Several states have argued that the 65-mph speed limit has no justification and have refused to enforce it. The federal Department of Transportation (DOT) believes that the 65-mph limit saves lives. To illustrate its contention, the department regressed the number of traffic fatalities last year in a state (Y) on the state’s population (X₁), the number of days of snow cover (X₂), and the average speed of all cars (X₃). It found

Does reducing the average speed of cars have an impact? All other things being equal, how many lives would be saved in a state if the average speed were reduced from 75 to 65 mph?

561 = 1.4 +.00029X = .00003 + 2.4X + 10.3X3 = 1.1 = .62 562 563 y|x R = .78 Adj. R = .78 .78 N = 50 = 36.1