Gov. Jay Inslee said the state was taking an important step in strengthening laws to protect people.
"No law can bring these loved ones back," said Inslee, who also was joined by lawmakers and law enforcement officials. "But we must move forward in preventing even more loss of life on our roads."
The Washington State Patrol said there are about 40,000 DUI arrests a year, with half made by the patrol and others by local law enforcement.
Under the new law, a driver suspected of a second impaired driving offense faces mandatory arrest and will have an interlock device installed on their vehicle within five days of being charged.
The state also will begin a pilot program in as many as three counties and two cities not in those counties to conduct daily alcohol monitoring of anyone convicted twice under the DUI law. Additional money will be put toward ensuring that local jurisdictions prosecute and punish more offenders more quickly.
"Today we refuse to be helpless victims of impaired drivers," Inslee said. "We choose to take action."
Most of the law takes effect Sept. 28, with some components, including the monitoring program, taking effect Jan. 1.
The measure is a scaled-back version of a plan that would have increased minimum jail times for offenders. Lawmakers, citing the costs of implementing the move in a tough budget year, revised the measure to remove some of the more costly elements, such as making driving under the influence a felony on the fourth conviction. The new measure maintains the underlying law that makes DUI a felony on the fifth offense within 10 years.
Dan Schulte, a Seattle man who lost his parents and whose wife and infant son were seriously injured in March by an driver who police say was drunk, praised the work of the Legislature but said there Is still room for improvement.
"I think that there's an opportunity to go even further than this, to lengthen the minimum jail sentences and to convict these drunk drivers as felons earlier in the process," Schulte said.
Mark Mullan, who has had previous DUI convictions, has pleaded not guilty to vehicular homicide, vehicle assault and reckless driving in the crash in which his pickup truck hit four members of the family as they crossed a street.
The crash killed Schulte's parents, Dennis and Judith Schulte, retired teachers from Kokomo, Ind., who had recently moved to Seattle to be closer to their new grandson, Elias.
Just over a week later, a Seattle woman was killed in a head-on crash with a suspected drunk driver who police said was going the wrong way on Highway 520 near the University of Washington.
"It's very fresh for us and it's pretty hard to be here, but we're very thankful that the Legislature and governor are working on this issue," said Schulte, who was joined by his sister, Marilyn Schulte, who wiped away tears during the event.
Schulte said his son is now home, but his wife, Karina, remains at a rehabilitation hospital with a long road ahead of her.
"The doctors describe it as a marathon," he said.
Also in attendance at the bill signing were Frank and Carol Blair, who have been actively working with law enforcement and legislative leaders to strengthen DUI laws since their daughter, Sheena, was killed by a drunk driver in 2010.
Frank Blair said the new law is a message to people who drink and drive.
"Your state is now going to hold you accountable, more than ever," he said.
The new DUI law is Senate Bill 5912.