Thursday's action from the U.S. Department of Transportation builds on a Feb. 25 order that targeted oil shipments for more stringent testing.
Testing has long been required to gauge the volatility of oil and other hazardous liquids. But there were no standards on how frequently that had to be done.
Transportation officials are now specifying that within the "reasonable, recent past" companies must have tested the flash point and boiling point of crude to determine how likely it is to ignite.
Officials are telling companies not to re-label crude as some other volatile product in an attempt to get around testing.