Waymo, Alphabet's (GOOGL.US) self-driving unit, said a driverless Waymo car collided with a cyclist in San Francisco on Tuesday, but the cyclist suffered only minor scratches and left the scene on his own. Waymo said it had called police to the scene and contacted relevant authorities regarding the incident.

Waymo said its self-driving vehicle came to a complete stop at a four-way intersection when a large truck entered the intersection. When it was Waymo's car's turn to move on, it moved forward.

However, the cyclist was behind the truck and was not visible to the cameras as they crossed the path of the Waymo vehicle. When the rider was fully visible, Waymo's vehicle braked heavily but was still unable to avoid the collision, the company said.

A spokesperson for the San Francisco Fire Department said via email that a 911 call was made to report the collision and the rider was not taken to the hospital. The San Francisco Police Department is investigating the incident, they said.

Waymo has been looking to expand its self-driving service in Los Angeles, where it is currently testing self-driving services and allowing invite-only rides in new vehicles.

But self-driving car companies have faced resistance from some lawmakers and citizens who worry that the vehicles are unproven and pose safety risks.

Last month, GM's Cruise self-driving car unit disclosed investigations by the U.S. Department of Justice and Securities and Exchange Commission after one of its self-driving taxis struck a pedestrian and dragged her 20 feet.

Last year, more than two dozen unions urged U.S. auto safety regulators to launch an industry-wide investigation into driverless cars, arguing that they were "unsafe and untenable in their current form" due to the large number of crashes.