In a pretextual stop (also called an investigatory stop), officers pull over people citing a minor issue, then start asking unrelated questions. University of Kansas professor Charles Epp in a study found that black drivers were three times more likely than whites to be subjected to "pretextual" stops, and five times more likely to be searched during them. However, Epp found no difference in the frequency and treatment with which black and white drivers were stopped for serious violations like speeding. The bias, however, was significant for stops over minor issues such as a broken tail light, a missing front plate or a failure to signal a lane change.