Does "They are driving with dangerous speed" sound unnatural?

Does the proposed answer "They are going at a dangerous speed" also include the process of finally reaching that speed / crossing some limit?

I think putting off an indefinite artice before "speed" should be accepted. It can be uncountable. You can see an example here: https://dictionary.cambridge.org/dictionary/english/speed

I have got this correct for thee time now and it is always reject as incorrect. Why?