The 7-2 decision affirms the legal victory of about 70 Brink's Home Security Inc. technicians, who were awarded back pay at trial after suing Brink's in November 2002.
Under the company's "home dispatch program," some Brink's security technicians started their day's work by driving a company truck directly from their homes to a job site.
When the day was done, they took the trucks home.
But Brink's didn't pay those technicians for their first and last trips of the day, unless the workers were behind the wheel for more than 45 minutes.
On Thursday, the Supreme Court said that policy violated Washington's minimum wage laws.
The majority opinion, written by Justice Susan Owens, said the technicians were clearly on duty and in a prescribed workplace while driving the company trucks - satisfying state rules that define working hours.
"The nature of Brink's business requires technicians to drive the Brink's trucks to reach customers' homes and carry the tools and equipment necessary for servicing and installing home alarm systems," Owens wrote. "In addition, the Brink's trucks serve as the location where technicians often complete work-related paperwork."
A concurring opinion by Justice Barbara Madsen agreed with the majority's result, but differed on the legal reasoning. Justice Mary Fairhurst signed both the majority and concurring opinions.
The court's two dissenters, Justices Richard Sanders and James Johnson, said the technicians weren't on the clock because the company trucks aren't necessarily a workplace.
The Brink's technicians, Sanders pointed out, didn't claim they were entitled to pay when driving a work truck to the company office - only when they were driving to and from a job site.
"Here, the technicians' principal work consists of installing and repairing alarm systems in customers' homes," Sanders wrote. "Hence, this work is done only at the homes of the customers and not while commuting."