I suspect the speed is the closing speed between the cyclist and whatever they hit? Reading the article, it is unclear and might even be solely the speed of the car.

Maybe best to try and find the original paper to clarify that one.

EDIT: http://bast.opus.hbz-nrw.de/volltexte/2013/692/pdf/41_Than.p... looks relevant, but I can't see the claims repeated in e.g. the Mirror article.