A road geek is for all intents a road hobbyist. That is to say, they are to
numbered highways what a
railfan is to
trains.
By way of a hobby, a typical road geek might amass numerous maps of given areas, and will typically focus on no more than a few areas - one may focus on three-digit interstates, interchange types, highway types (such as the US Highway or Interstate highway system), or even in some cases roads that fall under the jurisdiction of states, provinces, or whatever your country subdivides itself into (IE, California highways).