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
Chain rule for short, The derivative function is therefore: Another way of computing this derivative is to view the composite function as the composite of and "h". Applying the chain rule to this situation gives: This is the same as what was computed above. This should be expected because . Sometimes it is necessary to differentiate an arbitrarily long composition of the form formula_23. In this case, define where formula_25 and formula_26 when formula_27. Then the chain rule takes the form or, in the Lagrange notation, The chain rule can be used to derive some well-known differentiation rules. For example, the quotient