The loud screeching of tires pierced the air at the busy intersection of 17th and Mississippi Streets in San Francisco on the overcast afternoon of February 6th. Pedestrians turned to see a shocking sight – a self-driving Waymo vehicle skidding to an abrupt halt, unable to avoid colliding with a cyclist crossing the intersection.

The cyclist, still visibly shaken when speaking to authorities afterward, described a harrowing scene. He had been cycling along 17th Street behind a large truck also approaching the 4-way intersection. When the truck drove through, the Waymo began moving forward to proceed through the intersection as well. Unbeknownst to the driverless car’s detection systems, the cyclist was following closely behind the truck, occluded from view. As the cyclist emerged and turned left to cross Mississippi Street, the Waymo’s sensors suddenly detected the human and slammed on its brakes – but too late.

The cyclist collided with the front quarter panel of the Waymo vehicle at speed and was thrown from his bicycle to the hard pavement. The deafening squeal of brakes and scattering of debris stunned onlookers lining both sides of the dense urban intersection. The cyclist somehow escaped with only minor scrapes and bruising, but the startling incident has raised familiar concerns about the safety of driverless vehicles on public city streets once more.

While still rare, accidents keep occurring between automated test vehicles and human cyclists, scooter-riders and pedestrians on urban roads. Back in October, a Cruise autonomous vehicle dragged a pedestrian it had collided with 20 feet down a San Francisco street – leading California regulators to suspend the company’s driverless testing permit.

The Waymo collision comes on the heels of renewed calls by transportation unions and advocacy groups for urgent federal investigations and regulation of driverless testing on public roads across America. In a letter sent last November to the Department of Transportation, a coalition of 26 organizations wrote that “automated driving system-equipped vehicle operations are unsafe and untenable in their current form“.

As Silicon Valley powers ahead in the driverless race, regulators struggle to keep pace with ensuring safety. And humans continue to pay the price, as they collide with driverless cars clearly still learning to co-exist on roads not yet ready for our autonomous future.