15% of total traffic to U.S. retail web sites during the holiday shopping month of November will stem from smartphones and tablets, according to new data from the fourth annual IBM Coremetrics Benchmark report. Further, total November online sales will grow by 12-15% compared with November 2010, and among the driving factors is mobile commerce, the report says.

Record numbers of consumers will shift shopping activities from their PCs to smartphones or tablets this holiday season, the report predicts. In October 11.0% of traffic to retail web sites came from mobile devices, up from 4.2% in October 2010. Additionally, mobile sales continue to increase, reaching 9.6% of total online sales in October 2011, up from 3.4% in October 2010.

“This November holiday season will mark the true advent of the post-PC era with consumers demonstrating a heightened interest in adding mobile devices to their holiday shopping arsenal,” says John Squire, director of product management in the enterprise marketing management group at IBM Industry Solutions. “In response, savvy retailers must invest in delivering hyper-personalized, smarter commerce shopping experiences that are capable of building loyalty through multiple channels with exceptionally relevant promotions, free shipping and more.”

The IBM Coremetrics Benchmark report gathers data from the web sites of more than 500 U.S. retailer clients of IBM. IBM uses its web analytics to measures traffic, sales and other information.

The report also finds that shoppers using an iPad will lead to more purchases than shoppers on other mobile devices. In October 2011 the conversion rate for consumers using iPads was 6.8% compared with the overall mobile device conversion rate of 3.6%.