NEW YORK >> Investors flocked to Zipcar Inc. yesterday, its first day as a public company, sending the car-sharing service’s stock soaring 56 percent.

That performance means Zipcar had one of the best initial public offerings of 2011 in terms of first-day increases for its stock. It trails only Qihoo 360 Technology Co., which went public in March and climbed 135 percent that day.

Zipcar shares priced at $18 on Wednesday night, totaling $174 million. The stock had soared to $28 by the time the markets closed yesterday. That was far above the company’s original projections of $14 to $16 per share.

The warm reception also led investors who had already been backing the company to cash out more shares than they originally planned — about 3 million shares versus 1.7 million.

Investors were also closely watching the IPO of Arcos Dorados Holdings Inc., the largest franchisee of McDonald’s Corp. restaurants. Its stock climbed 25 percent.