DETROIT (Reuters) - Self-driving truck startup TuSimple said on Tuesday it raised an additional $120 million from investors to be used to expand long-haul services for truck fleets and jointly develop an autonomous commercial vehicle with truck makers and suppliers.

TuSimple said the new funds were part of a round of fundraising that was oversubscribed and reached a total of $215 million.

The company’s new investors include Chinese alternative investment firm CDH Investments, Hong Kong-based investment firm Lavender Hill Capital and Korean auto supplier Mando Corp. TuSimple said the latest investments bring its total funding so far to $398 million.

That same round of funding included an investment by UPS Ventures, the venture capital arm of United Parcel Service Inc. TuSimple is also conducting road tests for UPS’s supply-chain business on a busy stretch of highway covering a little over 100 miles (160 km) between Phoenix and Tucson.

TuSimple’s other investors include Chinese online media company Sina Corp and U.S. chipmaker Nvidia Corp.

Companies from Silicon Valley tech firms to traditional carmakers are racing to put fully commercial self-driving vehicles on the road. Efforts by robotaxis companies such as General Motors Co unit Cruise and Uber Technologies Inc have stumbled because it is difficult and costly to develop self-driving cars capable of anticipating and responding to humans in urban areas while picking up and dropping off passengers at random locations, at random times.

But self-driving trucks are seen as an easier proposition as most run consistently on predictable, revenue-generating highway routes around the clock, often early in the morning when driving conditions are ideal.

“Our goal is still to have a factory-produced (self-driving) truck by the 2023 time frame,” TuSimple Chief Financial Officer Cheng Lu told Reuters.

Until the company has its own specially developed truck on the road, the startup’s aim is to expand the service it is currently running for 18 truck freight customers in the United States using retrofitted trucks.

TuSimple currently has a driver and engineer in each truck cab on freight runs while its self-driving technology is tested out.

TuSimple also ran a two-week test for the U.S. Postal Service earlier this year transporting mail across three southwestern states.