Toll Lane Thickness Half of GP Lanes
The I-77 toll lanes will be constructed with about half the thickness of standard general purpose lanes. According to the contract between NCDOT and Cintra, the general purpose lanes are currently 32 inches thick, including an aggregate base course and a subgrade. In contrast, the toll lanes will be 17 inches thick, with no subgrade.
Asphalt on the toll lanes will be half as thick as regular lanes, 6 inches vs 12 inches. Here is a summary of the different layers comprising the two:
This begs the question: how can one part of the same road be only half the thickness of the other and not fall apart?
The answer can be found in the Bible of asphalt paving, the American Association of Highway and Transportation Officials (AASHTO) 1993 handbook on pavement design. The toll lanes are to be built according to the AASHTO standard. Among other things, the handbook methodology takes into account traffic type andvolume.
Two things are at play here. First, you may recall that trucks are not allowed on the toll lanes. This greatly decreases wear on the private toll lanes, maintained by Cintra, while placing all of the heavy wear and tear on the public lanes, maintained by the taxpayer. So the type of traffic means thinner lifts.
Second, the contract allows Cintra to use their traffic volume projections. This information is unavailable to the public because it is proprietary. However, according to the credit rating agencies the toll lanes are expected to carry 4-6% of total traffic when they open, escalating to 10%. So the toll lanes carry far less than their fair share of traffic volume.
With a light loading and light volume, the road bed need be only half as thick. This is a tacit admission that the toll lanes will be underutilized vis a vis the general purpose lanes.
Nevertheless, this allows toll advocates to claim- with a straight face- that the toll lanes are being built to the same standards as the general purpose lanes. While technically true, most people would take that to mean we’re getting the same road as we normally would. In reality, we’re only getting half.
The consequences could be dire. If the project defaults (perhaps we should say “when the project defaults”?), advocates tout that the NCDOT can buy it back “for pennies on the dollar.” But what exactly are we buying?
Say some happy day a few decades hence the state decides to open up the lanes to all traffic. With no subgrade layer we would have to rip out the toll lanes all the way down to dirt and repave completely.
The public is contributing over $150 million to this project. If it defaults the NC taxpayer will bailout the lenders. In addition to paying tolls we’ll watch our gas tax dollars being spent elsewhere.
And we’ll end up with half a road.