There's quite a large discrepancy between this image and OPs image. This image says the survival rate of 30 mph (48 kph) is 60%, while OP's image says 50 kph it's at 20%. I wish they included a source for the data that could explain it.

This is the same issue I take with braking distance scales. They often vary wildly, and some don't even follow a quadratic increase in distance like you'd expect.