Walk outside and give the treadmill a break

One thing that I've noticed at the gym is how, on perfectly beautiful days, people are indoors getting their cardio on a treadmill.  Now, I understand the convenience, etc., of a treadmill but why not walk outdoors instead? According to The Green Book, even a 25% shift in time on the treadmill to time outside would conserve about 60 kilowatt-hours of energy per year.  If an additional 10% of Americans who walk as their number one form of exercise  did so outside, it would save as much energy as it would take to drive a car 40,000 miles! That's just over three years worth of driving mileage for most people who own a car.  Incredible! Source: The Green Book