PHOENIX — Even on weekends, there’s a constant stream of cars that drive through the downtown Phoenix building where one of Maricopa County’s several secure drop boxes allow voters to return their ballots.

Some people get out of their cars and take selfies as they’re dropping off their ballots. They are among over 1.2 million voters whose ballots have already been counted this year, surpassing the overall number of early ballots cast in the 2016 general election.

Voters in this battleground county and state, where a tight Senate race could flip to Democrats, have efficiently — and on a huge scale — used mail voting for years. Roughly 80% of Arizona voters cast their ballots early by mail or by dropping them off in secure boxes.

In some ways, Arizona, like the few other states with long traditions of secure mail voting, was primed for a successful election in the year of the coronavirus, when some voters are afraid of casting ballots in person.

By Wednesday, Maricopa County had processed over 1.2 million ballots, most of which were mail ballots that can be submitted via postal service or dropped off at official locations. The county says 80,000 people have cast early ballots in person. The county began tabulation last week.

Alex Gulotta, Arizona state director for All Voting is Local, a national voting rights group, said this intense of a turnout is good news for Election Day because officials can start processing those early ballots before Nov. 3.

“Elections officials have been doing their part, voters are doing their part, and therefore we’ve started on a path that will have a smooth election. We just need to keep doing it,” Mr. Gulotta said.

Susan Levy, of Phoenix, took a picture while dropping off her ballot at a downtown drop box in mid-October. Ms. Levy is a longtime mail voter but decided to drop off her ballot instead of mailing it because she didn’t want to risk delays, she said.

“I wanted to make sure it was counted,” Ms. Levy said.

Brittany McPherson and John Bain, a married couple, went on a date at a park, where they filled out their ballots together — a tradition — before driving to drop them off at the downtown drop box.

Both have a long history of voting by mail.

“It makes me feel safer, like it’s not gonna get lost in the mail,” Ms. McPherson said.

Although most voters cast their ballots by mail, the county is still expecting to see high Election Day turnout, roughly 200,000 to 300,000 people.

That’ll be a test for the county’s revamped elections department.

Voting rights advocates say a number of multimillion-dollar changes, including new tabulation machines and more staff, have helped improve the Election Day process just four years after a catastrophic presidential preference election, when the county sliced the number of polling locations, resulting in extremely long lines. Voters ousted the longtime elections chief that year.

This year, the county is abandoning the traditional assigned precinct model and plans on opening around 170 vote centers where any voter can cast a ballot on Nov. 3. It’s already opened some centers for early in-person voting and will implement longer and weekend hours at others.