How much depth of soil?

This reminds me of a traffic circle that the City of Beaverton, Oregon, hired me to do a planting design years ago.

That one was rediculous. It was to be placed over the hardtop, with just a couple feet of soil, no irrigation system and no drainage.

It was at the mercy of whether neighbors would drag a hose out to water it.

Keep the plants short for safety.

A couple fit of soil over asphalt should be more than adequate, unless the bed sits in a depression that collects water.

City of Seattle has used Viburnum davidii successfully in intersection mini-parks. Otherwise, it seems they regulate traffic circle plantings in some way. You may want to contact appropriate personnel before proceeding.