SAN FRANCISCO, May 17 (Xinhua) -- California Highway Patrol (CHP) police are more concerned about the growing problem of the impact of smoking marijuana on vehicle drivers, which has caused several deaths since last year, a TV report said Wednesday.

"The Golden Gate division of the CHP is now on the lookout for signs of marijuana impairment," the local TV KTVU said, quoting police officer Vu Williams in San Francisco as saying that "it's now a growing problem."

"We may smell alcohol or marijuana coming from the car. We may see their speech is somewhat impaired or it may be a bit slower and drivers may not be able to respond to our questions," Williams said.

The standard field sobriety test, which is similar for both alcohol and marijuana, is now used to determine which drivers are impaired behind the wheel.

Buzzed driving has caught the attention of the CHP after a series of deadly crashes, the TV report said.

On Christmas Eve of 2017, a CHP officer was killed when he was rear ended by a car driving more than 100 mph (about 160 km per hour) on a freeway, and the driver later admitted to using marijuana.

On Tuesday, a 21-year-old driver under the influence of marijuana killed three people, including two kids, in a five-car crash on a freeway in San Jose, an economic, cultural, and political center of Silicon Valley and the largest city in Northern California.

According to CHP statistics, 197 DUI (driving under the influence) arrests were made for the nine Bay Area counties in 2017.

There are already 87 arrests for marijuana DUI from January to mid-April in 2018, the CHP figures show.

California passed Proposition 64 in 2016 to legalize the recreational use of weed in the state, which took effect on Jan. 1, 2018.

Up to now, a total of 29 U.S. states have adopted medical marijuana laws and several states have adopted the most expansive laws legalizing marijuana for recreational use.