B. should be controlled and slowed.
When there is an accident site on a highway, it is important to control and slow down the traffic flow around the area. This is done to ensure the safety of both the accident victims and the emergency responders. By controlling the traffic, it helps prevent further accidents or incidents at the site and allows emergency personnel to effectively respond to the situation. Slowing down the traffic also reduces the risk of collisions and provides a safer environment for everyone involved. Option A, "should be prevented entirely during incident response," is not practical as completely preventing traffic may not be feasible in all situations and could cause significant disruptions. Option C, "is unsafe until a safe zone is created," is partially correct as the area around the accident site should be deemed safe before allowing traffic to pass through, but it does not encompass the entire answer. Option D, "should only be open to rescue vehicles," is too restrictive, as traffic control measures often allow for the passage of essential vehicles and prioritize the safety of all individuals involved.