The user would select the relevant traffic light from a location-dependent (via GPS) list.
Such a system would present a great opportunity for municipal authorities to recoup some of their costs of maintaining the roads, with the cost of each message to a traffic light costing a small amount (perhaps from a prepaid account). The fee would need to be sufficient to cover the comms put into each of the traffic lights, and to contribute toward the cost of maintaining roads.
Another potential feature would be automated messaging to particular sets of lights (the ones that annoy you the most!) based on geographic location (via GPS) and speed (via GPS).