Metric spaces in which many triangles are degenerate

Richmond and Richmond (1997) proved the following theorem: If, in a metric space with at least five points, all triangles are degenerate, then the space is isometric to a subset of the real line. We prove that the hypothesis is unnecessarily strong: In a metric space on n points, fewer than 7n2/6 suitably placed degenerate triangles suffice. However, fewer than n(n−1)/2 degenerate triangles, no matter how cleverly placed, never suffice.