You will be asked 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.
There are also roadside tests available for drug impairment. There are, additionally, field sobriety tests given to measure your balance and response to instructions. The officer who stops you may also notice the odor of alcoholic beverage in your vehicle or on your person. (In Virginia, having an open container of alcoholic beverage in the car with you is prohibited}.
The officer, will, if he or she determines that you appear to be under the influence, offer you further tests to determine accuracy of results. If you refuse to take the offered tests, you can be further charged with “refusal” which carries additional penalties and additional loss of license if you are convicted. When you drive on a public street or highway, in Virginia, you are implying that you consent for the taking of breath and/or blood samples.