Apologies for wandering off topic, but is this correct - this definition of a structure wasn't covered in the Air Law I did recently
The definition was established in the high court when the CAA prosecuted the kind and very popular instructor/examiner Eric Thurston, flying from Stapleford Tawney He was found not guilty. Eric was conducting a PPL Skill test. There is no definition in the ANO on what constitutes a 'structure'.
The judge however decided to define, although not central to the case, that which he considered too vague, and to be of assistance for the future. A road is a structure there for the purpose of conveying people and therefore it is not unexpected to find people and vehicles, so no defence. A waterway in some cases is the same. If you fly within 500ft of a person or vessel in a busy estuary or river when it is known to be used by people, such as a pleasure boat even at the last minute then there can be no defence. If I remember it correctly he also clarified: should the flight be crossing a field of crops then it could be a defence, should a romantic young couple pop up unexpectedly from amongst the crop. It is reasonable to expect the field of crop to be normally free of people. He did not consider that the fencing surrounding a field counted as a structure for this purpose.