Yeah, probably just keep straight on this prefectural road, and...
Literal
Yeah, probably this prefectural-road straight [with-で]...
A trailing sentence that leaves the directions hanging — a natural speech pattern where the speaker trails off while thinking. 県道 is a specific category of road in Japan's road hierarchy (国道 national → 県道 prefectural → 市道 municipal). The で here is instrumental-adverbial, marking まっすぐ as the manner/means of travel: 'with straight' = 'straight ahead'.