Median barrier

A Jersey barrier or Jersey wall is a modular concrete or plastic barrier employed to separate lanes of traffic. It is designed to minimize vehicle damage in cases of incidental contact while still preventing the crossover case of a head-on collision. Jersey barriers are also used to reroute traffic and protect pedestrians and workers during highway construction, as well as temporary and semi-permanent protections against landborne attack such as suicide vehicle bombs. A Jersey barrier is also known in the western United States as K-rail, a term borrowed from the California Department of Transportation specification for temporary concrete traffic barriers, or colloquially as a Jersey bump. Plastic water-filled barriers of the same general shape are also now commonly called Jersey barriers.

Note: most of the original barriers constructed in NJ in the 50s and early 60s were not "modular" they were poured in place the way curbs are. Many of the first installations (route 46 in Bergen County and Passaic County for instance) were much shorter than the heights discussed here, typically about two feet. Some dividers on county or local roads may have been lower than that since they replaced a raised concrete rumble strip that would dissuade but not prevent traffic crossing from one lane to another. Even route 46 had the rumble strip in many places before the higher barrier was gradually installed. These lower dividers should be visible on many old photographs. When the Bergen Mall was first opened in Paramus these rumble strip dividers were extensively used on the roadway (Forest Ave) that separated the grocery stores from the mall proper. Taller barriers have the added advantage of blocking most ongoing headlights.

The Jersey barrier, also called New Jersey wall, was developed in the 1950s (introduced in current form in 1959), at the Stevens Institute of Technology,New Jersey, United States, under the direction of the New Jersey State Highway Department to divide multiple lanes on a highway. A typical Jersey barrier stands 32 inches (81 cm) tall and is made of steel-reinforced poured concrete or plastic. Many are constructed with the embedded steel reinforcement protruding from each end, allowing them to be incorporated into permanent emplacements when linked to one another by sections of fresh concrete poured on-site.

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Wikipedia