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?