It's called Long Shoals Road, it provides access to Lake Julian and it has its own interchange (exit 6) with I-26. Most cartographers blow 146 off, but the state shows it in all its splendor (1999 official map, right).

The Durham Freeway was born by 1970. (It shows up as proposed from Chapel Hill Street south on the 1969 official map.) Originally it ran only from Chapel Hill Street southeast to Alston Avenue (N.C. 55) and was called the East-West Expressway.  

When the freeway opened it had no numerical designation. This isn't too unusual for freeways in North Carolina. Charlotte's Brookshire Freeway (nee Northwest Expressway; see I-277) wasn't originally numbered. Fayetteville's All-American Freeway still isn't. 

Around 1973, the East-West Expressway was extended south to its current terminus in RTP. This was done around the same time I-40 opened between the freeway and Raleigh. This stretch of 40 was disjointed from the rest of 40 -- the transcontinental 40, whose main portion originally ended in Greensboro -- for many years. If you were to take 40 out of Raleigh back then, 40 would end at today's Exit 279, and you'd continue on the E-W Expressway (which wasn't very E-W at that point) into downtown Durham. 

The Durham Freeway was never part of I-40, even though for many years it seemed like 40's logical extension. That the freeway never received an Interstate designation is again consistent with N.C.'s numbering practices: The Raleigh Beltline and Asheville's I-240 weren't Interstates at the time, either, and some similar urban freeways still aren't. However, "To I-40" signs did show up on the Freeway by the early 1980s.[1] 

Around 1975, the E-W Expressway was extended about a mile "east" to end at a traffic light at Erwin Road.  

By 1983, the still-unnumbered road was named the Durham Freeway. The name change may have come years earlier, but the earliest map I have which uses "Durham Freeway" dates from 1983. Signs pointing to the "East-West Expressway" would remain around Durham into the early 1990s. 

Around 1987, once work had progressed on extending I-40 west through RTP and towards Chapel Hill, the Freeway was designated as N.C. 147, and was signed as a north-south highway.  

Construction to extend the Freeway north to I-85 started in 1990. The Freeway would no longer provide access to Erwin Road. During the first stage of construction, all traffic exited at Swift Avenue. Early in 1992, the Freeway was finished north to U.S. 15-501. Later that year, the entire road received exit numbers for the first time (see below). 

By 1997, N.C. 147 was finished all the way north to I-85. The map above, which shows the final stretch as proposed, is the 1995 official state map.

OK, what you really want to know is why this one got a number. According to the state maps (1999 at right), 149 doesn't even reach the couple of extra miles into Plymouth -- so what gives? Well, 149 is called Weyerhauser Road. Neraby is a Pulp Mill Road. The route crosses the CSX Railroad, and it passes along something the DeLorme calls "Industrial Waste Ponds". Ha! Looks like it's a major conduit for North Carolina's forestry-industrial complex! Could the state be turning a blind eye to clearcutting and waste dumping by giving this road a number? If you want, call 149 the Exploitation Highway, but don't do it near any environmentalists.