Rendering “area:highway” would help where people have mapped those, but that’s far from widespread. It is however possible to create a map with any width highway you like. There’s a series of widths (and casing widths) at https://github.com/gravitystorm/openstreetmap-carto/blob/master/roads.mss#L63 - try experimenting with different values there. If you want help with setting things up have a look at https://ircama.github.io/osm-carto-tutorials/ , https://switch2osm.org/manually-building-a-tile-server-16-04-2-lts/ and https://github.com/gravitystorm/openstreetmap-carto/blob/master/INSTALL.md etc.