I am 99% sure it was filmed here: http://www.google.com/maps?saddr=Minden,+LA&daddr=Homer,+LA+71040&hl=en&ll=32.810281,-93.088357&spn=0.000002,0.002747&sll=32.692555,-92.790527&sspn=0.625247,0.883026&geocode=FQSs8QEdTY5w-ilzFBXsVRIxhjH2dDHsYKty3w%3BFQVd9AEdGhV0-imn1DYMX_8whjHrQOQFASCyGw&vpsrc=6&mra=ls&layer=c&cbll=32.810282,-93.088046&panoid=sK8-5FdWwebWtb68Ew7V7w&cbp=13,267.01,,0,4.96&t=h&z=19
That's on Highway 2, near Tanglwood Road, in Homer, Louisiana.
Compare the black mailbox to the one seen in the movie, where Sandra Bullock gets out of her car.
Look west, and you'll see the same hill at the end of the road that is seen in the scene.
The actual accident appears to have been shot farther up that hill to the west, on Highway 2, between Dean Road and Tanglewood Road, right here: http://www.google.com/maps?saddr=Minden,+LA&daddr=Homer,+LA+71040&hl=en&ll=32.81029,-93.098332&spn=0.000002,0.002747&sll=32.692555,-92.790527&sspn=0.625247,0.883026&geocode=FQSs8QEdTY5w-ilzFBXsVRIxhjH2dDHsYKty3w%3BFQVd9AEdGhV0-imn1DYMX_8whjHrQOQFASCyGw&vpsrc=6&mra=ls&layer=c&cbll=32.81029,-93.098522&panoid=37t7zA04uIvfZpEBa-_2ww&cbp=13,96.6,,0,12.22&t=h&z=19
Note the yellow, triangle-shaped sign on the left. This can be glimpsed in the movie earlier, when he's talking to her from his car. And in the scene where the truck explodes, the trunk winds up facing a small sign next to a slope. You can see both on the right side in the above StreetView.
The latitude & longitude appear to be around 32.81031, -93.09866.