Ask any driver of any tankers and they'll tell you the risk of a catastrophic explosion is very real.
"We're not allowed to keep it running," said one driver.
The engines must be turned off because explosions can happen when the engine is running.
In Brazoria County in 2003, Texas City in 2005, Tacoma, Washington in 2007 and in Oklahoma last summer, diesel truck engines were suspected of running out of control and triggering explosions.
At first, it sounds bizarre. How could a diesel engine start revving out of control with no way to stop it?
Well, experts say that in case after case, there was nothing wrong with those engines. It was what was in the air.
"Engine's running and there's some sort of release, flammable gas, that engine has the potential to pull that gas into the air intake," said B.T. Steadman with Vacuum Truck Rentals.
In Deer Park, B.T. Steadman's company owns trucks that are used to haul chemical waste. He said his company keeps 120 trucks at that location.
The danger, as seen in a video from Washington state, is that if an explosive vapor escapes from the truck's cargo or a tank nearby, the diesel engine, often left running to operate pumps, can start pulling the vapor into its air intake, causing the engine to run faster and faster.
Fehling: "Why doesn't the driver just turn the key off?"
Steadman: "In a diesel engine, turning the key off won't shut it off." Fehling: "Turning the key off won't shut then engine off?"
Steadman: "Not in a diesel engine."
Steadman says that diesel engines use compression, not spark plugs to ignite fuel, so the only way to stop a runaway engine is to shutoff the vapors getting sucked into its air intake.
On most trucks, there's no way to do that, but it can be done using a safety valve made right here in Houston, said Jogen Bhalla.
"It shuts down the air to the engine," said Bhalla.
An international company called AMOT makes the valve that cuts off the air and kills the engine if the engine revs beyond a safe limit.
Fehling: "Your Company has been making this product for a number of years?"
Bhalla: "Twenty-five plus."
The valves are reportedly required in Europe and Canada.
But even after three deaths in Brazoria County, 15 at BP and still others around the nation, the U-S has no requirement for the safety shut-off valves.
BP wouldn't answer questions about this, but some companies, like Steadman's, are taking action.
He said this company is putting the valves on 200 of his trucks at a cost of a couple thousand dollars each.
Meantime, the United Steelworkers Union (USW) is urging government regulators to make the safety valves mandatory saying if they save one life, they'd be worth it.