Expanding network of roads, low airfare, rail travel and shipping lanes help to generate the Time Travel to Major Cities map. From the European Commission Global Environment Monitoring website:
The map shows how accessible some parts of the world have become whilst other regions have remained isolated. Accessibility - whether it is to markets, schools, hospitals or water - is a precondition for the satisfaction of almost any economic need. Furthermore, accessibility is relevant at all levels, from local development to global trade and this map fills an important gap in our understanding of the spatial patterns of economic, physical and social connectivity.
The geospatial data, sources, software information (ArcGIS 9.2 and ArcView 3.x, and others) and more Travel Map information can be found here.
Thanks joguldi for Twittering about this project.