Most users of Tesla’s autopilot software have experienced “phantom braking” while driving along the interstate. This often occurs approaching bridges and overpasses.

Conventional Explanations

Conventional wisdom explain phantom braking as ‘precautionary logic’ in Autopilot to avoid hitting a vehicle crossing the road perpendicular to the traveled lane. One of the first Autopilot fatalities occurred in Florida killing its driver Joshua Brown. Autopilot and its driver failed to brake for a tractor trailer crossing a Florida Turnpike. The theory behind phantom braking is that Autopilot confuses an up coming bridge or overpass as a vehicle crossing the lane of traffic so engages emergency brakes.

I have personally experienced this phantom braking. One can anticipate which bridges are most likely to trigger this behavior, if the road dips down just before the bridge, there is a high likelihood phantom braking will occur.

Need for an alternative explanation

One thing that has always puzzled me is that if autopilot has detected what it thinks is a crossing vehicle ahead, why doesn’t it come to a stop or at least a very low speed? The phantom braking is typically very brief and the vehicle may slow by 15-20 miles per hour and then accelerates back to its previous set speed.

Slowing down by 15 Mph on a interstate isn’t emergency braking. It is however annoying as heck, and could prove dangerous if someone is tailgating you.

An Alternative explanation based on speed limits and an overly aggressive braking response to them

Church Street bridge/overpass on I-65 South

Recent experience has lead me to believe that phantom braking can often be explained by a change in speed limits, especially if the change is wildly wrong.

On I-65 south in Nashvile TN just past Charlotte Avenue the road dips down ahead of the Church Street overpass. I have experienced phantom braking several time here. Recently I noticed that during one of the phantom braking events the vehicle displayed a speed limit of 30 instead of 55!! The speed limit reverted to 55 after going under the bridge.

Could it be that due to a GPS error the car thought I had turned onto Church Street, or maybe the ramp to church street and reduced speed suddenly? This would explain why phantom braking is momentary and why the car resumes its set speed a soon as it resets the speed limit to the actual road speed limit.

Autosteer Speed restricted

In a recent video from Bjorn Nyland he interrupts his monologue with the exclamation “Phantom Brake” as his Model 3 brakes suddenly approaching a tunnel. Conventional wisdom would say that this is due to Autopilot mistaking the tunnel approach as a vehicle crossing its path. However if you watch the video closely you notice that the speed limit drops by 10 km/h approaching the tunnel and displays a warning that it has restricted Autosteer speed. Just moments before this warning appears the car display shows that for a brief moment heavy regenerative braking was applied. Unfortunately the video is very blurry, no doubt due to the sudden braking, I also include that image below as well.

Max Regen for just a moment blurs video image

Feel free to watch Bjorn’s video. Advance to 10:12 to see where the Phantom Braking event occurred.

What should Tesla Do?

When speed limits reduce, autopilot should gradually reduce speed for the first second then progressively apply brakes if the speed limit reduction remains true. This would avoid unnecessary aggressive braking on interstates and highways. This can be very disconcerting for drivers and passengers alike.

Tesla should also eliminate speed limits on ramps to avoid GPS position errors triggering an inaccurate speed limit.