WASHINGTON, D.C. — Two of the four members of the National Transportation Safety Board have submitted their letters of resignation to President Obama.

Acting Chairman Mark Rosenker said he will delay his departure until a new chairman and an additional board member are confirmed. His term was set to expire at the end of next year.

Member Kathryn “Kitty” Higgins will leave her post Aug. 3. Her term was set to expire at the end of this year.

Last month, Obama nominated current NTSB member Deborah Hersman to be chairman of the agency.

Hersman, an NTSB member since June 2004, is no stranger to the pupil transportation industry. She has served as the board’s representative at major school bus accidents, and her credentials include a CDL with a school bus endorsement. She is also a certified child passenger safety technician.

Hersman’s nomination was awaiting confirmation by the Senate.

Both Rosenker and Higgins said they were leaving the NTSB to pursue opportunities in the private sector.

Rosenker joined the NTSB in March 2003 and has served as chairman or acting chairman since 2005.

During his tenure, the agency marked a number of major transportation safety achievements. Three significant items were removed from the NTSB's “Most Wanted” list: airliner fuel tank flammability, positive train control and rail fatigue.

The board also added important issues to the Most Wanted list under Rosenker’s leadership, including cell phone use by bus drivers, highway vehicle anti-collision technology, on-board highway vehicle recorders and school bus passenger safety.

In his letter to Obama, Rosenker said that the opportunity to serve in and lead the NTSB “has been the highlight of my entire 40-year professional life. It is an agency that truly makes a positive difference every day, saving lives by preventing accidents, not only in our great nation, but around the world.”

Higgins has been an NTSB member since January 2006. She was the board member on scene at several major accident investigations, including the September 2008 collision of a commuter train with a freight train that killed 25 in Chatsworth, Calif., and the ditching of a US Airways airliner in the Hudson River in January.

She also chaired the NTSB hearing into the fire on a motorcoach in Texas that killed 23 elderly patients being evacuated from Hurricane Rita in September 2005.