Here is the sentence again:
When Robert is riding his Honda motorcycle, he drives
recklessly, weaving dangerously through
rush hour traffic, Robert doesn't 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 a main clause. Weaving dangerously through rush hour traffic, Robert doesn't care ... begins the next sentence.
To fix this problem, you could put a period after recklessly and capitalize the W for weaving.