Most people take the partitions that divide the traffic on U.S. highways for granted. But these seemingly simple barriers are actually deceptively sophisticated: Their designs have been well-tested and tweaked to ensure driver safety on both sides of the road in the event of a crash. The most common name for these ubiquitous concrete slabs is “Jersey Barriers”—but why?

Concrete road barriers were first used in California in 1946; they replaced the standard (but weak) wood beam guardrails on the treacherous Grapevine section of the state’s Ridge Route highway—the home of the original “Dead Man’s Curve”—where the roads had a 6 percent downgrade that led to many head-on collisions. Then, in 1949, the state of New Jersey adopted comparable concrete structures and installed preventative parabolic median barriers on the Jugtown Mountain section of US Route 22 in Hunterdon County, which had a similarly hazardous downgrade to the Ridge Route highway.

These original barriers measured 19 inches high and 30 inches wide, with 2 inches buried in the road to provide stability. Each was anchored to the roadbed by steel dowels and consisted of a 2-inch thick outer layer of white concrete to make it more visible at night. Though the initial barriers were somewhat successful in reducing the impact of collisions, New Jersey state highway engineers continued to tinker with the design, creating progressively larger prototypes based on amounts of observed accidents (as opposed to performing controlled crash testing). Eventually, in 1959, they settled on a standard barrier height of 32 full inches above the pavement with a 24-inch-wide base. The base is 3 inches high and is followed by a 13-inch side slope before the barrier becomes vertical. These barriers would be implemented in various states, but would bear the name of the state in which they were developed.

Jersey Barriers are designed to redirect a crash, using the car’s momentum to absorb the impact and slide the vehicle up parallel along the side of the barrier to prevent a rollover. In high-speed crashes with small cars along Jersey Barriers, however, there is a greater likelihood that the car will roll over, so an alternate barrier was created. According to the Federal Highway Administration, the F-Shape barrier has the same 3-inch-high base, but features a side that slopes 10 inches above the pavement—three inches less than the side slope of the Jersey Barrier—and is thus able to better absorb proportional impacts from smaller chassis to prevent a rollover. Though the F-Shape is generally preferred, the use of Jersey Barriers—as well as other barrier designs, including constant slope, single slope, and vertical—are still acceptable, because they adequately pass crash tests administered by the National Highway Traffic Safety Administration.

Additional Source: Federal Highway Adminstration [PDF]; primary image courtesy of NYC Dot Flickr.