The Safety Cushion
A safety cushion can be determined very quickly and easily by using the time interval method. The time interval method is a formula for determining the distance between your vehicle and other traffic ahead. It is superior to other systems because it is self-adjusting for vehicle speed. To use it select an object ahead - an overpass, a traffic sign, or even a crack in the road.
When the vehicle you are following passes this object begin timing yourself by counting "one thousand and one, one thousand and two", until you pass the same object.
Under ideal daylight driving conditions, never follow any closer than two seconds. Driving during less-than-ideal conditions will require an increase in this time creating a larger safety cushion. Extra attention and caution must be exercised if light, weather, road, traffic, vehicle or driver conditions are adverse.
Special Situations Loading
Whenever a vehicle is towing an object, like a trailer, add 1 second following distance for every three metres (10ft) of additional length.
Traction and Visibility
When driving conditions become adverse, increase the following distance. rain and fog can make roads slippery and reduce your ability to stop. so add more seconds to the original following distance, giving yourself enough time to react to other hightway users. One of the most dangerous times to drive is when ice is at 0 degrees C (32 degrees F). For this condition keep especially alert and maintain a long following distance, as much as 12 seconds, behind the vehicle ahead. Or better yet, avoid driving altogether. While you may be able to handle your vehicle, allowing for other drivers' errors becomes next to impossible.
Slow down, make it easy for any tailgater to pass. If the driver does not pass, at least you have created a safer following distance for yourself.
You may become increasingly tired when driving long distances. This slows perception and reaction time. In this case increase the following distance from 4 to 6 seconds. And remember, pull off the highway and stop for a rest break every hour.