An increase in vacation day usage is showing a positive impact for American work culture, yet workers still have unused days, according to research from Project: Time Off.

In 2016, workers utilized an average of 16.8 days of vacation, an increase from 16.2 days in 2015.

"The issues facing our workforce around vacation culture, while alarming, also present clear opportunities and solutions. Americans are using more vacation, and the positive change can continue if American workers—particularly senior leaders—prioritize conversation, planning, and modeling of good vacation behavior," said Project: Time Off senior director and report author Katie Denis.

Despite this increase in usage, which provided an estimated $37 billion total impact to the U.S. economy, more than half (54 percent) of employees still ended the year with unused time.

In 2016, 662 million vacation days were left on the table, four million days more than 2015, according to the report. These forfeited days cost an estimated $66.4 billion in lost benefits, or $604 per employee.

"We need to put to rest the fallacy that 'work ethic' is equivalent to work martyrdom," said Cait DeBaun, Project: Time Off director of communications. "Not only are employees not getting ahead by sacrificing time off, these habits may also be harming their careers."

Project: Time Off estimates potential to create 1.8 million jobs and generate $70 billion in additional income for American workers if unused vacation time was not forfeited.

The reason for this increase, according to the report, is that more than half, or 54 percent, of American workers now are planning out their vacation day usage. Workers who set aside their time are more likely to use a majority of it, according to the report.

In addition, 75 percent of those who plan their vacations ahead of time tend to take a week off or more. Workers who do not plan their time off take an average of one to three days.

Project: Time Off's State of American Vacation 2017 report is based on data from 7,331 U.S. employees who earn time off as well as economic analysis conducted by Oxford Economics.