By 2017, 60% of all U.S. retail sales will involve the Internet in some way, either as a direct e-commerce transaction or as part of a shopper’s research on a laptop or mobile device, according to a new report by Forrester Research Inc. entitled “U.S. Cross-Channel Retail Forecast, 2012 To 2017.” Approximately 10.3% of total retail sales in the U.S. in five years will be online purchases, or $370 billion in web sales compared to $3.6 trillion in total retail sales.

In contrast, last year e-commerce accounted for just 5.2% of total retail spending in the United States, according to U.S. Commerce Department figures that include items rarely if ever bought online, such as gasoline and restaurant meals. And in 2012, less than half—46%—of total U.S. retail sales were either transacted directly or influenced by Internet research on PCs, smartphones and tablets, Forrester says.

Driving this e-retailing growth is the increasing ubiquity of smartphone ownership in the United States along with retailers’ investments in enabling e-commerce and mobile applications, such as mobile coupons, says Sucharita Mulpuru, a Forrester analyst and author of the report. By the end of 2013, 150 million (47.3%) of the country’s population of 317 million will be regular mobile Internet users, she says.

The categories that will be most influenced by Internet research in five years will be grocery, apparel and accessories, home improvement and consumer electronics, in particular through mobile activity like reading customer reviews while in the aisle, , the report says. Together, those categories will account for $1.1 trillion of the $1.8 trillion total web-influenced retail sales predicted for 2017. (The web-influenced figure does not include the e-commerce purchases of $370 billion.)

“The categories that have the lowest online sales are also the ones that see the greatest levels of online research and vice versa,” Mulpuru writes in the report. “In general, consumers in virtually all categories touch the web during some part of their purchase journey, but web sales (i.e., dollars spent online) tend to be strongest in categories where consumers don’t need to touch the products or have them immediately.”

For example, shoppers often buy consumer electronics online, but they more often buy grocery and home improvement items in stores—though they research those products online more frequently than consumer electronics, she says.

Retailers should focus on three main areas in order to prepare for the web-immersed shoppers of the future, Mulpuru says. First, retailers should make sure to support self-service mobile functionality for in-store shoppers or provide associates with mobile tools to help shoppers. For example, Wal-Mart Stores Inc.’s mobile app detects when a shopper is in the store and helps her find products or discounts, while Pacific Sunwear of California Inc. gives store associates iPadsto help shoppers order online products that are out of stock in stores.

In that vein, Mulpuru also recommends that retailers enable fulfillment of web orders using store inventory, and the reverse. “This approach leverages the increasing amount of research that happens online while taking advantage of store inventory that may be trapped in slower-turning stores,” she says.

Along with connecting web and store inventories, the third task for retailers to master before 2017 is to resolve price differences between e-commerce and store catalogs. “As more shoppers have smartphones and are able to access prices instantaneously, such discrepancies are likely to work against retailers, particularly if the pricing variances are large and don’t favor the customer,” she says.