The Pipeline and Hazardous Materials Safety Administration is required by law to respond to safety recommendations issued by the National Transportation Safety Board (NTSB), an independent federal agency charged by Congress with investigating significant incidents that involve hazardous materials. Recommendations usually address a specific issue uncovered during an investigation or study and specify how to correct the situation.

Open Recommendations
The NTSB has 6 classifications for open recommendations:

OAR - Open Acceptable Response
OAA - Open Acceptable Action
OAAR - Open Acceptable Alternative Action
OUA - Open Unacceptable Action
OUR - Open Unacceptable Response
UCL - Unclassified (until NTSB Classifies)

Closed Recommendations
NTSB closes recommendations when it has been determined that sufficient actions have been achieved. NTSB organizes its closed recommendations into 7 classifications:

CAR - Closed Acceptable Response
CAA - Closed Acceptable Action
CAAA - Closed Acceptable Alternative Action
CAAR - Closed Acceptable Alternative Response
CNLA - Closed No Longer Applicable
CUA - Closed Unacceptable Action
CUR - Closed Unacceptable Response

PHMSA's Office of Hazardous Materials updates the status chart when new information is available.

Status chart- by mode

Status chart- by classification