On Thanksgiving (Nov 24) an 8-car pileup occurred on the San Francisco to Oakland Bay Bridge. Nobody was seriously injured, but interest was raised because it was all started by a driver in a 2021 Tesla Model S. The driver, a 76 year-old San Francisco lawyer, told police he was using Tesla’s “Full Self Driving” mode and it malfunctioned, changing lanes and hitting the brakes hard in front of a line of cars. While the car would have actually been in “Autopilot” (a different system) this crash opens up some surprisingly interesting questions about how driver supervision of “pilot” style driver-assist systems should work, and who is at fault.

Tesla has two different systems which can take control of the car. The original one, “Autopilot,” is included with cars and drives highways while drivers keep their hands on the wheel and eyes on the road, ready to intervene if the system does anything wrong. This is known as advanced “driver assist” or ADAS. Owners can also buy an upgrade to this called “Navigate on Autopilot” which adds the ability to do automatic lane changes, among a few other features.

For even more money, some Tesla owners have pre-bought Tesla’s eventual “full self driving” system, a product which is not yet ready, though Tesla lets customers try out the early prototype in what they (very incorrectly) refer to as a “beta” test. While the system, when eventually delivered, promises right in the name to be an actual self-driving system, the prototype is not that, and needs very diligent monitoring with regular intervention by drivers. It works on city streets, and does not operate on highways. If you turn it on, and it enters a highway, it switches to the older Autopilot system automatically, which might have confused the driver in thinking that the FSD system was engaged, though the switch is quite obvious on the display screen.

Both systems require driver supervision and can’t be bought or turned on without a lot of warnings about that, though as we know, many people — even lawyers like this driver — click to agree to warnings and then get lax. To reduce that, Tesla requires drivers apply regular force to the wheel, and recently started watching their heads with a camera to assure they keep eyes on the road. Sadly, there have been a number of crashes with Autopilot where drivers didn’t pay attention to the road, with some fatal results. Some drivers deliberately don’t pay attention, and a few even try to defeat the system that nags you if you don’t touch the wheel. With the FSD early prototype, drivers actually pay more attention because it makes mistakes so often you would crash almost every time you went out if you did not, as you would with a basic cruise control if you turned it on and didn’t watch.

(Elon Musk recently announced that they will soon allow drivers with lots of experience supervising the FSD prototype to only be monitored for attention on the road, no longer getting nagged for not keeping hands on the wheel. Many competing systems do not require hands on the wheel and only watch the eyes.)

According to the driver, he had activated FSD and the car was moving on the lower deck of the Bay Bridge leaving SF. It sought the left lane, something it does if the driver has asked it to drive well above the speed limit — drivers can configure this behavior. If it finds the lane it is in is too slow, it will automatically change lanes to the left. If it’s too fast, it will move right. It also moves to take upcoming exits. Drivers are notified of lane changes and are told to check the road and blindspots when it signals one, and to abort the lane change if it’s unsafe. The system, however, is fairly good at checking the blindspots — or at least often is good enough — and so some drivers get lax at this.

The Tesla selected the 2nd from left lane of the 5-lane bridge before entering a tunnel on Yerba Buena Island after crossing the first span out of SF. The driver claims he (or the car) was driving 55mph, slightly above the 50mph limit but hardly unusual — in fact it’s slow for the left hand lanes.

Then the car did something quite odd. It signaled to change lanes to the left, into the fastest lane. According to the driver, it did this, but then suddenly braked for no apparent reason. (The police report, which includes examination of the video, says the car braked before changing lanes, which appears correct.) The car behind, who was cut off, slams on the brakes and apparently hits the Tesla, though they claim they did not. Same for the next car, but other cars behind, going faster, do hit, and push other cars forward until everybody gets hit, in one case car #6 gets pushed up on top of car #7 when