Make the ambient noise less. Hybrids aren't completely silent, just quieter than everyone else. So it would be like an arms race, only for the quietest car.

Then you set a fine and issue tickets for driving a car that makes enough noise to drown out the sound of the next car. Eventually they make no more noise than bees. Then you lower the speed limit cause you can't hear a bee till it's close by, so everyone has to go slower.

Now that all cars are very quiet, you put special transponders in cars that send out a signal. The signal is received by stereo headsets in the area, each with a computer that calculates the position and speed of the car relative to the wearer and modifies the sound to express a) the individuality of that car, e.g. two cars in the area have distinct sounds b) the speed and c) the direction of the car. Soon anyone who chooses to wear these headsets can tell instantly where cars are, how fast they are travelling, and their direction. But people who don't want to hear cars, like sitting on their porch for example, don't have to listen to them.