Weaving or an unsafe lane change under Vehicle Code 21658 is one of the most common reasons officers give for a DUI stop, which makes this modest traffic rule surprisingly important to a DUI case. I am Joel Brand, and here is what the statute actually requires and why it matters.
The text of the law
Vehicle Code 21658. Whenever any roadway has been divided into two or more clearly marked lanes for traffic in one direction, the following rules apply: (a) A vehicle shall be driven as nearly as practical entirely within a single lane and shall not be moved from the lane until such movement can be made with reasonable safety. (b) Official signs may be erected directing slow-moving traffic to use a designated lane or allocating specified lanes to traffic moving in the same direction, and drivers of vehicles shall obey the directions of the traffic device.
The key phrase: "as nearly as practical"
The heart of the statute is subdivision (a), and the operative words are "as nearly as practical." The law does not require perfect, ruler-straight lane-keeping. It requires staying within the lane as nearly as practical and changing lanes only when it is safe. That qualifier matters enormously, because it means brief, minor movements within a lane, the kind every driver makes, are not automatically violations. The statute targets genuinely unsafe lane movement, not ordinary drift, and that single qualifier is the foundation of most successful challenges to a weaving-based stop.
Weaving is not automatically impairment
Officers often treat any weaving or drifting as a sign of a drunk driver, but that leap is not supported by the statute or the science. Sober drivers drift for many reasons: adjusting the radio, wind, road camber, a momentary glance, fatigue, or simply the normal small corrections of steering. Courts have recognized that not every minor weave within a lane even justifies a stop. The gap between an actual unsafe lane movement and ordinary, lawful driving is where many of these cases are won.
Why it matters in a DUI
In a DUI, the lane-change or weaving observation is usually the officer's justification for pulling you over. If that observation does not actually amount to a violation, the basis for the stop is in question, and if the stop was unlawful, the evidence that followed can be suppressed. So whether you truly violated 21658, or merely made the small movements any driver makes, can decide the entire case.
Challenging the stop
When weaving is the basis for the stop, I compare the officer's description to the dash-cam and body-cam footage and to what the statute actually prohibits. Time and again, the video shows ordinary driving rather than the unsafe movement the report describes. Where the claimed violation does not hold up, I bring a motion to suppress under Penal Code 1538.5, and suppressing the stop can unravel the DUI entirely.
Weaving within a lane versus crossing the line
There is an important legal distinction between weaving inside your own lane and actually crossing the line into another. Pronounced, repeated weaving across lane lines is far more likely to justify a stop than slight movement within a single lane. Many reports blur this line, describing minor in-lane drift in language that suggests something more dramatic. Pinning down exactly what the video shows, in-lane movement or actual line-crossing, and how often, is central to testing whether the stop was lawful.
Points, fines, and the record
As a standalone infraction, an unsafe lane change adds one point to your driving record under the DMV point system, plus a fine. On its own that is minor, but when it accompanies a DUI it adds to your record and can be folded into the overall resolution of the case. Keeping the point off matters for insurance and for the DMV's negligent-operator thresholds, where accumulating too many points in a set window can trigger a separate license suspension.
How it fits the larger defense
The lane-change issue is one piece of attacking the lawfulness of the stop, the single most powerful category of DUI defense. It works alongside challenges to the field sobriety testing and the chemical evidence, and it can expose the broader police mistakes at a DUI stop. See my top DUI defenses and the defenses guide.
What the video usually shows
The single most useful piece of evidence in a weaving case is the dash-cam footage, and it routinely tells a different story than the report. An officer writing up a stop at the end of a shift may describe "weaving all over the road" when the recording shows a car holding its lane with the ordinary, minor corrections every driver makes. Or the report may omit that the road was unlit, the lane markings were faded, or there was wind or a curve that explains the movement. I pull every second of available video and compare it frame by frame to the officer's account, because that gap between the words and the recording is where reasonable doubt about the stop lives.
When weaving is the only basis for the stop
Some DUI stops rest entirely on a brief weave. Those are the most vulnerable, because the entire prosecution depends on whether that single observation justified pulling you over. Courts have suppressed evidence where the only basis was minor, momentary drift within a lane. If the weaving was slight, isolated, and within the lane, the argument that it did not justify the stop is at its strongest, and winning that argument frequently ends the case. That is why I treat a weaving-only stop as a priority target rather than a minor detail.
Stopped for weaving and arrested for DUI? Let's talk.
Whether that stop was justified is often the heart of the case, and it is exactly what I review against the video. Use the free case analysis on this page, or call me directly at (888) 271-6644. I answer my own phone, 24 hours a day, 7 days a week.