Here is the item again:
When Robert is riding his motorcycle, he drives recklessly,
weaving dangerously through rush hour traffic, Robert does not care
that he might leave a kneecap or chunk of thigh speared on a car bumper.
The problem here is a comma splice; two complete sentences are joined with a comma alone. This is an illegal move in writing. He drives recklessly is one sentence. Weaving dangerously through rush hour traffic, Robert does not care ... begins the next sentence.
To fix this problem, you could put a period after recklessly and capitalize the W for weaving.