Can you identify this "USO" (Unidentified Stationary Object) -- For some years I've been seeing odd "installations" (for lack of a better term) on interstate highway pavement. These things appear to be eight-sided octagons or possibly six-sided hexagons. Whichever the case, these seem to be "irregular polygons" since some of the the six (or eight) sides seem to vary in length. It appears as though the asphalt was cut - in whichever shape. In some cases, the center of the polygon seems to be a different shade of black/grey. Are these some kind of sensor points, or perhaps a kind of traffic counter?

I would say traffic sensors. Have a look at https://www.fhwa.dot.gov/publications/research/operations/its/06139/chapt5b.cfm and see if these are what you mean. The actual shapes are referred to as squares or diamonds, but they seem to cut the corners off. This is probably to avoid too sharp a bend in the detector loop.

For cyclists at traffic lights, here's a good tip on how to trigger the sensors link https://www.youtube.com/watch?v=Sj-mNB6dLkk

As to the apparent colour difference, this could be an optical illusion similar to the ones illustrated here: https://www.mentalfloss.com/article/652720/best-pop-culture-christmas-ornaments?utm_content=infinitescroll1 The waterproofing is fairly shiny and can affect the viewing of the inside and outside of the shapes. A similar thing occurs in philately, where is is sometimes difficult to find the actual colour of a stamp if it has a heavy cancellation on part of the surface, or if it is a two colour printing with light and dark areas. (This is a problem with older stamps - modern printing methods may completely lose a colour on occasion or be out of place with one, but rarely cause distinctions affecting value like the difference betweeen yellowish-green, yellow-green, and greenish yellow. No kidding, those are really distinct colours in stamps of the UK in the early 1900s.)