Chapman: Dictionary of American Slang
super slab n phr truckers by 1970s
A superhighway; an interstate highway

http://www.sbc-rides.com/StreetRides_LB/hwy10...
Highway 101 from Santa Barbara to San Louis Obispo is one of the most scenic sections of Hwy 101 in Southern/Central California. However, it is a U.S. interstate and to some extent, super-slab.

http://www.koaa.com/news/view.asp?ID=3277
Proposed Super Slab fuels debate
Residents of eastern El Paso County are angry about a possible toll road that's proposed along the front range. The 210-mile road from south of Pueblo to near Fort Collins would be privately funded and would run about 25 miles east of I-25.