The concept isn't as far-fetched as it sounds. Some hybrids have a significant electric-only range -- the latest X5 can drive for just under 50 miles without touching its combustion engine, for instance. That could be enough to operate in emissions-free zones, and you'd still have the gas motor for when you need to visit more distant areas.

Whether or not cities like the idea is another matter. BMW is showing the technology to German cities under pressure to ban diesels, but it would need broad approval for this to work. What good is driving through one city if you're not allowed to drive in another? On top of this, the approach would only be viable for hybrids that can both operate in an electric-only mode and have a healthy range. The previous-generation X5's 14 miles likely wouldn't pass muster. Still, this might be vital to keeping some hybrids relevant until their owners are ready to go all-electric.