A Course Following Action can only be performed at an Intersection
An example of a trap based on this concept would be arriving at what looks like a T intersection with Rally Rd. However, the road to the left is marked "Dead End." Based on the definitions, the road to the left does not exist. So there is only one way to leave the intersection - the road to the right. This point ion the orad is no longer an intersection as defined. You could not execute the instruction "Right at Rally Road" at this location. You would have to proceed to the next side road right that exists and execute the instruction there.
Which leads to the next concept....