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Keep your driveway safe and clear

Property owners and retail businesses know that keeping driveways free from parked cars and warning vehicles that someone might be pulling out make for safe passage from point A to point B. Signage goes a long way toward making sure everyone gets where they’re going without a hitch.

Parking and tragedies of the commons

What do free parking and public grazing land have in common? They're both examples of a commons – of shared resources that individuals might be tempted to abuse. Whenever the public shares a valuable resource, and using it doesn't cost an individual anything, people will be tempted to exploit that resource as much as they can, leading to a tragedy of the commons – a situation where runaway consumption leads to total collapse.

Imagine a town has an area where anyone can take their cattle to graze for free. Since the villagers aren't paying to use that land, there's nothing to stop anyone from letting their livestock chew up more and more grass – but when too many people take advantage, their livestock may kill the grass and make the field useless for everyone. The problem is, even if one kindly farmer opts to limit how much their livestock grazes on that field for the common good, there's no reason to expect that the other farmers will do the same thing, since once he's gone, they'll just benefit from even more aggressive grazing, creating what economists call a free rider problem.

The Tragedy of the Commons

Philosophers and economists have been arguing about tragedies of the commons like overgrazing, overfishing and parking scarcity for centuries.
Like grazing space, free parking is susceptible to tragedies of the commons. If parking is free, even if there are alternatives, people will make heavier use of their cars until there isn't any more parking left, and everyone just drives in circles waiting for a space. To make matters worse, if my neighbor chooses not to buy a car and we're competing for parking spots, since she's put herself out of contention for that juicy spot right in front of my house, I actually have more reason to go out buy a car – nullifying her civic-minded act. So even doing the right thing can result in undesired results!

The 17th-century political philosopher Thomas Hobbes remarked in his book Leviathan, "…there must be some coercive power to compel men equally to the performance of their covenants." This must have sounded like wishful thinking in a time when there was little central power to regulate common goods! But by 1968, the landscape had changed considerably, and ecologist Garrett Hardin kicked off the contemporary study of commons with a paper, The Tragedy of the Commons, and used parking as an anecdotal example:

During the Christmas shopping season the parking meters downtown were covered with plastic bags that bore tags reading: "Do not open until after Christmas. Free parking courtesy of the mayor and city council." In other words, facing the prospect of an increased demand for already scarce space.the city fathers reinstituted the system of the commons.

Solutions to tragedies of the commons generally rely on a governmental authority carving up the shared resource and imposing prices that vary according to how heavily we use them. In the case of parking, that means parking meters or time limits, both of which impose costs on people who would drive when they didn't need to if it weren't just a little bit of a pain.

What does this have to do with driveway signs? Everything! First off, when parking supply is free but not plentiful, people are more likely to park in open spots where they shouldn’t – this is called spillover. Secondly, to a point, tiny risks that a driver faces when parking her car – tow-away zones, meters, lanes with instructions she must follow, driveway signs – impose an imperceptible cost on the driver, and these costs can eventually reduce the number of cars on the road.

In the grazing example earlier, imagine farmers knew that every time they fed their flock on the commons, there was a 1 in 1,000 chance that a sheep would wander off – if all else remained equal, they might decide to pay a small amount to graze on a risk-free lot, leading to less overworked commons. We're vastly oversimplifying, but limits on parking (even ones that may annoy drivers a little) support the automobile ecology keep parking sustainable for everyone.