They are a form (or better said a set) of
Fresnel lens. It shapes the light beam in the same manner, designed to distribute the light from the lamp into the desired directions.
The aim of the design for road lighting applications is to illuminate a stretch of road as evenly as possible, assuming the fixtures are arranged in a row, with distances about 4x their height (that is the standard the vast majority of streetlight optical systems are designed for, if there is any beam shaping at all). Usually it means forming two main lobes about 70deg off-vertical (one into each direction of the road) responsible for illuminating the more distant parts of the road, supplemented by the (rather scattered) nondirectional light illuminating the area close underneath to the lantern.
The refractor in fact forms the twin Fresnel lens structure (one for each lobe) utilizing direct light from the rather point-source arc of the HID lamp, so mainly the light going below horizontal. The part of the light going above horizontal gets reflected by the lanterns reflector and then, because it come from an angle the lenses are not designed for) scattered by the prisms.
Second aim is to distribute the light path into any desired direction (but mainly the throw lobes) over as large as possible surface of the fixture, to reduce the perceived glare, but that usually came for free with the refractor based optics (no direct light from the arc to the road).
With the reflector based (mainly the FCO) lanterns, the lobes are formed using accurately shaped reflector surfaces, the direct light from the arc then takes care of the close area below the fixture. The glass is then flat, not influencing the light pattern at all. Usually this type is less efficient in terms fo required lumen output per given illuminated area and minimum illumination level, but because of the flat glass, there is no spilled light above the horizon at all.
