CHICAGO, Nov. 29 (Xinhua) -- The U.S. holiday shopping season got off to a healthy start as retail sales increased 3 percent over 2007, Shopper Trak RCT, a Chicago-based research firm, said Saturday.

    ShopperTrak, with 20 years of retailing analysis expertise, said preliminary sales estimated at a daily rate for Black Friday totaled 10.6 billion U.S. dollars. Retail traffic was heavy throughout the country as shoppers responded to early openings and numerous door buster specials and sales promotions.

    Black Friday, so named because surging sales on that day ensure profits for retailers, is one of the busiest shopping days of the year. This year, however, retailers are not so sure about their profits as they are prompted to slash prices in an attempt to lure customers.

    At a New York Wal-Mart store, a worker was trampled by a throng of unruly shoppers moments after the store opened early Friday.

    ShopperTrak warned, while this is an encouraging start for retailers, there's no guarantee these deep discounts will continue after Black Friday weekend.