A police officer cannot just randomly pull a vehicle over to check if its driver is intoxicated. The U.S. Constitution, as interpreted by the courts, requires that the officer have reasonable suspicion the driver might have committed a crime before making a traffic stop.
Reasonable suspicion of DWI can be based on any number of observations. Some of the more common driving behaviors that will justify a stop include weaving, crossing the center line, drifting across lanes, braking frequently, driving too slowly, and narrowly missing objects or other vehicles. An officer can also have reasonable suspicion of drunk driving if he or she pulls a vehicle over for an unrelated violation and then observes the driver may be intoxicated.
Some states allow sobriety checkpoints, in which police stop cars in some predetermined order and look for impaired drivers. These arguably constitute an exception to the reasonable suspicion requirement. However, sobriety checkpoints are illegal in Texas, based on the state's interpretation of the U.S. Constitution.
Reasonable suspicion gives the officer the right to stop and briefly detain a driver for further investigation. Reasonable suspicion alone does not justify an arrest. To make an arrest the officer needs to meet the stricter standard of probable cause. Probable cause means the officer has enough evidence to believe the driver probably committed a crime; reasonable suspicion means there is evidence the driver might have committed a crime.
One of the things a DWI defense lawyer will look for in the arrest record is whether the officer was justified in pulling the vehicle over. If the officer lacked sufficient reasonable suspicion to make the stop, the DWI charge can be dismissed.
Source: Findlaw.com, "What is Reasonable Suspicion for a DUI Stop?" accessed April 19, 2015