A key concept in modern sociopolitical thought (note 1). describing the phenomenon in nature and human society where what is an advantageous behaviour for an individual in isolation becomes a disaster if it becomes everyone's common behaviour. Probably the simplest example is theft, theft is rational for the individual because s/he profits without expending effort; but in a culture where everyone steals from everyone else people lose the incentive to work and eventually there is nothing left to steal because no-one's doing anything to generate wealth; so everyone is poorer (more 1). A tragedy of the commons is a state of chaotic, degenerate and self defeating competition that degrades an essential resource (in the theft instance the resource is peoples' motivation, in relation to cars the resources are urban life, the economy and the natural environment). Some have called the same phenomenon "the paradox of aggregation"; paradoxical because rational becomes irrational, theft changes from being an action that makes you richer to one that makes you poorer (more 2).
Tragedies of the Commons (TOCs) are the hardest problems humanity has ever had to solve, because of the nature of these problems where "a common behaviour is the main problem". In a TOC all individuals would benefit from the restraint of all other individuals, but would suffer from their own individual choice of restraint if the others were not restrained. Those who unilaterally exercise restraint suffer doubly, they are deprived of the benefit of the resource yet suffer equally the tragedy wrought by other's lack of restraint, and may be wiped out. Hardin called this the "self eliminating" characteristic of conscience.
TOCs threaten survival so one way or another they must be solved, historically one group usually springs up to appropriate the behaviour as their exclusive privilege by force, reducing the destructive propagation in an unjust way (more 3). To solve TOCs humanity evolved the centralized authoritarian state (more 4) ; the only human institution that can effectively solve them; democracy, markets, education and experience are much less effective (more 5) (more 6). The state makes and enforces laws that restrict the amount of the activity that takes place through licencing or quotas, or bans anyone from doing the unbeneficial action by making it a crime. TOC is the basis for the old parental chide "what if everyone did that!". The avoidance of TOCs is the basis for almost all enduring moral coda and all enduring imposed laws (religions have often reaffirmed some of them, as in some of the ten commandments, but the coda are older and have outlasted many religions, mainly because religions pollute the coda with egocentric and often prurient and unnecessary restrictions on behaviour, leading to their ultimate loss of legitimacy), in fact they are the principle reason for the existence of laws and of the state per se, of government with police and courts and officials distinct from "the people" (more 7).
This is the reason this site calls for an enforced statutory ban on the car, and doesn't bother calling for individual restraint. If the car is a problem, it is a tragedy of the commons, as it is an individually rational behaviour and a common behaviour that culminates in collective loss. Thus the only effective means of countering it is a ban, not voluntarism and exhortations and not market mechanisms. The only point of dispute is whether the car represents a serious enough threat to our mutual well being and survival to justify such drastic measures. Are it's effects bad enough to justify calling it's use a crime? I obviously believe they are, and hope the rest of the site provides sufficient reason for others to concur with this judgement.