If an officer doing an investigation or a stop has reasonable suspicion that the person driving or operating a vehicle is under the influence of drugs or alcohol, he must then determine whether there is probable cause to believe that the person is legally impaired. The officer may ask you to perform several field sobriety tests to measure your balance and response to instructions. The officer may also make notes of your appearance, speech, and demeanor, including noticing an alcoholic beverage in your vehicle or on your person. (Keep in mind that in Virginia having an open container of an alcoholic beverage in the car with you is prohibited.) In addition, the officer may ask you to give a preliminary breath test at the site of your arrest to determine the level of alcohol in your system. If the preliminary test administered at the roadside registers a blood alcohol concentration of 0.08 % (by weight by volume} or more, or 0.08 grams or more per 210 liters of breath as indicated by a preliminary breath test, you have met the threshold for being charged.
However, if you are under 21 years old the threshold for being charged. is 0.02 (see the zero-tolerance section ahead). The on-site preliminary breath test is not mandatory and a person’s refusal to take such a test cannot be used in any prosecution. Also, the preliminary breath test results cannot be admitted into evidence in any prosecution for DWI, however, they can be used as evidence to support the officer’s probable cause determination. There are also roadside tests available for drug impairment.