Here is some interesting information on car fires from the US Fire Administration (USFA->FEMA->DHS) and the National Fire Protection Association.
From 2008-2010 "Approximately one in seven fires
responded to by fire departments across the nation is a highway vehicle fire
. This does not include the tens of thousands of fire department responses to highway vehicle accident sites.". The leading factors in ignition where "mechanical failure" (44.1%) and "electrical failure" (22.3%). 1
The actual number of highway car fires in that period was approximately 582,000, or an average of over 500 car fires every day
on American highways.2
In this accident which involved an electric car a large piece of sparking metal debris was run over by the car and thrown up with enough force to slice through the cars stored energy compartment, in this case one of the batteries. The driver was alerted via the display to a problem and instructed to pull over immediately due to the fact that one of the batteries was now leaking and smoldering. A short time later the burning ember reached critical temperature and was able to ignite the softer materials in the adjoining 'frunk', the carpeted front side trunk located where most cars have an engine. The other 15 battery compartments, having not been skewered by a giant metal spike, remained unharmed due to the firewalls and other protection, as did the passenger compartment.
If the owner had been driving a gas powered car and that metal spike had instead been driven up into the gas tank, ripping it open and showering the fuel with sparks as it was dragged along the highway, would the driver have had any warning other than a loud bump and then the passenger compartment being consumed by flames?
This is not the first Tesla fire, there was another involving the Roadster resulting in a recall of 439 vehicles. The source of the fire in that instance was not the advanced battery at all, it was one of the old style 12V lines (Tesla vehicles still include a regular 12V battery for lights/instruments and 'ignition') being in a bad position near a headlight and susceptible to damage that could spark a fire. Going back to the statistics above we have over 100 car fires each day (22.3% of 500) caused by those 12V wires and components being damaged and shorting out. For example Honda recalled over 140,000 (non-hybrid) Fits in the US this year because the wiring in a 12V door switch could get wet, short out and start a fire. GM had the same problem last year and had to recall almost half a million vehicles.