These are all examples of highway route designation signs.  They tell you what
Highway you are on and who owns and maintains the roadway.  Such as California
Interstate 280, or Santa Clara County G6, aka SanTomas Expressway.
Above are all examples of reflectors and mile post markers.  They either help you see
the contours of the road, or tell you where you are.  
Reflectors reflect light and are either white, right side of the road, or orange, left
Mile post markers tell your position on a road or highway.  Take the sign with the
96 on top.  This sign is on Route 96 in Siskyou County, exactly 55 miles from the
southern or western most point on that road in that county.