Road tunnels may require a makeover if the hydrogen economy takes off. Cars fuelled by hydrogen have been toutedf for their  potential to cut carbon dioxide emissions. Now Yajue Wu of the University of Sheffield, UK has built a computer simulation of a hydrogen car crashing inside a tunnel. Unlike gasoline, which pools  and ignites on ground, escaping hydrogen would create a high velocity "jet flame " stretching  upwards for many metres. In a paper  that appeared in Transportation Research C, Wu found that this 2000 c  flame would seriously damage tunnel ceiling structures and wreck fire sensors and sprinklers.

A spokesman for BMW has said that in a crash test the fuel tank on its prototype hydrogen car has never been breached.