Thornton police say 46-year-old Elizabeth Ward was spotted by an off-duty Arvada officer going in and out of her lane on I-25 as she approached West 136th Avenue.
The Arvada officer followed Ward and contacted Thornton police.
When Thornton police arrived, Ward's car was parked on the right side of the interstate, with the driver's side still partially in traffic, according to an arrest report.
When an officer asked why Ward was on the side of the road, she replied "I'm drunk," the report says.
Police say Ward refused the voluntary roadside test but opted for a blood test instead.
Ward is a DUI enforcement officer for Boulder police. Her arrest came less than three weeks after another Boulder officer's arrest on suspicion of DUI.
Ward is on administrative leave.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)