Texas police vehicles must be inscribed with lettering and coloring that identifies the vehicle as a law enforcement vehicle unless the county government exempts itself from this requirement. With an exemption in place, unmarked vehicles may patrol the city streets. The goal of using unmarked cars is to catch aggressive drivers in acts they would not commit openly around a marked police unit.
TL;DR (Too Long; Didn't Read)
If many Texas counties and cities, you can be stopped and cited from an unmarked police car. The police officer must have reason to suspect that you have committed, are committing, or are about to commit a crime.
Texas Law on Unmarked Vehicles
Under the Texas Transportation Code, all municipal vehicles must have the name of the municipality and the title of the department to which the vehicle belongs inscribed on the vehicle. So, a police car in the city of San Antonio would have the words "San Antonio Police" written on its side. Citizens should be able to read the inscription at a distance of 100 feet. However, local governments can pass an exemption allowing unmarked cars to patrol the city streets. The Corpus Christi police department, for example, has used unmarked vehicles for traffic enforcement since 2017.
Police Need Reasonable Suspicion to Pull You Over
When a city is exempted, police officers are permitted to patrol in unmarked vehicles and to pull you over from an unmarked vehicle. However, the police officer must have reason to suspect that you have committed, are committing, or are about to commit a crime or a traffic violation. If you're stopped by an unmarked vehicle and the officer is not in uniform, she must present her badge and/or her police identification card.
Unmarked Vehicles Cannot Pursue
While unmarked police cars can stop you, they generally cannot pursue you if you refuse to stop or attempt to flee the scene. By the Texas Transportation Code, a police officer in pursuit must be in full uniform and in a vehicle marked as an official police vehicle. In this situation, it's likely that a marked unit will be called out to initiate the stop.
What To Do If You're Pulled Over
Unmarked police cars raise the possibility of someone impersonating a police officer, which is a safety concern. In most counties, unmarked police vehicles still have a siren and flashing red-and-blue lights so it's not difficult to tell that a police officer is pulling you over. If you are worried about who is stopping you, the police advise that you drive to a well-lit public area such as a gas station or the parking lot of a convenience store. You can signal to the officer that you know he is behind you by turning on your hazard lights. Call 911 to check that a police officer is attempting a traffic stop in your location. Don't open the window until the police officer has shown his badge or other ID.