Parking structure fire protection has historically been light, and for defensible reasons. An open parking structure with sufficient natural ventilation was treated as a low hazard: vehicles were mostly steel, fires were infrequent and slow to spread between cars, and open sides removed smoke and heat effectively. NFPA 88A still distinguishes open from enclosed structures on exactly this basis, with enclosed structures generally requiring sprinkler protection.
The vehicles have changed considerably since those assumptions were formed. Modern vehicles carry far more plastic in bumpers, body panels, interior components, and under-hood parts, which produces substantially higher heat release rates and more rapid vehicle-to-vehicle spread than the test data underlying older design assumptions reflects. Documented incidents in parking structures over the past decade have involved considerably more vehicles per event than historical experience predicted.
Electric vehicles add a distinct and more difficult problem. A traction battery entering thermal runaway cannot be extinguished by conventional means, requires very large water volumes over extended duration for propagation control, and can reignite hours after apparent knockdown. When that occurs on an elevated deck, surrounded by parked vehicles, with charging infrastructure nearby, it is a materially different scenario from the vehicle fire the structure was designed around. The practical questions are where charging is located, what separation exists, and whether the structure can deliver the water volume propagation control requires.