when should you increase your following distance to four seconds?

It is recommended to increase your following distance when conditions are wet, frosty or if you are towing a trailer.

The following distances on the speedometer of most vehicles represent – either by themselves or in combination with other markings on the speedometer – reasonable and prudent driving distances at various speeds under average weather conditions. When harsh winter road conditions exist, you should follow a greater distance for comfort and safety. Don’t forget that drivers departing from slower-moving traffic have an added responsibility of allowing extra room before entering onto a faster roadway so it may be advisable to double these allowances.