Distance of W 20 from Warrington can be 19.5–20.5. "Just over half a mile further on" (call it 0.6 miles), that will have come down to 18.9–19.9. But the sign says 19, so in fact it must be 18.9–19.5. Half a mile further on, that will have come down to 18.4–19.0. But the sign says 18, so in fact it must be 18.4–18.5.