Medians are nice on the widest of boulevards. They break up the expansiveness of otherwise contiguous lanes. They make a four lane boulevard seem more like a pair of two lane roadways. Berms and other obstacles within medians limit the potential for head on collisions with traffic from opposite sides of the medians. Trees shade and cool some of the pavement when the weather gets warm. Besides all that, medians that are modestly landscaped simply look nice.
Notice that I said ‘modestly’ landscaped. There really is no need to get carried away with landscapes in medians. No one is really looking too closely at them anyway. People are driving past them, and really should be paying more attention to the road ahead rather than what is blooming to the side. Even passengers who are not driving probably are not seeing much of what goes into median landscapes. Color in such landscapes is nice; but no one cares if the color is provided by plants that are expensive and consumptive to maintain, or plants that can more or less survive on their own. It other words, resources should not be wasted on medians. Expensive and consumptive public landscapes should be installed only in parks or other places where they can be seen and appreciated.
Then there are those who must perform the maintenance. It is not safe for them. It will of course be necessary for crews to go out to maintain medians sometimes, and sometimes they might need to block a lane to do what needs to be done; but they should be out there as little possible. They should not be out there deadheading roses, pruning wisteria or planting petunias. They certainly should not be mowing lawns that no one can use! High maintenance features, like formal hedges, fountains, espaliers, trellises, arbors and beds of seasonal annuals, have no business out in medians! Such features require too much attention from those who must interact with traffic to attend to the maintenance.
Turf uses too much water anyway. It is useful in parks and athletic fields, but should be limited to situations where it can actually be useful for something. It is not useful in medians.
Trees are perhaps the best features of median landscapes, but even they are often not well thought out. They should be proportionate to the roadways that the get installed into, and get high enough for adequate clearance above truck traffic. Vertical clearance is not important if small trees can fit between the curbs of wide medians, but such wide medians should probably be outfitted with larger and taller trees. Trees in medians should exhibit complaisant roots that are less likely to damage curbs and pavement.
Landscape design takes serious work; and there is a lot to consider when designing landscapes for medians.