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
Littlewood's rule rejected. If a continuous distribution formula_14 is used to model the demand, then formula_15 can be calculated using what is called "Littlewood’s rule": This gives the optimal protection limit, in terms of the division of the marginal revenue of both classes. Alternatively bid prices can be calculated via Littlewood's model is limited to two classes. P. Belobaba developed a model based on this rule called Expected marginal seat revenue, abbreviated as EMSR, which is an formula_17-class model Littlewood's rule The earliest Revenue Management model is known as "Littlewood’s rule", developed by Ken Littlewood while working at British Overseas Airways Corporation.