Use option tigris_use_cache to tell tigris to cache
Census shapefile downloads. This is TRUE by default. e.g.
options(tigris_use_cache=TRUE)
Use option tigris_refresh to force a refresh of cached tigris
Shapefiles. e.g. options(tigris_refresh=TRUE)
Use option tigris_year to change the year for which you'd like to download data.
e.g. options(tigris_year = 2013). The default year for the package is 2015, the most
recent year for which data are available.