Driving northbound on I-25, as I was looking at the mountain range and scattered cloud coverage, I spotted an abnormally bright white soft, almost cloud-like object. I thought it to be a cloud until it moved suddenly North.I rolled my window down, held my arm out and noted it was due West out my window, about 45 degrees from the horizon (my fist height with my thumb pointed upward) above the tree line, about 8" above the mountains.I looked at the road and saw a MM 260 sign, put on my turn signal and quickly pulled into the shoulder to try to take a photograph with my cell phone (Unfortunately, none of the photos nor video turned out!)I called my husband to tell him what I was watching at 11:24 a.m. and after hanging up continued to watch this extremely bright white oval to cigar shaped object for about 20 more seconds . . . it moved SO QUICKLY . . . faster than any jet or flying aircraft I'd seen before and there was no vapor trail in sight . . . then stopped and I thought it had disappeared when I'd blinked, however, it was now almost due North and nearly out of sight.As for a size guess, when my arm was outstretched it was the width of my thumb nail (~ 3/4") and ~ 1/3" in height.When I exited I-25 at Prospect Road I noted the size of the street lights up close, next to my car and figured that to be the size I'd seen looking above my thumb with my arm outstretched, if that helps.There IS an airport along I-25, but as I said, I've never seen any aircraft including the Blue Angels fly at that speed, be able to stop, hover and take off again, almost disappearing from sight due North now within a couple of seconds.Thank you!