Add Country-Specific Road Speeds from OSM

[andresfchamorro]

Looking into the methodology behind the friction surface (Malaria Atlas Project, Weiss et al), I found out that they created a table of country, road type, and speed limit based on OSM attributes.

Here is the paragraph describing their road data processing:

Firstly, OSM roads data were extracted in October 2019 and converted into a rasterized surface of road types whereby the fastest road type in each pixel took precedence. In a second processing step, the OSM data were analyzed to determine the median speed limit value for each road type in each country (Supplementary Table 1). These values were subsequently used to attribute road speeds to specific pixels using a lookup table. In cases where a road type was present in a country, but associated speed limit information was unavailable, a global average speed limit for that road type was used instead. https://www.nature.com/articles/s41591-020-1059-1#MOESM2

I propose that we look into the quality/completeness of this table, and use it to replace default_speeds, or at least have it as an option. The table is available through the paper in the Supplementary Tables 1–3 link.

Thoughts?

@d3netxer @bpstewar