All states have laws that prohibit drivers from making unsafe lane changes. State laws generally require a driver on a road with marked lanes to operate the vehicle within a single lane and to move to another lane only at a time and in a manner that is safe.

This means a driver may not cross more than one lane of traffic at a time, must allow a safe distance between the driver’s vehicle and other vehicles, and must signal the lane change with a blinker, as required by law. The failure to do so may result in a ticket or citation for an unsafe lane change.

The penalty for making an unsafe lane change usually includes a fine and demerit points on your driver’s license or record.

As with many traffic violations, the officer issuing the citation and the prosecutor have significant discretion in determining whether there was a violation of a traffic law and whether the violation constitutes a certain offense, as defined by the state legislature in the traffic code or in a municipal ordinance.

This discretion to issue a ticket or citation and prosecute the charge against the driver is balanced by the discretion of the jury or judge in determining whether the prosecution met its burden of proof sufficient to convict the driver (for a criminal offense) or find the driver responsible (for a civil infraction, violation, or offense).

In California, the Vehicle Code Section 21658(a) stipulates that a vehicle must be driven as nearly as practical entirely within a single lane and should not be moved from that lane until the driver has ensured that the movement can be made safely. This means that drivers are required to make lane changes cautiously and only when it's safe to do so, which includes not crossing multiple lanes of traffic at once, maintaining a safe distance from other vehicles, and signaling the intention to change lanes using the vehicle's turn signals. Failure to adhere to these rules can result in a citation for an unsafe lane change. The penalties for such an infraction typically include a monetary fine and may also result in demerit points being added to the driver's record. In California, traffic enforcement officers and prosecutors have discretion in issuing citations and pursuing charges, while judges or juries have the final say in determining whether the evidence presented meets the burden of proof for a conviction or finding of responsibility, depending on whether the offense is criminal or civil in nature.