After more than a dozen major crude oil train derailments in the last few years, the Department of Transportation is updating its shipping rules.

The new rules phase out the easily-punctured tank cars known as DOT-111s over the next three years. They also set a 50 mile per hour speed limits for crude oil trains, and require trains carrying more than 70 cars of crude oil to have better brakes so they don’t crunch accordion-style during a derailment.

U.S. Transportation Secretary Anthony Foxx said in a press conference that although most crude oil shipments currently arrive safely at their destination, that’s not good enough.

“Accidents involving crude and ethanol that have occurred have shown us that 99.9 percent isn’t enough," he said. "We have to strive for perfection.”

The rules didn't satisfy many safety advocates though. Powder River Basin Resource Council organizer Megan Taylor says while they're a step in the right direction, they don’t go nearly far enough.

“We feel like these speed restrictions are too high, especially when considering that the newer model tank cars—the CPC1232s—actually derailed and exploded while traveling only 24 miles per hour in Lynchburg, Virginia,” she said.

Taylor added that the rules don't address crude oil volatility, which critics say is key to reducing the risk of fiery derailments and raise new issues about whether the movement of crude oil will be disclosed to emergency responders.