Two Self-Driving Waymo Taxis Get Confused By A Pickup Being Towed Backwards, Crash Into It

The idea of a robotaxi is quite appealing. It’s a car that takes you where you want to go, and neither you, nor anybody else, has to worry about driving. The reality of robotaxis is altogether different. Many of us are concerned about systems that are incapable of dealing with the whole gamut of often-chaotic road conditions. Waymo’s recent escapades certainly don’t help in that regard.

Titled “Voluntary recall of our previous software,” Waymo’s Chief Safety Officer Mauricio Peña’s new entry on the company’s blog explains a recall report the company filed with the National Highway Traffic Safety Administration (NHTSA). The filing was made in response to a hilarious and embarrassing incident on December 11, 2023 involving two of Waymo’s self-driving robotaxis.

According to Waymo, one of its robotaxis was operating in the city of Phoenix when it came across a pickup truck facing backwards on the road. The company alleges the vehicle was being “improperly towed” and that “the pickup truck was persistently angled across a center turn lane and a traffic lane.” When the Waymo robotaxi hit the pickup under tow, the tow truck driver didn’t stop after the collision, and continued traveling down the road. Mere minutes later, a second Waymo vehicle hit the same pickup truck under tow, at which point the tow truck driver elected to stop. Here’s Waymo’s full description of events:

On December 11, 2023 in Phoenix, a Waymo vehicle made contact with a backwards-facing pickup truck being improperly towed ahead of the Waymo vehicle such that the pickup truck was persistently angled across a center turn lane and a traffic lane. Following contact, the tow truck and towed pickup truck did not pull over or stop traveling, and a few minutes later another Waymo vehicle made contact with the same pickup truck while it was being towed in the same manner. Neither Waymo vehicle was transporting riders at the time, and this unusual scenario resulted in no injuries and minor vehicle damage.

Just imagine, you’re driving your truck with a pickup in tow behind you, and you feel a little something from behind. You look in the mirror and spot a Waymo vehicle, but assume you maybe just imagined the jolt. You get back to driving down the road, only for another Waymo to show up and again hit your consist from behind. You’d start to think these robot taxis were out to get you or something.

As covered by TechCrunch, both crashes caused only minor damage to bumpers and a sensor. The crashes were reported to police the same day, and the NHTSA on December 15. There were no reported injuries as a result of the crashes, and neither Waymo vehicle was carrying passengers at the time. Waymo put the problem down to the strange towing configuration, which confused its autonomous vehicle software. It apparently could not accurately understand or predict the motion of the tow truck or the pickup behind it, which led to the crash.

Here’s the company’s explanation of why these Waymos crashed into the truck, per the aforementioned blog entry:

Given our commitment to safety, our team went to work immediately to understand what happened. We determined that due to the persistent orientation mismatch of the towed pickup truck and tow truck combination, the Waymo AV incorrectly predicted the future motion of the towed vehicle. After developing, rigorously testing, and validating a fix, on December 20, 2023 we began deploying a software update to our fleet to address this issue (more here on how we rapidly and regularly enhance the Waymo Driver’s capabilities through software updates).

[Editor’s Note: The fundamental problem here is one I’ve discussed before: automated vehicles have no idea what they’re actually doing. They’re computers, following sets of instructions and reacting to sensor/camera inputs, but they, of course, lack any sort of consciousness or understanding of what they’re doing. And that sort of general understanding of the world around you is actually quite important to the task of driving, and it’s something that we humans do without even thinking about it. 

Looking at the description of events, it seems that it’s just a case of a truck being towed backwards. No human driver would have been confused by this; anyone capable of operating a car would understand that they were looking at a car being towed, and would understand how that affected the motion of the car. This isn’t because humans were calculating the physics of towed vehicle masses or whatever, it’s because we’ve all seen towed cars before and get how that works. 

We can call them “edge cases,” but really they’re just cases. Things like this happen every single day on the roads, and humans deal with them wonderfully, because we have an understanding of the world and what we’re doing in it. Can AVs be programmed to have some sort of conceptual model of the world that will allow them to make sense of potentially confusing situations like this one, with the backwards-towed truck? I’m not sure. But it’s not a concept we can ignore. – JT]

Waymo implemented a software fix for the problem, rolling it out on December 20 last year. The full fleet had received the update by January 12.  “Our ride-hailing service is not and has not been interrupted by this update,” reads Waymo’s blog entry.

It’s an interesting decision to make in the context of the accident. On the one hand, nobody was hurt in the twin incidents, and damage was minor. Plus, if Waymo’s account is accurate, it was an oddball situation which they might not reasonably expect to see again any time soon. At the same time, when two cars crash in the same way just minutes apart, you might consider shutting things down until a fix is out.

Based on conversations with the NHTSA, Waymo decided to file a voluntary recall report over the matter. However, this terminology is somewhat confusing, as Waymo didn’t really recall anything. It simply updated the software on its own vehicles over a period of a few weeks. Instead, the report really serves as a public notification that Waymo made a software change in response to the incident.

It’s true that Waymo hasn’t seen quite as much bad press as Cruise. The latter GM-aligned company has had to contend with one of its autonomous vehicles dragging a stricken pedestrian along the road for 20 feet. But it’s faced its own woes. A Waymo vehicle recently hit a cyclist, prompting investigation by California regulators. Worse, the company saw a mob go wild and destroy one of its vehicles in Chinatown just a few days ago.

But ultimately, all these robotaxi operations will need to sharpen up their act. Crashes like this one are the sort of thing that even a poor human driver can avoid. An inexperienced driver is generally smart enough not to drive into a pickup truck dangling from a tow hook, nor would they drag a pedestrian along the road after running them over.

The problem for these companies is that it’s not the regular driving task that’s hard to master. It’s challenging, sure. But the real problem is the strange edge cases that humans deal with every day. You can’t expect every road hazard to have a big flashing orange light or a stop sign, but you have to be able to deal with them anyway.

Waymo has actually published papers on this very topic, including one entitled Measuring Surprise In The Wild. It discusses methods for using machine learning models to “detect surprising human behavior in complex, dynamic environments like road traffic.” An ability to appropriately handle novel situations in traffic would be a major boon to any autonomous driving system. In comparison, the only other solution is for companies like Waymo to imagine every conceivable situation ever and provide appropriate countermeasures for such. Obviously, a more general ability to handle surprise is more desirable.

It’s clear from this incident that Waymo isn’t quite there yet; it’s just learned from one more strange situation and stuck that in the training files. Here’s hoping the robotaxis don’t start ganging up on broken BMWs or rusty Jeeps, lest The Autopian staff shortly end up in the firing line.

Image credits: Waymo