The median of a road is typically considered to be part of the recovery zone (engineering / DOT term). The shoulder is also part of the recovery zone. The recovery zone is intended to afford a vehicle an area to safely and without obstruction come to a complete rest in the event a it leaves the travel lane. Check your state DOT regulations for more information. Clear sight distance may also come into consideration (vertical visibility at the head of median – Google FDOT Standard Index 546 for graphic illustration). DOT’s are quite sensitive about what is allowed within the median. Even if this is not a state road the local municipality follows their guidelines, more than likely. Unfortunately the regulations all point to safety and landscape comes second (or third) when compared. The rationale behind these regulations is somewhat flawed. I have never been provided a report of crash tests or incidents that suggest objects (landscape or hardscape) have caused an accident or resulted in a more severe accident. On a professional level the liability would be a compelling reason to refrain from placing certain objects in the median. Imagine trying to justify the placement of boulder in the median for aesthetic reasons when a person may have sustained physical harm because when their car went off the road and made impact with an element that your design called for.