Question: what shape is the standard stop sign according to the highway code
[{'answer': 'octagon', 'provenance': [{'wikipedia_id': '557643', 'title': 'Stop sign', 'start_paragraph_id': 3, 'start_character': 143, 'end_paragraph_id': 3, 'end_character': 150, 'bleu_score': 1.0, 'section': 'Section::::Specifications.\n'}]}]