Nothing. Conventional cars are designed not to catch fire when they run over road debris, especially not cars built within the last two decades. At worst, debris may cause a flat if it punctures one of the tires.
That's dismissive hyperbole. Debris regularly causes serious damages to all vehicle types. If the object in question, as in this case, is hard enough and hit with enough force to penetrate the battery, it's hard enough and hit with enough force to seriously damage any system it hits. It may not crack the engine block, but it can destroy radiators/suspension/brakes/fuel tanks/etc. The worst case scenario is most certainly not "a flat".
That's just not true. I had about $2k in damage on a Mini Cooper (2006, stock) that hit the remnants of a blown out semi truck tire in the road. Similar situation to the blog post -- a pickup truck cleared it but I did not. I was not in a safe position to swerve.
The tire chewed up the lower front bumper and punctured the radiator. I had to immediately pull over and have the car towed. It happened to hit below the foam & all the fancy crash reinforcement on the bumper. Good times.
That was just 70lbs of steel-wrapped rubber. I can easily imagine a trailer hitch as described taking out a radiator or oil pan on a modern car. Unlikely to cause a fire for sure, but it'll certainly disable a vehicle.
"In 2004, a AAA Foundation for Traffic Safety study revealed that vehicle-related road debris caused 25,000 accidents—and nearly 100 deaths—each year."
http://en.wikipedia.org/wiki/Road_debris#Examples
See my other comment upthread for an example of a gasoline car catching fire because of cardboard boxes.