Two-second rule two seconds as inadequate, and recommend a three-second rule. German law requires a minimum 0.9 second distance but when tested under relaxed conditions researchers found that their test subjects spent 41% of the test time at following distances under 0.9 second. The United States National Safety Council suggests that a three-second rule—with increases of one second per factor of driving difficulty—is more appropriate. Factors that make driving more difficult include poor lighting conditions (dawn and dusk are the most common); inclement weather (ice, rain, snow, fog, etc.), adverse traffic mix (heavy vehicles, slow vehicles, impaired drivers, pedestrians, bicyclists, etc.), and
The Bruun Rule tests of the theory and concluded that "the concept henceforth be known as "Bruun's Rule."""" Per Bruun has since published works to clarify the limitations and practicality of his model, including "Review of Conditions for Uses of the Bruun Rule of Erosion" in 1983, and "The Bruun Rule of Erosion by Sea-Level Rise: A Discussion on Large-Scale Two- and Three- Dimensional Usages" in 1988. These publications outline the rule's limitations and practicality and stress that, though this model is intentionally two dimensional, it is applied to three dimensional environments in practice. Bruun states that the rule "must be subjected to