Most main streets have a co-ordinated traffic light system, timed to allow vehicles that have hit one green to pass through uninterrupted for as long a stretch as possible.
That said, if as a driver, your route involves turning onto a side street, your luck with the lights gets scrambled.
The timing of the lights is usually pre-programmed and in phases, accounting for variables such as offering independent lefts, rush-hour, etc. There are newer real-time reactive systems that account for unexpected traffic volume and the like, but for the most part, the lights have a state machine controller that pre-accounts for several traffic-related variables before spitting out an optimal light-changing schedule.
Given such a case, how about feeding the timing sequence of these lights into your GPS unit? That way, when mapping a route to your destination, the unit would consider the posted speed limits on all of these roads, traffic conditions (another real-time GPS input easily available these days) and how far you are from the next traffic light. The unit would then map a route that would take you through the most green lights, even if it meant a bit of a longer distance, with the goal being either fuel economy or a shorter drive time.
There you go, more $$ I've just given away :)
P.S. On a tangential note, I'd read UPS delivery trucks have routes mapped out for them that have no left turns so that the truck never gets stuck at a light. Saves a lot of time, PLUS the driver doesn't have to worry about explaining the shorts to whoever pulls up alongside at a light.