What is the point where you can make a U-turn called? Can we use "turnaround"?
Indeed you can use turnaround:
3 : a space permitting the turning around of a vehicle
It is a more general term, however. If somebody simply says "turnaround," it may not be clear that they are referring to "a break in the median" specifically (as opposed to a different type of turnaround).
In British English the land between the two lanes is called the central reservation and the wall or fence put on that land is called the traffic barrier So this could be described as "a break in the traffic barrier".
On UK motorways making a U-Turn is strictly forbidden, and there are no breaks in the traffic barriers. On some dual-carriageways, there might be a break in the central reservation to allow traffic from a side road to access both carriageways, but there are usually signs prohibiting U-turns at these points too.