I remember being suprised when I saw my uncle leave his keys in the ignition and leave his car when I visited him on San Juan Island. The community there allows people to do that (still not without risk, of course). I agree with @whatamidoing, this comes from a shared sense of responsibility for others in the community and their property. Although a utopian idea that works best in small places, it could occur on a much larger scale if that kind of consensus could take place among people.