The Old Black Canyon Highway is a leftover from before the time of I-17 being built.
Originally build in the mid to late 1800’s, it was a stagecoach route from Prescott to Phoenix that was known for its rough ride. There is a paved section that still exists in Prescott, and most of the road through Black Canyon City is also paved, but just before merging onto I-17 at the south end of the city, it makes a turn to the right becoming a dirt road. Today this dirt trail gives access to private property and the Black Canyon hiking trail, but also leads you into a very skinny, winding, and hilly off-road trail that will lead you into Table Mesa Recreation Area.
For more details see the Trailsoffroad.com web-site.