like this: Motorists are randomly selected — either by a uniformed police officer or a private contractor working for the National Highway Traffic Safety Administration — and waved into a parking lot, where they are questioned about their drinking and driving habits, asked to take a breath test, and offered money if they provide saliva and blood samples or agree to answer a more extensive written survey.

Federal officials stress the survey is both voluntary — a large sign at each survey site says so — and anonymous, with local police enlisted to provide security and divert selected motorists from the flow of traffic. Any driver found to be impaired is offered a ride home or put up in a hotel.