NACDL

It is not illegal to drink and drive. It is, however, illegal to drink an amount of alcohol that causes one to become impaired or above the legal “per se” limit. As such, a great deal of evidence admitted in a DWI case is based upon a police officer’s opinion that the suspect’s driving ability is impaired by alcohol. A DWI arrest is mostly based on subjective assessments made by the arresting officer, where the accused is often left defending a “he said, he said” situation. The officer describes the individual’s driving behavior, the interaction roadside, the arrest, the ride back to the station or hospital, and the breath or blood testing factors. To mount a successful defense, the motorist must refute these subjective opinions.

In reaching the decision to arrest, the officer often relies upon physical roadside “testing” to assess the driver’s sobriety. Most of these “tests” are based on a series of field studies commissioned by the National Highway Traffic Safety Admin