Here’s a patent for a blind spot detection system. It says, “A BSD warning should be triggered when a stagnating vehicle is detected, but should not be triggered by a stationary object, e.g. a guardrail, a concrete barrier, or a utility pole. Failure to distinguish between these two limits the effectiveness of the BSD system and leads a driver to mistrust warnings produced by the system.” So I think my guess was correct, in that the system is designed to ignore things like concrete barriers.
As to how this occurs, the patent later says, “Some stationary objects that are commonly encountered on roadways, such as guard rails and concrete barriers, are continuous structures and, as such, the objects will remain in the blind spot for a longer period of time. In such situations, the system may not be able to differentiate between a continuous stationary object and a vehicle stagnating in the blind spot of the host vehicle. For example, in the situation illustrated in FIG. 3, the right blind spot sensor 105 detects the presence of an object 303. The vehicle 303 is operating in the blind spot of the host vehicle 301 at nearly the same speed as the host vehicle 301 (i.e., stagnating). Because the speed of the vehicle 303 relative to the host vehicle is zero, the right blind spot detection sensor 105 cannot distinguish if it is a continuous stationary object (like a guard rail) or a stagnating vehicle based on the method of FIG. 4.” So it sounds like the system ignores continuous stationary objects like guardrails parallel to the car.
