While a hydrogen-fueled future seems perfect, it might not seem so great if it results in a jet flame heading towards you at a high velocity. According to researchers/buzzkills at the University of Sheffield, a computer simulation of a hydrogen car crashing inside a tunnel revealed some not-so-fantastic side effects. Whereas gasoline leaks onto the ground and then ignites, hydrogen would escape into the air as a 2000 °C flame that would extend out for many meters (likely straight up).
This would result in some serious damage to the tunnel's ceilings and fire-suppression system. Reps for BMW pointed out that the fuel tank of their prototype hydrogen 7-series has never been breached, which sounds like a challenge to me. [New Scientist]