They are out there in every city, just waiting for the right person to cross their path.
Sometimes they rove the streets looking for prey, other times they hide in darkness.
Who are they? Police looking for drunken drivers.
How can you avoid them? Simple: Don't drink and drive.
Police throughout the region will be out this week hunting for impaired motorists, with the goal of keeping the streets safe for law-abiding residents.
Thinking of ringing in the new year with a few beers? Go ahead, but don't get behind the wheel, police say.
Since Dec. 18, when law enforcement agencies throughout Alameda County began their extra enforcements as part of the Avoid the 21 program, there have been nearly 500 arrests.
Most of them were made by Alameda County Sheriff's Office deputies and California Highway Patrol officers, but dozens of others were made by local police officers in the Fremont-Hayward area.
One of the highest local arrest totals has been in Newark, where six drivers have been removed from the streets, including three late Monday, police Cmdr. Bob Douglas said.
"The Newark Police Department is aggressively enforcing DUI laws," he said. "We are fully supportive of the Avoid the 21 campaign and are out there hunting for DUI (suspects)."
Newark officers will conduct a sobriety checkpoint starting at 7 p.m. Saturday and lasting until 2 a.m. Sunday at Thornton Avenue and Newark Boulevard. While that is the only checkpoint slated for the area this week, all other departments will increase staffing levels to better enforce driving laws.
In some cities, such as Union City, teams of officers also will conduct DUI warrant sweeps.
"If you've got a warrant, we'll be looking for you," Union City police Lt. Kelly Musgrove said.
Reach Ben Aguirre Jr. at 510-353-7011. Follow him on Twitter at www.twitter.com/benaguirrejr.