My hands-free test drive of a Chevrolet Traverse onbusy highways was going surprisingly well. The SUV’s driver-assist wizardry displayed a comforting arc of green lights on the steering wheel. With my hands in my lap, I even relaxed a little as the truck turned on its blinkers and changed lanes on its own to rumble around slower traffic.

But on the return leg of a jaunt to Baltimore for coffee, a driver blasted up on our tail while we were in the left lane of the Baltimore-Washington Parkway. That’s when things with the General Motors Super Cruise system got a bit tense.

The car’s computer braindecided (whether to make way for the onrushing car, or for some other reason) to shift into the right lane. In that same moment, the speeder swerved toward the same space, to pass on the right.

A series of odd, slightly alarmed sounds came out of my mouth: “Weep, woop, whoa, okay.”

I grabbed the wheel and nudged my SUV firmly back to its lane. Had it continued tochange lanes, the vehicle behind wouldn’t have had much time to respond to the movement of the two-ton Traverse. A similar situation played out a little earlier on a wider stretch of the parkway that has three lanes; that time, the passing driver shifted over another lane before I took the wheel.

In both cases, it was a startling reminder that this technology required me to exercise a high degree of vigilance - even while riding along in a mode that functions like autopilot.

As a Washington Post transportation reporter, I’ve covered how the future of driving will require more trusting partnerships with machines. I wanted to test out a system myself. Car companies, with little government oversight, are pushing the limits of computer-assisted driving and even full vehicle autonomy, promising to radically transform commutes. I needed to see firsthand the delicate balance of danger and promise.

GM loaned me a Chevrolet Traverse for a week at no cost. Compared with my aging Mazda 3, it was a serious upgrade in just about every respect. It has cameras to help with parking, a huge touch screen on the dash and, of course, the Super Cruise driver-assist system. Super Cruise is what’s known as a Level 2 system, some versions ofwhich let drivers take their hands off the wheel but require them to keep eyes on the road. (Level 3 systems, as they are called, allow eyes off the road in some situations. And at Level 4, which includes Waymo’s robotaxis, the vehicle is considered fully autonomous with no human driver.)

Friends and colleagues had a range of responses - from terror at the thought of going hands-free to fascination. I agreed, in conversations with my wife, not to use Super Cruise with the kids in the car. Nonetheless, the first evening I had it, my 8-year-old son hopped behind the steering wheel figuring he could be captain of a self-driving car.

His outsize expectations mirror in some ways those of adult drivers, experts say. We still don’t have a great vocabulary for this technology. One of the first things I did after getting the Traverse was dig through the manual figuring out how to activate the system. Yet I still found myself wondering on the road what Super Cruise could and couldn’t do, like when I reached stop lights at the end of U.S. Route 50 before the system told me it was switching off.

GM is careful not to advertise the system as being a safety feature. A pamphlet in the vehicle warned quite the opposite, saying that Super Cruise “does not prevent crashes or warn of possible crashes” before listing a host of other things it can’t do, like respond to traffic lights. Aimee Ridella, a GM spokeswoman, described the company’s approach as “very conservative.”

“We’re really careful about safely deploying the tech,” she said. The systemwill work on only major routes that GM has pre-mapped, a network that ison course to cover 750,000 miles of road in the United States and Canada by the end of the year.

On my first outing, I put Super Cruise to the test on the Capital Beltway in the early part of the afternoon rush hour. If it could take the stress out of that sort of driving, I thought it might really prove its value. But the system wasn’t linked up with the digital map I was using for navigation; when the highway split,the car steered toward the wrong set of lanes. Acceleration in stop-and-go traffic felt unnervingly brisk. With cars weaving and merging, I didn’t feel confident letting go of the wheel for long.

The next test was the drive to Baltimore, a journey I made hundreds of times as a commute to an old job. I picked up photographer Craig Hudson, and as soon as wemerged onto I-95, I activated Super Cruise with the push of a couple of buttons on the steering wheel. I didn’t have to touch the wheel again until it was time to take the off-ramp near Fort McHenry.

When the system worked well, it had a magical quality to it. I got to Baltimore with none of that vague tiredness that can come from driving for an hour or more. Hudson marveled as he took pictures of me sitting back in my seat, hands off the wheel. I could have been twiddling my thumbs or - what safety experts fear most - answering texts or scrolling social media on my phone.

Ridella acknowledged that events like the questionable lane changes we experienced on the way back to Washington can feel “uncomfortable” but said the vehicle’s radars and cameras are looking behind. Ultimately, she said, the driver can make the call to take over: “You’re in charge.”

GM has disclosed more than 30 safety incidents involving Super Cruise to federal regulators at the National Highway Traffic Safety Administration. None involved a fatality or a serious injury. Ridella said the company’s own reviews have led it to conclude that the system itself was not responsible in any of them. The public incident reports make it hard to reach an independent conclusion. In many, the system disengaged a few seconds before a crash. Was an inattentive driver the problem, or did the system somehow put them in a situation where they found it impossible to avoid a collision? The reports don’t include enough detail to say.

Other manufacturers’ systems have been linked to deaths, and NHTSA has open investigations into fatalities involving Tesla and Ford.

It is thefew seconds during the handoff between human and machine that safety experts say poses particular risk. The Super Cruise system monitors the driver’s eyes and is designed to provide escalating alerts if it detects you’re distracted. I never got an alert, but after about 15-20 minutes with the system engaged, I began to notice my concentration slipping. I wondered if I would have become progressively more distracted had I not checked myself.

David Aylor,a vice president at the nonprofit Insurance Institute for Highway Safety who studies the systems, said that as they take on more tasks - like automatically changing lanes - drivers can be lulled into becoming overly reliant.

“This in-between stage where the driver and the car have to work together is a challenging one because humans are not good at monitoring technology,” Aylor said.

Industry leaders say the dangers can be managed as companies roll out more tech lettingdrivers take their eyes off the road. Mercedes has already enabled it in some cars on roads in California and Nevada. It’s a goal GM chief executive Mary Barra laid out for the company in remarks last week.

Will I give it a try? Maybe. But my experience using Super Cruise suggests even a brief lapse by computer or driver could have serious consequences.