How Things Work: Highway Barriers

You have somewhere to go. You jump in the car and maneuver onto the nearest interstate. Whatever your destination, the National Highway System has probably helped you get there at some point. These highways allow direct, fast, and safe movement across the country; however, the high level of safety we have today on our highway systems hasn?t come easily.
When designing highways, minimizing the severity of off-road accidents is important. Unlike in video games, stunt bonuses don?t exist, normal people don?t drive down the sidewalk, and your Honda Civic probably can?t take a shortcut off a bridge. Fatalities resulting from these ?road departures? can be reduced in several ways.
On highways, median barriers between opposing traffic reduce fatalities from road departures. A highway-bound departure becomes especially dangerous in urban areas when the probability of hitting opposing traffic is high: More cars and less wiggle room means a lot more chances.
The concrete median may appear to be a simple construction, but these are no ordinary walls ? they?re super walls! The goal here isn?t just to stop vehicles from entering opposing traffic. A giant pit between the lanes would accomplish that. Concrete medians prevent median crossings, protect the occupants of the vehicle, minimize damage, and redirect the errant vehicle back to where it belongs.
The New Jersey Turnpike Authority pioneered the ?New Jersey shape? barrier, now adopted nationwide. The barrier is
such that a vehicle?s tire
will rise up on the shallow slope of the barrier?s base, directing the vehicle away from the median in
glancing collisions. When a vehicle collides at steeper angles, the steel-reinforced barrier can absorb the force
of a collision with a semi-
trailer without sending flying pieces of concrete into
surrounding traffic.
Fatalities from road departures on the outer edges of highways result from hitting trees, rollovers, and various ?road hardware? such as
telephone poles, stoplights, and mailboxes. The most common roadside protections are W beams and cable barriers.
W beams act as metal cushions that absorb impact by crumpling. Cable barriers deflect vehicles using wires tensed at each end.
Head-on highway collisions pose a new problem: Barriers prove more deadly than beneficial. Something is needed to bring the vehicle to a stop
at accelerations tolerable to the passengers. John Fitch, a famous race car driver
during the ?50s and ?60s,
solved this prolem with
the Fitch Highway Barrier
You might not know them by name, but you?ll probably recognize the yellow barrels with black lids sitting between highways and exit lanes. These sand-filled barrels are set up in rows along the most likely line of impact to a hazard. The vanguard barriers contain the most sand. Each successive barrel contains less. When a vehicle collides with the barrels, they shatter, and the
vehicle decelerates smoothly instead of violently hitting a barrier. Fitch barriers are used widely because of their cost efficiency and crash-worthiness.
There are many safety features in place to protect you on the highway, but
remember that safety isn?t
only engineering ? education and awareness are just as important. So don?t drive drunk, aggressively, or after a night?s worth of Physics II problem sets. The highway barriers are there, but they are better
seen than experienced.