The 74-year-old Threlkeld, who lived in East Hampton, was killed when his 2008 Mini Cooper collided with a propane tanker at the intersection of Cross Highway and Route 27 at 8:18 a.m., police said.
Police Chief Edward V. Ecker Jr. said it was fortunate that the collision didn't cause an explosion. The crash remains under investigation.
The driver of the 2008 Peterbilt tanker truck, Earl Fryberger Jr., 57, of Coatesville, Pa., was not injured in the crash, police said.
Threlkeld was pronounced dead at Southampton Hospital.
Threlkeld had worked for CBS for more than 25 years, with a stint in the middle with ABC News, and was a regular contributor to "CBS Evening News" and "Sunday Morning."
CBS colleague Bob Simon, now a correspondent on "60 Minutes," said he and Threlkeld were frequent competitors in stories around the globe.
"He was always first-rate," Simon said. "Richard was old school in the best sense. He really didn't give a damn about being on camera. He didn't do many standups. He always figured there was more interesting footage than himself."
Threlkeld was born in Iowa, and grew up in Burlington, Ill. He graduated from Ripon College in Wisconsin with a bachelor's degree in political science and history, and earned his master's in journalism from Northwestern University.
He joined CBS News in 1966 and retired 32 years later, returning briefly in 2004 to help celebrate the 25th anniversary of "Sunday Morning."
Threlkeld is survived by his wife, Betsy Aaron, a former CBS and CNN correspondent; a brother; two children; and two grandchildren.