This Accessibility map from the JRC displays estimated travel times to the nearest town with a population greater than 50,000 in the year 2000. It is clipped to the EDENext extent and projected in lat/long WGS84 at a scale of 0.008333 degrees or ~1km. It is available as a geoTIFF (.tif) format.
They define accessibility as the travel time to a location of interest using land (road/off road) or water (navigable river, lake and ocean) based travel. This accessibility is computed using a cost-distance algorithm which computes the "cost" of travelling between two locations on a regular raster grid. Generally this cost is measured in units of time. The cells in this raster grid contain values which represent the cost required to travel across them, hence this raster grid is often termed a friction-surface.
JRC website: http://bioval.jrc.ec.europa.eu/products/gam/download.htm
Nelson, A. (2008) Estimated travel time to the nearest city of 50,000 or more people in year 2000. Global Environment Monitoring Unit - Joint Research Centre of the European Commission, Ispra Italy. Available at http://bioval.jrc.ec.europa.eu/products/gam/ (accessed 07/11/2011)