It was approximately 10pm [pacific] on Thursday, March 24th. We were driving about 60mph north on US 97 in Washington state, near the city of Toppenish by Dry Creek. There were 4 people in our car and we were discussing how pretty the stars were and how Halley's Comet tails might be seen later in April, when all of a sudden, the two front passengers [myself and the driver] saw a large'ish [penny-sized] pulsating/twinkling orb fairly low in the sky, travel from [above] the front of our car to the back and then disappear. It came out of nowhere; it just appeared. I blinked a few times and asked the driver if he saw it and we both agreed we saw the same thing and described it the same way as well. Our passengers did not see it, but heard me exclaim and look to the rear of the car to track its path. We were at milepost 49 and the UFO was to the west of us. I'm not sure what I thought it was, everything ran through my mind from airplane lights do a possible drone. Nothing fit logically. I tried to report it online via my cell phone, but reception was bad and it was cumbersome. I let it go until today, April 1st [not a joke!] because it is sticking with me. I know we saw something unusual that we've never seen before.