It's unclear, but the practice has become commonplace in smaller cities—think Melbourne, Florida, and other select cities in Connecticut, Pennsylvania and North Carolina—according to a ProPublica report. Some of these cities are using DNA samples to create private databases.
The issue of consent is a fraught one: If randomly pulled over and asked for a DNA swab, is a 15-year-old equipped to agree without parental consent? The law has yet to catch up to the nascent concept of people consenting to provide a DNA sample. From ProPublica:
. . . The notion of collecting DNA consensually is still so new that the ground rules remain uncertain. Who can give such consent and what must they be told about what they're consenting to? Who decides how long to keep these samples and what can be done with them? Maryland's Supreme Court is the highest to rule on such a case, saying in 2015 that law enforcement could use DNA voluntarily provided to police investigating one crime to solve another, but that case didn't take on DNA collected outside of an investigation, in chance street or traffic stops.