Recommendations issued to PHMSA are organized as follows: 

1. Hazmat Transportation
   1. Open Recommendations
      1. Aviation
      2. Highway
      3. Intermodal
      4. Rail
   2. Closed Recommendations 
2. Pipeline Transportation
   1. Open Recommendations
   2. Closed Recommendations

About NTSB Recommendations

PHMSA is required to respond to safety recommendations issued by the National Transportation Safety Board, an independent federal agency that investigates significant incidents involving hazardous materials. Recommendations usually address a specific issue uncovered during an investigation or study and specify how to correct the situation.

You may search for recommendations from the NTSB database here. 

Open Recommendations
The status of a recommendation is open until NTSB determines that sufficient action has been achieved to mark it as closed. NTSB classifies responses to their open recommendations as follows:

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 they have determined that sufficient responsive actions have been achieved. NTSB classifies responses to their closed recommendations as follows:

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CNLA - Closed No Longer Applicable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