If you're one of the lucky few allowed to evaluate a BMW Hydrogen 7, you're free to roam about the country - albeit with a few restrictions.  The Port Authority of New York and New Jersey isn't allowing the hydrogen-powered sedan to drive in a number of tunnels.

The Port Authority is banning the BMWs - along with all vehicles carrying compressed flammable gases - from both the Lincoln and Holland Tunnels.  Hydrogen 7s will, however, be permitted on a number of bridges, including the George Washington Bridge - albeit only on its top deck.

The restrictions stem from the fact that hydrogen is extremely flammable.  The Hydrogen 7 uses compressed hydrogen as a gasoline substitute to fuel its 6.0-liter V-12 engine.  Although the hydrogen is stored onboard as a super-chilled liquid, the Port Authority fears the possibility of it returning to a gaseous state and potentially causing a fire or explosion.  Given a tunnel's limited access and air supply, such a disaster would be difficult for rescue workers to control.

BMW, however, has extensively tested the hydrogen system to prevent such an event from happening.  The hydrogen is so well insulated that a block of frozen ice would take over a decade to melt within the tank.  Any vapor that should happen to build up within the tank is released through a pair of roof vents in a controlled manner to reduce the chance of explosion.

While 100 examples of the Hydrogen 7 have been manufactured, only twenty were imported to the U.S.  Overall, the entire fleet has logged over a million miles without any serious incident.

Source: New York Times