When your vision is blocked by poor visibility on road (e.g., fog, heavy rain).
When following or driving next to vehicles that block your view ahead (e.g. trucks, SUVs, buses). You need the extra room to see around the vehicle and to the sides.
On slippery roads. If the car ahead slows or stops, you will need more distance to stop your car.
When it rains or snows. Stopping distances increase on snowy and wet roads.
When following motorcycles. If the motorcycle falls, you'll have to avoid hitting the rider. Motorcycles fall more often on wet or icy roads, bridge gratings, railroad tracks, and on gravel
When the speed increases and the traffic flow is fast. In such cases, you need a space of at least five seconds.
When merging on an expressway.