What is the lowest speed crash for you to get whiplash?

Varies: That depends alot on the physics involved including are you large or small, restrained or unrestrained, what position your head & neck is in, what size vehicle you are in & what size vehicle hits you, are the vehicles at the same height at the time of impact, is your vehicle moving or stopped, are either of the vehicles designed to absorb the energy of the impact rather than transmitting it

Unclear: I have seen neck injury from low speed accidents, about 5-7 mph. There are too many variables to sort out, such as the restraint devices, size of the vehicles, positioning at time of impact, etc. I think often these issues wind up in courtrooms, not in scientific research.