I like the idea
It would have to use different algorithms for electric and gasoline, I would think, as stop-and-go might not affect them the same.
More than that, until surface street traffic and traffic signals are taken into account, it isn't going to be very effective. I have a GPS device with traffic, and it sometimes suggests using the access (service/frontage) road when the freeway is backed up, not aware of the backup on the access road. And once off of the highway and on city streets, it severely underestimates the effect of traffic signals. Neither the FM/RDS system, nor the new HD (Garmin calls it "3D") traffic provide surface street information.
Google traffic does provide the surface data, so perhaps we aren't too far off.
