A driver can be ticketed, blamed for a crash, or left fighting an injury claim within hours if this rule is misunderstood. An improper lane change happens when someone moves from one lane to another without making sure the move can be done safely, or without giving proper notice to other drivers. That can include drifting across lane lines, cutting into another vehicle's space, changing lanes in traffic too quickly, or failing to signal before moving over.

In practical terms, this violation often becomes a key fact after a wreck because lane changes happen fast and leave little room to react. On roads like I-90 near Missoula or Butte, where curves, grades, weather, and heavy trucks can tighten timing, a bad lane change can trigger a sideswipe, force another driver off the road, or start a chain-reaction collision. A citation may support an argument that the driver was negligent, though it does not decide the whole case by itself.

For an injury claim, the issue can affect fault, insurance payments, and how damages are divided. Montana follows a modified comparative negligence rule, so a driver's share of blame matters. Montana law also requires a driver to stay within a lane unless the movement can be made safely, and to signal continuously during not less than the last 100 feet before turning or changing lanes under the Montana Code Annotated. After a crash, quick action matters because vehicle damage, dashcam footage, and witness memories disappear fast.