CHANDLER, Arizona -- The introduction of Google's Waymo self-driving cars in the test city of Chandler, Arizona has sparked a new kind road rage: human versus robot.

In the past two years, there have been at least 21 instances documented by local police involving people harassing Waymo vehicles since the cars began sharing the road with regular drivers in the Phoenix suburb of 240,000.

Those instances include a man waving a pistol at a Waymo vehicle as it passed his driveway, tires slashed while idling in traffic, thrown rocks, and a Jeep that ran a Waymo car off the road six times, according to a review by the Arizona Republic.

Waymo, the self-driving car division that spun out from Google X, has been in Chandler since 2016, perfecting the autonomous vehicle's ability to use an array of cameras and computer programs to start, stop, accelerate, change lanes, turn, and more. During road tests, a person sits in the driver's seat for when safety calls for human intervention.

The vehicles, easily noticeable with clunky hub of Light Detection and Ranging (LiDAR) technology on top, are a constant sight in neighborhoods and on the city’s surface streets.

Go a mile in Chandler and it’s nearly impossible not to spot a Waymo vehicle. Waymo's fleet logs more than 25,000 miles a day on public roads.

“Over the past two years, we’ve found Arizonans to be welcoming and excited by the potential of this technology to make our roads safer," Waymo told NBC News in a statement. "We believe a key element of local engagement has been our ongoing work with the communities in which we drive, including Arizona law enforcement and first responders.”

For some, the cars are no longer a novelty and have been accepted as any other vehicle on the road.

“I drive by them. They don’t bother me,” said Sarah Miranda.

But other local residents say the cars make them uncomfortable.

Kevin Ridley, a retired technical writer in Tempe, told NBC News he prefers not to drive next to Waymo cars and does his best to avoid being stuck next to or behind one in traffic.

“They scare me,” he said. “I don’t think any amount of technology can replace the human decision-making process... How many people will be hurt or killed as we learn the limitations or missed parameters of the programming for self-driving cars as they are released to the public?”

Waymo said in a statement that “keeping our drivers, our riders and the public safe is our top priority.”

Drivers are advised to call the police if they ever feel unsafe. Each vehicle is also equipped with a hands-free button that connects the human test drivers to Waymo dispatch, who can help them handle a situation with police or alert the entire Waymo fleet for safety or security concerns. The company says instances of police involvement in Arizona and California have been rare.

Rubber to the road

Arizona has become a popular testing ground for self-driving vehicles, thanks to both its lack of moisture and regulation. In 2015, Governor Doug Ducey signed an executive order that said "it is in Arizona's interest to support the development of these technologies, by allowing testing and operation of self-driving vehicles on certain public roads, in order to advance the technology.”

And more self-driving initiatives in Arizona have been unveiled just this month. Kroger and Nuro, a Mountain View, California company, announced they will start testing grocery delivery in Scottsdale, another Phoenix suburb, using an autonomous vehicle called the R1. The delivery truck doesn’t have a steering wheel or seats.

Local Motors said its electric, driverless shuttles, called “Olli” will be tested next year at the East Valley Institute of Technology in Mesa, Arizona.

Not all of the self-driving technology tests have gone smoothly.

In March, the State of Arizona barred Uber from testing its self-driving cars on public roads after a pedestrian was killed while crossing the street in the path of an Uber vehicle. Uber resumed testing of its autonomous vehicles on Thursday, but is currently limiting the test to a one-mile loop in Pittsburgh, Pennsylvania.

A yellow light flashes

Americans are still cautious about the self-driving future. A Brookings survey found that 21 percent of adult internet users said they would ride in an autonomous vehicle, compared to 61 percent who said they would not.

Andrew Maynard, director of the Risk Innovation Lab at Arizona State University's School for the Future of Innovation in Society, said that while many people are curious and excited about the cars, he expects there will be more incidents like those in Arizona.

"Sadly, I think we probably will see more incidents involving humans harassing self-driving cars," Maynard told NBC News. "This is something the companies operating these vehicles and authorities need to take seriously."

"This sort of public backlash, even if from a minority of people," said Maynard, "could come back to bite them."