Everyone knows they are there to protect us from driving off the road, but what happens when a car slams into the front of a guardrail?
There are systems in place to help absorb the blow, to slow a car safely to a stop and to save passengers from harm--at least that's the theory.
One man says that isn't happening all of the time; dozens of people have been killed or seriously injured when a guardrail suddenly pierces the front of a car and impales its passengers like a metal spear.
When guardrail end caps are hit head on, they are supposed to flatten and roll the guardrail away from a vehicle, safely slowing it to a stop.
But, does it always work that way?
Jay Traylor was on Interstate 40 in North Carolina when he dosed off and hit a guardrail.
The guardrail ripped through his car and into the passenger compartment.
After six surgeries, Traylor is now a double amputee.
Attorneys for Traylor filed a lawsuit against the guardrail manufacturer, trinity industries of Dallas, Texas, last year.
Trinity declined an on-camera interview request from Fox News in Dallas, but it has called all allegations against its guardrail terminals false and misleading.
Traylor's case is scheduled to go to trial next January.
The latest round of testing may help Trinity's defense.
The Federal Highway Administration released a video from tests of the Trinity ET Plus Guardrail System.
The tests began in San Antonio late last year and the results were recently announced.
While the cars take an obvious beating , researchers say the passengers in these collisions would have survived without serious injury, so the guardrail end caps passed.
In a written response to the test results, Trinity writes, quote: "Today's data release by the Federal Highway Administration (FHWA) validates what we have consistently said...The ET Plus is a robust end terminal system that performs as designed."
After more than 20 years in the guardrail manufacturing and repair business in Virginia, Josh Harman tells a different story.
He says he started to notice something wrong with some of the guardrails he was repairing.
"It was a terminal I did a repair on," said Harman.
Harman noticed guardrails end caps made by trinity industries weren't working like they were supposed to.
"The vehicle literally rolled laterally across the top of the guardrail, ejecting the occupants and killing the entire family," said Harman.
In a video of the 1999 test of the original Trinity guardrail end cap system, the truck hits the guardrail, the end cap flattens and curls away from the guardrail and the truck comes to a safe stop.
But after 2005, Harman claims that didn't happen anymore.
The guardrails were left locked up inside the guardrail cap, sometimes spearing the inside of the passenger compartment.
Harman started taking photos, posted them on his website www.failingheads.com and started measuring trinity end caps.
We recently joined Harman as he measured guardrails along US 60, east of Phoenix.
Within just a few miles, we found what Harman says are three different versions of Trinity's guardrail end caps.
He showed us the differences in the width of the guardrail cap exit gate.
Changes in the width of the feeder channel from five to four inches.
According to Harman, the original model worked flawlessly.
"This model is a work of art. I've repaired terminal accidents that people shouldn't have went home and they walked home the literally didn't have a scratch on them," said Harman.
But, just down the road, Harman shows a second version that he claims Trinity started selling in 2005.
Harman alleges the company changed it's end caps again in 2013 and returned to a model closer to the original.
He showed us one of those too.
This brings us back to the most recent testing done by the Federal Highway Administration.
Harman claims the feds only tested the third version of the guardrail, the one Trinity fixed.
His attorney, Steven Lawrence, had this to say about the tests:"The Federal Highway Administration cannot waive a magic wand that makes the four-inch ET Plus safe. Trinity's fraud will cause serious injuries and deaths across the country as long as this product is on the road."
Trinity maintains it has made only one version of its guardrail end cap, not three as Harman alleges.
Again in a written statement, Trinity writes: "There is one version of the ET Plus extruder head. Any claims to the contrary are purely false and misleading."
Last October, Harman took Trinity to court and won a $525 million verdict.
The federal court jury agreed with claims made by Harman's legal team that trinity industries changed its guardrail specifications without notifying the federal government.
The judge has not confirmed the verdict, but if he does, Harman's attorney says 70 to 75 percent of the award goes to the federal government and 20 to 25 percent to Harman.
During the case held in this Texas court, trinity argued that Harman was suing out of spite, that Harman's own business stood to gain by the decision.
Harman owns 37 percent of Spig Industry, which manufacturers and repairs guardrails.
But on March 17, Spig filed for bankruptcy protection after an expensive patent fight with trinity in 2013, a case that was settled for an undisclosed amount.
In the meantime, Harman says he now spends almost all his time on the road investigating crashes involving trinity end caps.
"One man investigating this looking into this, I'm over 200 victims, both fatalities, losing legs, limbs. I've got everything from a widow who lost her husband to a gentleman who lost his wife and three little girls who lost their mother," said Harman.
He hopes someday the thousands of trinity guard rail end caps still on the road will be replaced.
"No terminal is 100 percent perfect, but no terminal should be failing at the level this terminal is failing," said Harman.
The Arizona Department of Transportation says there are more than 3,000 Trinity ET Plus guardrail caps on highways in our state, but it's frozen the installation of new trinity caps for now.
Among those guardrail related accidents Harman has investigated is the death of Mark Hraneck. He was killed last April on the Loop 303 near Grand Avenue, northwest of Phoenix.
The Wickenburg Sun reported police believe Hraneck was killed when he fell asleep, and hit a guardrail.