'Project: Time Off:' 61 percent of workers don't use all of their vacation time for fear of looking replaceable

MILWAUKEE -- Workers in the United States aren't using all of their vacation time, and the reason might be a fear of looking replaceable.

"Project: Time Off," an initiative of the U.S. Travel Association told CNBC Americans used to be better at taking time off.

During the 1980s and 1990s, employees burned through 20 days of vacation per year on average. Currently, it's about 17 days.

Still, more than half of American workers aren't even using all the vacation time they do have, and 61 percent of employees surveyed said it's a fear of looking replaceable that has them showing up at the office every day. Fifty-six percent insist their workload is too heavy.