Jonathan D. Woods / msnbc.com

Taxi driver Cliff Adler in New York on Friday, September 2, 2011.

On the morning of Sept. 11, 2001, Cliff Adler was en route to a dealership to have the air conditioning fixed in his yellow cab. Like many taxi drivers that morning, he first heard about the attack on the World Trade Center on the radio. He could not imagine that anything larger than a hobbyist pilot’s airplane had crashed into the north tower at 8:46 a.m.

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But then he turned onto the West Side Highway, several lanes of north-to-south traffic from which drivers can see the end of Manhattan. “I could see a huge hole in the north tower,” he remembers. It was American Airlines Flight 11. “And while I was sitting in traffic, I saw a ball of flame shoot out of the south tower.” That was United Airlines Flight 175, which crashed at 9:03 a.m.

Adler felt compelled to help. He was born on an Air Force base in Germany, but his father’s family traces their roots to New York City back to 1890, when Adler’s grandfather emigrated from Romania as an infant. In his early 50s, Adler knew he didn’t have the physical energy to volunteer at ground zero. Instead, he decided to volunteer his cab.

In the hours after the attacks, Matthew Daus, head of the city’s Taxi and Limousine Commission, put out a call asking for drivers to transport volunteers, emergency responders, blood donors and others involved in reacting to the attacks. Adler was one of the drivers who lined up on Lexington Avenue outside of the National Guard armory to shuttle passengers free of charge.

Soon he relocated to Pier 94, where families of victims went to receive assistance in applying for death certificates, as well as other social services. More than a dozen agencies set up at the pier, including the Federal Emergency Management Agency, the Red Cross and the Salvation Army.

For about a month, when the pier was open 24 hours a day, Adler arrived every day at 7 a.m. and stayed until 11 p.m. He continued to take passengers when the pier switched to a 12-hour schedule. Free of charge, he drove victims’ family members, Red Cross volunteers, cops – anyone who needed a ride. He estimates he took a half-dozen paid fares over two months. “I was upset like a lot of people,” Adler says. “There were a lot of people who tried to help out and I was just one of many.”

One of Adler’s most vivid memories is of a woman in sweat pants, clutching a manila envelope against her chest, who left the pier looking stunned. Adler offered the woman a ride to her home downtown. “She was sitting there dead quiet,” remembers Adler. “I felt obliged to say something. I said, ‘You lost somebody.’ ” It took her a moment, but she responded that she had lost her husband, brother, brother-in-law and several friends. She had also watched the attacks from the window of her apartment. “We both broke down crying.”

Adler still has difficulty explaining why he worked such long hours at the pier. “I really don’t have an answer for it,” he says, “but I knew I couldn’t stop it.”

The altruism devastated him financially. Adler paid for the taxi’s gas without reimbursement. After missed payments, the credit union threatened to foreclose on the loan for his medallion, the aluminum plate affixed to every cab in the city that permits drivers to pick up street hails.Adler still has difficulty explaining why he worked such long hours at the pier. “I really don’t have an answer for it,” he says, “but I knew I couldn’t stop it.”

“I went nearly completely broke,” says Adler. “I probably didn’t need to stay that long ... but they would just call and say, ‘We’ve got family members coming out.’ They wanted someone they could trust.”

Adler soon returned to picking up paid fares, but the taxi industry had suffered a severe blow to business.  The Sept. 11 attacks led to a 50 percent drop in yellow cab fares through at least October 2001, says Alann Fromberg, deputy commissioner of public affairs for the Taxi and Limousine Commission. The sharp decline partly had to do with the closure of the financial district during the clean-up effort. The annual number of tourists, 35 million, also remained flat until 2003. Tourist spending dropped from $21.3 billion in 2000 to $18.5 billion in 2001 – and stayed flat until 2004, when it reached pre-Sept. 11 levels and increased to $24.3 billion.

Ten years later, the city’s tourism industry has more than recovered from the attacks. In 2010, 49 million people visited New York City and spent $31.5 billion. Fromberg says the commission does not track how many tourists take taxis, but he knows the drivers of the city’s 13,000 yellow cabs are the face of New York for many visitors.

“You could not meet a group of people who are more in tune with their clientele and more in tune with where their role puts them,” Fromberg says. “They take it seriously. You can just tell they actually care.”