Whether or not this feature will come to the U.S. within the same timetable as in the U.K. is unknown. But given the manufacturer's reticence to introduce such gas-saving measures as diesel engines and four cylinders in the U.S. market (conceivably fearing the dilution of its Ultimate Driving Machine image in the power-hungry USA), we'd expect a slower timetable compared to the one in Europe, where the price of gas makes U.S. numbers look like a bargain.
Just like the stop/start systems in hybrids such as Toyota's Prius and GM's Tahoe, the gasoline engine shuts down when the vehicle is stationary. Unlike those vehicles, however, the engine must re-start immediately as it is the only means of propulsion, unlike a gas/electric hybrid. Unknown is whether BMW uses a traditional starter motor in this setup, or an enlarged alternator/starter as in some hybrids.
Predictably, the stop/start feature finds its greatest fuel economy benefit around town, where high cycle stop-and-go driving is common. Presumably, stoplights would offer the greatest fuel savings for the driver of a BMW with this technology.--Colin Mathews