Under what condition is it illegal to change lanes?

Changing lanes is illegal when there is a solid line beside your lane because solid lines indicate that crossing them is prohibited. Solid lines are used to signal that it is unsafe or inadvisable to change lanes in that area, often for reasons such as upcoming intersections, pedestrian crosswalks, or other hazardous conditions. This regulation is meant to promote safety on the road by discouraging sudden or unexpected moves that could lead to collisions.

In contrast, driving under the speed limit does not inherently affect the legality of changing lanes, nor does signaling to indicate a lane change. Oncoming traffic might create a situation where changing lanes is ill-advised, but the presence of a solid line is the definitive rule that makes lane changes illegal in that scenario.