Between 1980 and 2000, the average CO emission factor of the vehicle fleet in Hillsborough County, Florida, dropped by almost half, from about 65 to 34 g/vehicle-mile. However the total miles driven in the county by all vehicles increased by 60 percent during this same time period. Did countywide emissions of CO go up or down, and by how much (Assume N= 0)? Vehicle exhaust is getting cleaner through a combination of engine improvements, emission control technologies, auto re-design, and fuel reformulation, but what other solutions can you think of?