General Motors Co's self-driving unit, Cruise Automation, has more than doubled the size of its test fleet of robot cars in California during the past three months, a GM spokesman said on Wednesday.

As the company increases the size of its test fleet, it has also reported more run-ins between its self-driving cars and human-operated vehicles and bicycles, telling California regulators its vehicles were involved in six minor crashes in the state in September.

"All our incidents this year were caused by the other vehicle," said Rebecca Mark, spokeswoman for GM Cruise. In the past three months, the Cruise unit has increased the number of vehicles registered for testing on California streets to 100 from the previous 30 to 40, GM spokesman Ray Wert said.

Cruise is testing vehicles in San Francisco as part of its effort to develop software capable of navigating congested and often chaotic urban environments. Investors are watching GM's progress closely, and the automaker's shares have risen 17 percent during the past month as some analysts have said the company could deploy robot taxis within the next year or two.