Concrete barrier 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.

The distinguishing characteristics of the Jersey Barrier are two. For one thing, they sometimes have steel cable or bar loops at each end, offset vertically on each end by a few inches and projecting out. This way a bar can be pushed down through two sets of loops to link adjacent units together for extra strength and to minimize any breaks in the outward surface of the line - see below. More important, however, is the design of the base. They're not oblong, and not a wedge. The bottom third of the barrier slopes outward at a slightly shallower angle than the top third, so they look a bit like the Eiffel Tower in cross-section, except the two sections meet at a hard angle rather than a curve.

This angle is there for two reasons, as far as I can tell. First, it concentrates more of the mass of the barrier at the bottom, making it more difficult to overturn. Although they're very heavy (being concrete and rebar) they're meant to stand up to vehicle impacts carrying lots of energy. The second reason is that the shallower angle at the bottom will, if the vehicle hits the barrier at an oblique angle as it will if it's in a lane guided by the barrier, cause the tire to ride up the barrier rather than allow the vehicle body to strike it directly. Much of the car's energy can be diverted by causing it to 'jump' as the tire rides up the low angle, and in that moment, the reduction may be enough to 'softly' divert the car from an oblique approaching angle to an oblique departing angle - essentially, bounce it softly, while minimizing direct impact on the barrier. Sure, the car's tires and even fenders or sides may get chewed up, but it's much less likely it will strike the barrier and 'catch'. The linking system means it's unlikely that any of the barrier ends will be projecting out far enough to present a perpendicular surface for the car to hit.

You will often see evidence that a Jersey barrier has performed its design function in a swatch of rubber spread along the side. This indicates a tire has struck the barrier and been directed along it. Sometimes the rubber mark is even a rough arc, showing where it hit, then rose up the side, then slid back down.

So although they're very simple objects, made in great number from cheap materials, their design is very important.

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