The real Sparkwood and 21 is located just steps from Ronette's Bridge at the intersection of Meadowbrook Way SE, SE Reinig Rd and 396th Drive SE. That location, coincidentally, is the intersection of three roads that hold 3 key locations; the Sheriff's Department (396th dr SE) and sawmill, the Twin Peaks sign spot (SE Reinig Road) and Ronette's bridge (Meadowbrook Way.) Map HERE.
If you look closely, you can see Ronette's bridge in the background of the wide shot of Sparkwood and 21 when Laura jumps off James' bike. In reality, James Marshall rode to this spot on the Sheriff's Station road and if he veered left from where he is in the still he would be on the Twin Peaks sign road.