NuTonomy, a spinoff from the Massachusetts Institute of Technology, announced Friday that the Singapore public can now book self-driving taxis through an app by Grab, the biggest ride-hailing company in Southeast Asia.

The two companies announced a year-long partnership.

Passengers will have to select the "robo-car" option on Grab's smartphone app, which has been downloaded more than 20 million times. Passengers have to be older than 18, book in advance and sign away any liability over safety. Rides will be free for at least two months.

"We will be combining nuTonomy's self-driving car software with Grab's app, with their proven fleet-routing technology and their mapping capabilities," said nuTonomy CEO Karl Iagnemma.

The trial will be limited to a 6.5-square-kilometer (2.5-square-mile) business and residential district called One North and exclude highways outside the test area.

"If a trip requires travel on roads outside of One North, the safety driver will take control of the vehicle for that portion of the trip," a joint statement by Grab and software company nuTonomy said.

The robo-taxis will be modified Renault Zoe and Mitsubishi i-MiEV electrics - with a human safety driver in front who is prepared to take the wheel and a researcher in back who watches the car's computers.

"It's an evolution to identify where are the easy parts, where are the trickier parts where we need to spend more time," he said. Iagnemma would not say how many rides nuTonomy provided in the trial period but said thousands signed up for the invited trial within the first 48 hours. The company said there had been no problems.

Grab CEO Anthony Tan said that the move would help supplement "underserved areas of Singapore" with an "innovative driverless commuting option."

The company expects its fleet of half a dozen driverless cars to double by the end of the year. It plans to make its entire Singapore taxi fleet self-driving by 2018.

The move comes just over a week after US company Uber started a trial of self-driving cars for travelers in the east coast city Pittsburgh. In the US there have been at least two fatalities involving self-driving cars.

jar/msh (AFP, dpa, AP)