When you drive the car at night there is an analogous situation that unfolds. When the traffic is less and the speeds are relatively low even a dim tail light will suffice and other drivers on the road will know of the presence of your car. But if the visibility is not good say due to a fog and the traffic is fast as on an expressway, then suddenly you need bright tail lights because the dim ones may go unnoticed. At high speeds when the visibility is not good you do not see much of the car ahead of you at all. All you will be able to see are the tail lights.