Walking speed is "almost the perfect measure." This review summarizes the strong psychometric properties of walking speed and robust evidence for using this clinical measurement. Walking speed is easily measurable, clinically interpretable, and a potentially modifiable risk factor. For these reasons, using WS as the sixth vital sign is both pragmatic and essential.
Fritz, S. & Lusardi, M. (2009). White Paper: Walking speed: the sixth vital sign. Journal of Geriatric Physical Therapy, 32(2), 2-5. doi: 10.1519/00139143-200932020-00002