So by "normal", I meant the radar-based systems in normal cars, also known as AEB. They are usually mounted low on the front fenders or grille. Semi truck trailers are tall, have a huge empty gap between them and the road, and just high enough to not be seen by the radar. From the radars POV it appears like the road is unobstructed.

Obviously this isn't hard to detect with full LIDAR systems in self driving cars because they are on the roof and you have full depth information. This is why I do not mention "self driving cars" in my parent comment.