This is a fascinating exercise with a disappointing argument and pre-determined conclusion.

The writer notes that higher speed increase likelihood of an injury in the case of an accident. There is no comment on the factors that influence the likelihood of an accident and how these change with speed. By this logic, every road should have a 20mph limit - clearly this would do more harm than good overall.

A better analysis would take into account the utility of all relevant parties (including drivers). This would mean superior solutions such as those that maintain car speed as well as improve cyclist/ped safety could be considered rather than simply constructing an argument to fit one solution the writer has proposed without any evidence that it would even work.