Driving Observations Made by Police in DUI Stops

Posted on October 16, 2014

Drink and Car Key

Most of the time, police officers base DUI stops on behaviors they observe. As most San Francisco DUI attorneys can confirm, these observations are often the most critical factor to juries. To prove the charges against you, the prosecution is likely to emphasize your “speeding,” “swerving” or other observed driving behaviors.

If certain behaviors are observed, how significant were they? In studies performed by the government, many of these behaviors fall well below the statistical level that most would consider proof beyond a reasonable doubt. The NHTSA Study Detection Guide outlines a list of behaviors that the police may use to detect impaired drivers. Experienced San Francisco DUI lawyers with knowledge of the “science” in this guide can use it to refute the significance of supposed bad driving behaviors and can even use it to support the probability of sober driving.

The Detection Guide lists four categories of observed behaviors: braking and speeding issues; judgment issues; vigilance issues and issues with maintaining proper lane position.

Cues outlined in the guide predict a DUI about 35 percent of the time. If weaving is observed, the likelihood of a DUI is at least 50 percent. If weaving and another behavior is observed, the likelihood jumps to at least 65 percent. If there are two additional cues beyond weaving, the likelihood of DUI is at least 50 percent. Single-cue behaviors like accelerating for no reason, driving off the designated road and swerving all have probabilities of at least 70 percent.

Officers use the Detection Guide to detect impaired driving, to articulate observed driving behaviors in their reports and to support their testimonies at trial.

For additional clarification regarding impaired driving cues, visit these NHTSA sites:


Your lawyer may choose to contrast the NHTSA’s calculations with the observations that are being used to support the case against you. Also, comparing the actual definition of terms like “weaving” and “swerving” to the behaviors that were observed by the arresting officer may be useful.

Hire an Experienced DUI Lawyer Now

San Francisco DUI attorneys are indispensable when it comes to protecting your rights after being charged with a DUI. Before doing anything else, call Silveira Law at (415) 795-3890.