Elon Musk:

In 2013, two extremely unusual Model S collisions resulted in underbody damage that led to car fires. These incidents, unfortunately, received more national headlines than the other 200,000 gasoline car fires that happened last year in North America alone. In both cases, the occupants walked away unharmed, thanks to the car’s safety features. The onboard computer warned the occupants to exit the vehicles, which they did well before any fire was noticeable. However, even if the occupants had remained in the vehicle and the fire department had not arrived, they would still have been safely protected by the steel and ceramic firewall between the battery pack and the passenger compartment.
 It is important to note that there have been no fire injuries (or serious, permanent injuries of any kind) in a Tesla at all. The odds of fire in a Model S, at roughly 1 in 8,000 vehicles, are five times lower than those of an average gasoline car and, when a fire does occur, the actual combustion potential is comparatively small. However, to improve things further, we provided an over-the-air software update a few months ago to increase the default ground clearance of the Model S at highway speeds, substantially reducing the odds of a severe underbody impact.
 Nonetheless, we felt it was important to bring this risk down to virtually zero to give Model S owners complete peace of mind. Starting with vehicle bodies manufactured as of March 6, all cars have been outfitted with a triple underbody shield. Tesla service will also retrofit the shields, free of charge, to existing cars upon request or as part of a normally scheduled service.