I know that a land mass completely surrounded by water is called an island. Is there a separate term for a piece of land, say a garden, in the center or a road or highway?

That space is generally called the median.

If it is isolated from other similar pockets, and the limits of the island don't correspond with cross streets at both ends, the words "island" is also used. In the St. Louis area, "island" refers to one thing, "median" to another, usually a separation between traffic travelling different directions on the same street. Medians may or may not have dirt and plants, while islands usually do. The few islands that are used at or near crosswalks and are for pedistrians are usually called pedestrian islands.