Mine consistently brakes on the freeway where another road goes off: no signs, no road markings, no traffic. No brains....
One common and consistent cause of undesired sudden braking seems related to the Autopilot Speed Limit "relative" setting. Ours is set to Relative = +6 mph, reasonable under most conditions. When engaged, cruise control immediate accelerates to 6 mph over the speed limit (unless slower traffic ahead). But traffic flow on many freeways is 10-15 mph over the posted limit at off-peak times. So when cruise control is engaged the car initially accelerates to 71 mph (in a 65 zone), and must be manually bumped up if a higher cruising speed is desired.
"Phantom" or undesired braking can occur when transitioning to a new freeway, even where the speed limits for both freeways are the same, if the cruise speed was set to something other than posted + relative (65 + 6 = 71). Apparently Auto Pilot sees it as a "new" speed limit, and resets cruise speed to Speed Limit + Relative. If cruising faster, the car brakes aggressively down to 71 mph. It happens so consistently in some spots that I've learned to expect it and just bump the speed back up to the desired setting. But it's certainly annoying, both to me and any drivers behind.
I haven't figured out a good way around this. I could set Auto Pilot Speed Limit / Relative higher, maybe to +15. When engaged in light traffic, the vehicle would accelerate to 80 in a 65 zone. But it would also accelerate to 55 in a 40 mph zone, which is almost certainly undesirable. I suspect it would also ACCELERATE to 80 mph if cruising at less than 80 mph and transitioning to a new freeway. Anyone know for sure?
There's apparently no way to disable the Speed Limit feature; the only choices are Relative or Absolute. Neither is good in all cases, but Relative seems generally better. Maybe have 2 Speed Limit settings, one for freeways and another for surface streets. OR, allow a relative percent or "factor" instead of just mph/kph. A relative factor of 1.15 would accelerate the car to 75 in a 65 zone, but only to 46 in a 40 mph zone. Still not perfect, but better, although people with social science degrees would have to do math, and the vehicle would still accelerate or decelerate when the speed limit changes (good) or when transitioning to another freeway/autobahn, etc (usually not good). This may be one of those problems to which there is no good solution.
And while I do observe THIS phenomena predictably, it's clearly not the only cause of phantom braking, just the most consistent.