With Black Friday looming, everybody’s looking for a deal. But slashed prices for flat screens, video game consoles and cashmere mittens can’t compare to the best way to save dough over the holidays: keeping your car in park.

It’s not just that you’ll have to limit your shopping haul to what you’re willing to bring back with you on the Metro, or that you’ll pocket a sizable chunk of change by not having to fill up your tank or pay for parking tickets. It’s also that you’ll be contributing to the societal savings that come from pumping less exhaust into the air and from shrinking countless waistlines.

A study published this month in Environmental Health Perspectives projected that if we could get just the people in the Upper Midwestern U.S. to bike rather than drive for half of their trips that are five miles or less, that would be worth $7 billion in reduced health care costs and improved air quality.

Add in walking and you’re looking at even more value. By my definition, that includes the hoofing it you do on either end of your transit trips. Last week, I’d forgotten I’d thrown a pedometer in my purse. When I fished it out, I was amazed to find that just carrying it to the train between my apartment and my office (and back), I’d clocked about 5,000 steps — which is, depressingly, more than the national average of steps taken all day.

A couple miles isn’t much, but multiply that by the millions of other riders getting a move on and you wind up with massive sums.

So maybe that’s why the figures quoted in recent transportation news stories haven’t fazed me. Sure, it’s slated to cost $103 million to rebuild the Capital Crescent Trail to make way for the Purple Line, a $1.93 billion project. The Silver Line out to Dulles has a current price tag of $2.8 billion. And the proposal to add streetcars to Arlington and Fairfax estimates the initial costs to be in the neighborhood of $160 million.

That’s a lot of zeros. But when we look at those costs, we have to factor in the huge economic benefits that come from creating walkable neighborhoods, including more foot traffic for businesses and easier access to employment opportunities. There’s also the not-so-insignificant issue of increased property values.

These projects are far from free, but when you crunch the numbers, they could wind up being a bargain. And the best part of all? They don’t require waiting in line at Best Buy at midnight.