(CNN) — Workers in the United States left a record number of vacation days on the table last year, equating to billions in lost benefits, according to research from the U.S. Travel Association, Oxford Economics and Ipsos.

A total of 768 million days went unused in 2018, a 9% increase from 2017. Of those, 236 million were completely forfeited, which comes out to $65.5 billion in lost benefits.

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Fifty-five percent of workers reported that they did not use all of their vacation days. If American workers used their time off to travel, the study says, the economic opportunity amounts to $151.5 billion in additional travel spending and would create 2 million American jobs.

One of the main culprits for the increase in unused vacation days is that the number of earned days off is increasing faster than workers are using them. Workers did not use an average of 27.7% of their earned days off in 2018, up from 25.9% in 2017.

Although there was a large increase in unused vacation days, on average, American workers took more days off in 2018: 17.4, up from 17.2 off in 2017. However, that is a significant drop from the 20.3 average vacation days used from 1978 to 2000.

The study says one reason for the small increase in average days used is that employment is strong and benefits improved after the tax reform of 2017. Americans are also earning more paid time off, from 23.2 days in 2017 to 23.9 days in 2018.

"When I see how many vacation days went unused, I don't just see a number -- I see 768 million missed opportunities to recharge, experience something new and connect with family and friends," U.S. Travel Association President and CEO Roger Dow said in a statement.