The use of unmarked police cars in traffic stops brings a significant amount of concern. As a result, some states restricted these stops and some courts invalidated them.
The concerns range from potential Constitutional violations to issues of safety. The Constitution requires that police officers follow a number of rules when making a routine traffic stop, such as having probable cause for it. Making the stop in an unmarked car, however, is not unconstitutional. Still, because of reports of criminals pretending to be police, some people suggest that police should not use unmarked cars for routine stops.
Few jurisdictions restrict the use of unmarked police cars. In California, the state Legislature enacted a law against speed traps, but the state's district court ruled that it did not ban traffic stops by unmarked cars. Texas does not have a specific law regarding the use of unmarked police cars in routine stops. Additionally, the state court system has not ruled that they are illegal.
One way of ensuring safety when being pulled over by an unmarked police car is to drive to a well-lit and populated location such as a gas station. At that point, the officer should be willing to produce his identification. The driver can also request that another officer is present.