PHMSA 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.
The NTSB has five classifications for open recommendations. Also, the NTSB closes recommendations when it has been determined that sufficient actions have been achieved. The NTSB organizes its closed recommendations into 11 classifications.
OAA - Open Acceptable Response
CEX - Closed Exceeds Recommended Action
OAAR - Open Acceptable Alternate Response
CAA - Closed Acceptable Action
OUA - Open Unacceptable Response
CAAA - Closed Acceptable Alternative Action
ORR - Open Response Received
CUA - Closed Unacceptable Action
OAR - Open Await Response
CUAN - Closed Unacceptable Action/No Response Received