Bridge held up by the cars that cross it

Think about it: a bridge that collects energy from cars to hold itself up. Basically the bridge is a giant treadmill, so that the cars that move across it generate energy. When you go onto the bridge, there's a bunch of little rollers linked to generators built into the road. Driving across them would "steal" some of your speed and pipes it into turbines on the bottom of the bridge that push it up. Energy could also be stored for later, and might even be supplemented by solar panels.

The resistance of the rollers can also be adjusted individually, so speed limits can be strictly enforced. Going to fast? Nope! You're just donating more energy to the bridge.

Once electric cars take over, and this idea becomes the norm (as it inevitably will), power can be pulled directly from the batteries of the cars.

If you think about it, this isn't that much different than normal bridges. Typically, when you pay to cross a bridge, you're really paying for the materials whose energy keeps the bridge standing. In this way, the money you spend on gas that goes to powering the bridge is paying for the energy needed to keep it standing.