Based on a TED talk by someone from Google[1]. I think having the car apply the brakes when there’s a possible disturbance causes the car to apply the breaks too much and makes for a really uncomfortable ride.

I think a major effort in self driving is solving the Goldilocks issue of reacting properly to impending accidents, but also not apply uncomfortable breaking if it’s not needed.

Seems like it was too insensitive at that distance.

[1] https://www.ted.com/talks/chris_urmson_how_a_driverless_car_...