Stop/start systems are becoming more widespread in today's new cars, as manufacturers aim to improve fuel economy and lower emissions. On its face, it's a great feature to have. When you roll to a stop, these systems cut the engine out. That way, you save fuel (and stop noxious greenhouse gas emissions) when you aren't moving. But -- and there is a but -- these systems also have a tendency to lurch the car back into life when you take your foot off the brake.