Half of the most trafficked PC Web sites, as defined by Nielson Online’s top 20 list from July, don’t make the cut on mobile, a study released today by Bango found. With 5% of visitors to these sites now coming from mobile devices – a 1% increase from last year – these PC Web sites are not adapting to the mobile browsing experience with sites that are either not user-friendly or not working at all.

Bango surveyed these companies (see chart below) and tested their mobile Web sites on the Motorola V3 Razr and the Nokia 6300 on AT&T in the United States, although it was Apple’s iPhone that brought visibility to the problem. The increase can be attributed to the consumer awareness that Apple brought to browsing, according to Andy Bovingdon, vice president of product marketing for Bango. He said that the iPhone caused a ripple effect of other handset manufacturers, increasing the capabilities of their handsets to support better quality Web browsers as well as operators’ increased transcoding of content for mobile.

“They will take a traditional standard PC desktop Web site and reformat it on the fly to make it work better on mainstream handsets that perhaps don’t have such good Web-browsing capabilities,” Bovingdon said. “That means that anybody with a handset they purchased in the last four years now has the capability to browse pretty much any Web site they want through the URL on their mobile phone.”

The companies themselves are the ones that haven’t kept up with the carriers and handset manufacturers. Many companies questioned by Bango didn’t even know how much Web traffic came from mobile handsets, which Bango estimates is between 3% and 10%. The company today launched an Analytics for PC product to track these mobile phone users as well as garner information on their handset, network and country of origin.

A common problem is that companies who do have a mobile Web site set it up at a .mobi URL, but consumers are unaware of this. So when they go to the traditional PC Web address, the experience is subpar. Optimizing the site for mobile doesn’t take longer than a few weeks, Bovingdon said, and it’s primarily a factor of screen size. Companies need only to optimize their site for five profiles, recommended by the Mobile Marketing Association to cover 95% of all handsets, he added.

“It’s surprising that more companies aren’t doing it yet,” Bovingdon said. “It comes down to the unknown. People don’t realize that mobile is important and is easy to account for. It’s not something where you have millions of different handsets and a lot of hard, complex work to get it working.”