Other mistake: When Neal checks his airline ticket it says he departs from N.Y.C. J.F.K. but the movie claims to have him flying out of LaGuardia.
Other mistake: To go from Wichita, KS, to Chicago, they would go NE through KS, Missouri, Illinois. Kentucky is NOT on the route. They are pulled over by Kentucky State Police (Lenny). But this is the trip from Hell. Maybe they took the L O N G way East to Paducah, then back North to Chicago.
Answer: The reason was that the snowbank was on his side (the passenger side) when it should have been on the driver's side if they were driving in the right direction. Also and I'm not sure if you could see this in the film but the deflecters on the road which separate the lanes if you are going in the right direction shine yellow and if you are going the wrong way shine red.
Tobin OReilly