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Holiday travelers returning to California from the Las Vegas area caused a severe backup lasting for more than 20 miles.

The westbound lanes of Interstate 15 moved at a trickle Sunday as the Thanksgiving holiday weekend wrapped up. The Regional Transportation Commission of Southern Nevada says the backup reached 21 miles Sunday afternoon.

The Nevada Highway Patrol warns drivers not to use the shoulder to bypass traffic and says troopers are patrolling the area. Violations bring a $200 fine and potential insurance penalties.