Bandwidth doesn't vary much with speed (since people tend to maintain the same 2 second gap from the car in front of them), except at low speeds where the nonzero size of each car becomes significant.
Anecdotally, it seems like I see more wild driving when posted speeds are higher. People are more aggressive in higher speed zones, which inevitably leads to more crashes. Severe accidents being one of those things which can dramatically hurt traffic throughput.
