So you have maps that record the location of stationary signs etc. and then have the car stop if it gets a radar return from a stationary object (e.g. fire truck) that's not on your map? Then when someone puts up a new sign you're slamming on the brakes until the map gets updated.

Not being able to tell if a stationary object is something you're going to hit or not (like LiDAR or stereo cameras might do) is more relevant to this case than maps.