Safety is of paramount importance, but the guidelines are also useful to establish some minimums for trips. For example, a trip intended for advanced paddlers should not be planned for a pace of two knots. This can create boring conditions. Obviously the condition with the highest rating should generally determine the rating of the trip; however, the boundaries between ratings are not rigid. For example, if you are planning a trip on a calm, sheltered lake in late August, at a gentle pace, but you want to go 16 nautical miles, there is no reason the trip should not be classified as an intermediate trip. It would be a good idea to let people know that the trip is a little long for the classification, but there is no need to dissuade intermediate paddlers from participating.
The most important safety tool in kayaking is good judgment. The most critical criterion for safety is wave height, and it is the one that is the least predictable. It is never possible to be certain that conditions will not be worse than the guidelines for a rating level. You have to deal with probability. If there is a significant chance that the conditions will be worse than the guidelines, or if there is a slight possibility that the conditions will be a lot worse than the guidelines, you should upgrade the rating. In most cases it is the waves that pose the threat, not the wind. This is the reason that wave height is used in these guidelines rather than wind speed.
Exposure to the wind, fetch for waves to build, time of year and time of day are all things that can affect the potential for large wind-generated waves and are things to be considered in rating a trip. As the day of the trip approaches, weather forecasts begin to have pertinence to the probability that conditions will be within the limits for the classification of the trip. The day before and the day of the trip, weather buoy data becomes also becomes pertinent. At this point if the probability that wave conditions will exceed the rating of the trip, the rating should be upgraded or the trip relocated. A trip can be relocated to a place where sheltering conditions compensate for stronger winds.
Another thing about waves to consider is their steepness. A breaking one-foot wave is a greater threat than an eight foot swell. Swells with long periods gently lift you up and down without any tendency to tip you over. Of course the energy in these swells can become dangerous if you paddle into shallow water where the waves slow down and build up. For the wave height criterion in these guidelines, it is assumed that the waves are steep.