Nearly one in eight U.S. drivers was uninsured in 2015, putting insured drivers at greater risk in the event of an auto accident, according to a new study.

The study, directed by the Insurance Research Council (IRC) and co-sponsored by The Hanover Insurance Group, found that 13 percent of all U.S. motorists were uninsured in 2015, up from 12.3 percent in 2010, following a seven-year decline.

When an uninsured driver is at fault in an accident, insured drivers, or their insurance companies, often are left to pay for the resulting physical damage and health costs. Similarly, an underinsured driver may not have high enough limits on his or her policy to cover all costs of damage caused.

“The results of the survey sound an alarm,” said Daniel Halsey, president, personal lines, at The Hanover. “Uninsured motorists represent a significant risk to insured drivers. With the average cost of an uninsured motorist claim around $20,000, excluding any physical damage to the vehicle, the best approach is to make sure you have the proper insurance in place to protect yourself in the event of an accident.”

Drivers in 49 states are required to carry car insurance. Despite this, some drivers choose to violate the laws and drive without coverage.

The numbers of uninsured motorists varied between states, ranging anywhere from 4.5 percent to 26.7 percent, according to the IRC. Florida, Mississippi, New Mexico, Michigan and Tennessee were the top five states with uninsured motorists, while North Carolina, Massachusetts, New York and Maine had the lowest rates. While Massachusetts had one of the lowest rates, it experienced the largest percentage-point increase over a 10-year period.

“While some states saw significant drops in their uninsured motorists rates, overall, the rate is increasing nationwide,” said Elizabeth Sprinkel, senior vice president of the IRC. “This can mean added risk for all motorists.”