Mobile video advertising spending is expected to hit $159 billion across the globe by 2018, according to AOL’s State of the Video industry report.

AOL Platforms found that one-fifth (20%) of ad buyers increased what they spent on mobile video advertising in 2015. That’s compared to growth of 18% across roughly 300 advertisers since 2014 — indicating that spending is accelerating.

AOL also found that viewability is considered the most important factor in the success of a video ad, with audience targeting coming in second. Typically, ad buyers across the industry create video advertising campaigns with 50% to 100% viewability. The most common viewability rate seen by the media brand’s ad buyers and publishing partners was an average of 65% to 74%.

In all, ad buyers are shifting about 10% of their overall television advertising budgets to digital platforms. AOL noted digital video ad spend has continued to grow over the last six years, with 2015 seeing a 42% increase over 2014’s figures to hit a total $7.46 billion projected spend. That figure should increase to $13 billion by 2019.

“Brands and agencies are looking to digital to address the need for device-agnostic video consumption, as well as explore alternatives to the cost of TV advertising, which has increased 29% since 2012,” AOL wrote in its report.