Safety distance of approach (SDA)
The fundamental measure of ships passing is distance to closest point of approach (DCPA). Its value should be safety, it means:
DCPA ≥ DCPA min
DCPA = distance to closest point of approach
DCPA min = acceptable distance to closest point of approach.
The accident can happen; when above condition will not be performance. Knowing the number of entries of ships in a year (annual intensity of traffic), one can determine the probability of ships collision for one ship transit.