I am looking for the shapefiles of the intersetate highways of Puerto Rico in an easy to use geoJSON format. I found this page here http://www.baruch.cuny.edu/geoportal/data/esri/esri_usa.htm but when I unpacked could only find 50 states.
Even though PR is an island, there are 3 interstates.
Since I have promised, I will answer this question without waiting for its migration, if it will ever happen. Basically, I think that the best and latest data set that you can find now is this one - from the US official open data repository's TIGER/Line database. This page is generated, based on a relevant search (Puerto Rico), and might also contain some data sets of your interest.
Other potentially useful data sets include ones within U.S. Atlas TopoJSON repository (on how to use the data via R, see this nice tutorial) as well as this repository of U.S. major roads ESRI shapefile and geoJSON data sets (you have to check whether this repository contains PR data).
