[giantg2]

Tall stuff impedes vision. The gental slope around tends to be accommodating for tractor trailers. Short stuff, even just a regular curb inside of the sloped area should be enough to stop cars from driving through it. Some short flowers or plants can help draw attention.

[landemva]

You want to impede vision across so people focus on what is important for their merge.

[giantg2]

That's only potentially true with very large circles. Small round abouts are so small that you want to be able to see obstacles before you even get to the point of preparing merging.