Michael Tiemann here at OSCON has a challenge “Many car audio systems have a feature that increases the volume of the program to keep the signal level constant over the noise level. But with cars like the Prius, we can control the level of the noise based on the signal–a better approach. The first hack is to mod the Prius power management system so that in addition to battery level, acceleration, and speed, it takes GPS data into account…”
The second hack is to read ahead in the music program (and/or to pick the right next song from a shuffle playlist) so that the signal will be loud when it’s appropriate to charge the battery (with the engine) and the battery is sufficient for driving when the signal is low. On a flat road running at highway speeds, such a car could intermittently charge the battery when the music would mask it, and it would run silent when the music was quietest. In San Francisco, the hack would choose loud music for going up the hills and quiet music when coasting around.