"Part of our identity is we don't shy away from provocative marketing," said Mike Trigg, Hightail's chief marketing officer. The billboard helps to "grow awareness of the new name but also get in front of prospective partners, investors, media and prospective employees," he said.
Trigg said Hightail, which is based in Campbell, California, is promoting the idea that features offered by Dropbox and Box are some combination of unsecured and uninteresting, while Hightail provides safer options and allows users to do more with their documents and data. Representatives from Dropbox and Box declined to comment.
Plenty of people will see the billboard, especially during the morning and afternoon commutes, when traffic often slows to a crawl, a result of the booming tech market. Employment in the Silicon Valley region jumped 3.4 percent in May from a year earlier, tied for the third-biggest increase among U.S. metropolitan areas, according to the Bureau of Labor Statistics.
Read MoreThe paradox of doing business in California
It's no surprise that Trigg or Hightail Chief Executive Officer Brad Garlinghouse like the aggressive approach, created by local advertising agency Heat. Trigg was an early employee at Octane Software, which in 1999 took aim at Siebel Systems in a Highway 101 billboard before being acquired the following year by Epiphany in a bubble-era deal. And prior to joining Hightail, Garlinghouse was a top executive at AOL, which in 2011 put up a billboard on 101 saying, "Come work for AOL before your boss does." That was a nod to the talent battle in Silicon Valley, rather than an attack on a particular company.
For Box, the Hightail campaign should also look familiar. Much of Box's early promotional efforts were targeted at Microsoft's collaboration product called SharePoint, including a 101 billboard five years ago that read, "Box.net is like SharePoint, but without the servers, set-up costs, manuals, downtime, firewall restrictions, migraines" and on and on. Box still uses the tech corridor for billboard marketing, though currently to promote its annual conference, BoxWorks, coming in September.
Snagging users from Dropbox and Box is very much an uphill slog for Hightail, as both rivals have raised hundreds of millions of dollars more in financing. Dropbox has many times the number of users in the consumer cloud storage market, while Box has a more established presence among business users, with Gartner ranking it among the four leaders (along with Citrix, EMC and Accelion) in the enterprise file synchronization and sharing market. But Hightail does have 45 million registered users, many carried over from the YouSendIt days, when the company's cloud software gained popularity for helping people send large video and data files that were too big for e-mail.
Trigg said he's happy enough with the performance of the billboard to do more campaigns like it in the near future, along with television ads and online marketing.
"A lot of people mention it to us and say they saw it and got a chuckle out of it," Trigg said. "It got the response we were looking for."
—By CNBC's Ari Levy