Working on the same principles that transform electrical impulses in speaker wire into mechanical movements, or vibrations, in speakers and headphones - except in reverse - the equipment fitted to the bridges takes energy that would otherwise be wasted making sound and turns it back into a usable form.
It's not clear how much electricity can be generated this way, or how efficient the system is, but it's apparently cost-effective, at least in the long term, since it's being expanded to the whole metropolitan bridge system.
While not as whimsical as the Japanese taste for musical roads, the bridges are certainly more practical. But the feeling of setting the cruise control at 28mph, settling back, and letting the road play your tires like a musical instrument must be unique. If the two systems were to be put together, it would be a complete reversal of the speaker's electro-mechanical concept.