Stupid four-wheelers, pickups pulling campers, and short-distance trucks contribute quite a bit to congestion, too. Flatlanders who don't understand hills are a problem, as are people who can't maintain a steady speed even with cruise control. There are those who don't believe you can occasionally exceed the speed limit by a few miles an hour for a short stretch and take forever to pass a car with a 1 MPH speed differential (blocking a whole line of traffic in the left lane behind them). I-81 is the most schizophrenic highway around. At one point you can be in the middle of a large slug of traffic and a few miles down the road (with only maybe a local exit in the stretch) you can be almost alone. You can have someone blow past you at 80 and a few miles down the road pass them doing 60.
And huge differences by day. Many weekdays Mon-Thurs the 4 lanes work well enough if not fine. At least 20 weekends Fri-Sun per year, 6 lanes the whole length and 8 lanes in a few places would be required for free flowing traffic conditions. (considering all day Friday part of the weekend for this analysis).
Christiansburg to Troutville and around Harrisonburg and Winchester, plus the I-81/I-77 overlap, are places that should have 8 lanes.
If the Star Solutions plans had been a bit more reasonable in execution, we might have six lanes at least in several places where they are desperately needed, like from exit 118 to exit 150 (Christiansburg to Troutville) and around Harrisonburg and Winchester.
The Star Solutions proposal for dual-divided roadways was in 2002. Fluor Virginia Inc. in 2003 submitted a proposal to add two car-only lanes in the median of I-81 by 2011 and to pay for it entirely with tolls on cars and trucks on all lanes.
VDOT and FHWA conducted the Tier 1 Draft Environmental Impact Statement from 2003 to 2007, and it evaluated a range of alternatives besides those two.
VDOT's application to toll Interstate 81 under section 1216(b) of TEA-21 was approved by FHWA in 2003.