Oregon ranks first for access to voting, according to the Cost of Voting Index created by political scientists at Northern Illinois University, Jacksonville University and China's Wuhan University, Washington Post reports.

This is in part thanks to Oregon's automatic registration through the Department of Motor Vehicles and the state's vote-by-mail system, in which ballots are sent out several weeks before every election. (Most arrived in mailboxes last week.)

Gov. Kate Brown passed the New Motor Voter Act in 2015, which registers Oregonians to vote when they renew their driver's licenses.

The index looked at seven categories of voting laws and how they affected the "time and effort" it took to vote in each state. The categories included voter-registration deadlines, restrictions on registration and registration drives, pre-registration laws for people under 18 attempting to register in time for their first elections, laws that affect ease of voting (such as early and absentee voting), voter ID requirements and polling hours.

Following Oregon, the other top five states included Colorado, California, North Dakota, and Iowa. At the bottom of the list: Virginia, Tennessee, Indiana and Texas, with Mississippi in last place. Mississippi requires photo ID and prohibits early voting and no-excuse absentee voting.

The index also found that in most cases, states with higher voting turnout usually have less restrictive voting laws.

The researchers also recommended same-day registration laws, which Oregon does not have. Same-day registration would allow voters to register at the polling stations on election day instead of weeks before the election, to help level the disparity between states.