In its own tests, Google found that 69% of its visitors abandoned the Google+ experience when presented with a full-page interstitial.

Google’s rationale for disclosing its internal stats: “we are sharing this with the hope that you will reconsider the use of promotional interstitials.”

Now the company is planning to use force to encourage publishers to reconsider their use of interstitials. 

In a post on the Google Webmaster Central Blog, Google Search software engineer Daniel Bathgate announced that Google has updated its Mobile-Friendly Test “to indicate that sites should avoid showing app install interstitials that hide a significant amount of content on the transition from the search result page.”

Starting on November 1, sites that still have such interstitials in place “will no longer be considered mobile-friendly.” That’s a potentially big deal for publishers, as in April Google expanded its algorithm to include mobile-friendliness as a ranking signal.

So what does this mean for publishers?

Google is serious about mobile-friendliness

When Google added a mobile-friendliness ranking factor to its algorithm, it made it clear that it was serious about directing users to mobile sites offering quality users experiences.

Of course, some might question its judgment of what constitutes mobile-friendliness, but the message to publishers is clear: if you’re not thinking about user experience on mobile, you’re not thinking about the user.

There’s a good chance you’ll do something that loses referral traffic as Google continues to tweak and extend its mobile-friendliness ranking factors.

Driving app installs is only going to get harder

Getting users to download, install and use mobile apps has for many publishers become an incredibly costly exercise.

For obvious reasons, use of prominent interstitials on their own properties has been an attractive promotional tool for many publishers.

While Google’s data from Google+ suggests that full-page mobile app install interstitials are bad for business, other publishers argue that their experience with these interstitials isn’t nearly as negative.

One company that takes issue with Google’s position on interstitials is LinkedIn. In a blog post, Omar Restom, a mobile product manager at LinkedIn, revealed that LinkedIn achieved a 20% rise in incremental app downloads using interstitials with a bounce rate of just 26%.

These figures differ significantly from Google’s Google+ reference case, which saw a whopping 69% bounce rate with just a 2% increase in incremental app downloads.

According to Restom, Google’s bad numbers were a result of bad targeting and the fact that “no One Wants Google+.”

Restom says LinkedIn appropriately avoids showing interstitials to users who it knows already have its app, something Google apparently didn’t do, and claims LinkedIn “worked hard to make the interstitial page as delightful and interesting as possible.”

Unfortunately for publishers, even if their data shows that interstitials aren’t driving users away, continued use of interstitials could come at the cost of their position in the SERPs.

There’s a new double standard to complain about

Google is to many publishers a frenemy. The search giant drives a significant amount of traffic for many publishers, but it has also been accused of harming publishers to protect and further its own interests.

Already, Jeremy Stoppelman, the CEO of online reviews site Yelp and a frequent critic of Google’s tactics, is questioning Google’s motivations, pointing out that Google is still using prominent interstitials to promote its own apps and ironically just announced a full-screen app install interstitial ad product.

Google’s continued use of interstitials and its new full-page interstitial ad product lend credence to the argument that Google’s newest mobile-friendliness ranking factor isn’t just about promoting quality user experiences.

As ReCode’s Mark Bergen notes, there is “suspicion, percolating in the mobile industry now, that Google is trying to replicate its Web search position with apps.”

If this turns out to be the case, publishers that have previously criticized Google for behavior they see as abusive and anti-competitive can expect that the mobile SERPs will be the latest front in their battles with Google.

It’s a good time to look at interstitial alternatives

Right or wrong, Google’s decisions influence publishers and with Google suggesting that publishers use Smart Banners in Safari and Native App Install Banners in Chrome, publishers should probably take a look at these browser-specific techniques to promote their apps, especially if they decide to bend to Google’s will and eliminate prominent interstitials.