Traveling from L.A. to Sacramento CA, five witnesses in the truck total. Object first appeared due East while traveling north from Bakersfield, but before Fresno. Three occupants of the car were panicking so we could not stop to watch, but the object seemed to stay with us. The highway was busy, moving a little slow that night, though traffic never fully stopped. Nobody else seemed to pull over, and I don't believe the UFO was to blame for the traffic.
I was not the first person to spot the UFO. A passenger commented that there was a suspicious plane in the sky, and I looked around until I spotted it to the east from the passenger side, telling them after a few moments that the maneuvers it had been doing as I had watched it were not indicative of an airplane, and that it was a UFO.
Triangle was a black triangular shape in the sky, with three white lights on the bottom of the craft. As it flew over the highway, it blocked starlight, and it spun horizontally clockwise at roughly 1 rotation per 2 seconds, roughly.
Continued to spin as it crossed back over the highway and began to descend towards the orchard next to us, moving with us perpendicular to the highway. Object would then rise and repeat the same maneuver, dipping towards the highway. Came visibly close to a farmhouse in one orchard, perceived to be less than 200 feet away at one point when crossing over us, and the farmhouse. From the closest observation I had, the UFO was roughly 150 feet in diameter.
On some occasions it made sharp right turns, or fully reverse movement with only slight deceleration. It would maintain altitude when doing these maneuvers.
Motions appeared to be deliberate or for a test flight, as the object was moving fluidly and seemed to be bringing it low across the fields, and then ascending, repeating the action (descending/ascending while rotating) for roughly 20-25 minutes until it slowly ascended and then began to accelerate West-Northwest.
Acceleration was not instantaneous, though it eventually reached incredible speeds heading away from us (growing smaller rapidly) before growing too small in the sky to see over the course of roughly 30 seconds.
Attempted to take photos with a phone, but the images couldn't capture the three lights or shape of the object, which was quite dim (not much brighter than the brightest star in the sky at the time, despite light pollution from nearby cars in moderate traffic.)
Other passengers in the car were getting very spooked (teenage girls aged 16-18) and were growing more nervous when talking to one-another about the 'what would happen if(s)' should things progress beyond a sighting, as the object would hover quite close over the highway and I asked about pulling over.
I remained calm and watched the UFO for the entirety of the flight. It was only out of my field of vision a few times during the entire encounter, when it went behind the car to the South-East for a full minute.
The driver was focusing on the traffic, as the night had been particularly hectic with at least two drunk drivers threatening the road. She caught a few glimpses of the object, but was more worried about driving.
We talked about the UFO for a long time after the event that night, as there was still a few hours of driving needed to get home.
I have included a Paint sketch of the relative observed size(s) of the object in the sky.