usually 3 to 4 feet high, in the middle of a highway
when there is no median
. They are put in when lanes travelling in opposite directions would be right next to each other.
They are called Jersey barriers because they are frequently seen in New Jersey, where small two-lane highways have become four lane or six lane roads, and the Jersey barrier is used to maximize the use of available space.
Using a Jersey barrier in New Jersey turns a road into strange hybrid of a controlled-access highway and a smaller road. The road often still has traffic lights that stop the flow of traffic on it, but to make any turns you must use jughandles--special ramps connecting the main road to the cross streets, after which you would cross at the light or go on to the side street as appropriate. Jersey barriers also mean you cannot just make a left turn into stores anywhere along the roadway, so you frequently must do a U-turn at a jughandle, and go back down the road in the opposite direction to get there. Which gas station I go to is entirely dependent on which way I'm going and the way to get there with the least U-turns at any time.
While this may seem cumbersome, it does keep the traffic flowing smoothly, and eliminates the need for a suicide lane, and people making left turns across busy roadways and their problems.