Aurora's autonomous trucks are now driving at night. Its next big challenge is rain.

Over the next year, Aurora Innovation CEO Chris Urmson wants to “unlock” the Sunbelt of the United States, a southern route where its self-driving trucks will carry goods for companies like Uber Freight and Hirschbach Motor Lines. Aurora, which launched its driverless commercial self-driving truck service this spring, has already made some progress towards that goal. The company reported Wednesday in its second-quarter letter to shareholders that it now has three self-driving trucks operating commercially between Dallas and Houston and logged more than 20,000 driverless miles by the end of...