We propose a theory-inspired measure of the accessibility of a city's center: the size of the surrounding area from which it can be reached within a specific time. Using publicly available optimal-routing software, we compute these "accessibility zones" for the 109 largest US and European...

The Inca Empire was the last of a long series of highly developed cultures in pre-colonial South America. It stretched across parts of the current territories of Argentina, Bolivia, Chile, Colombia, Ecuador and the whole of Peru. The Inca Road was its 30,000-kilometer-long transportation system....