VDOT is carefully surveying Interstate 81 between Botetourt County and Christiansburg before adding more lanes.
The agency is in the middle of a two-year environmental study, which includes geological surveys.
Since that portion of I-81 sits partially on limestone, which is more likely to take in water and crack than other rocks, sinkholes are a real concern.
Geologist Bill Henika said geological maps are developed so the interstate's extension can be built safely.
"If you have these old fault zones where water is entering and dissolving away large sections, you have to plan on it being a rough zone," Henika said. "We're going to have to design this differently."
VDOT will likely wrap up the $500,000 economic impact study in 2014.
At that point construction is expected to begin.