Review the item:
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 comma between motorcycle and he is completely correct. When Robert is riding his motorcycle is a subordinate clause, so it cannot stand alone. He drives recklessly is a main clause.
When a subordinate clause introduces a main clause, you need a comma between them.