/vehicle/traffic_lights provides you with the location of the traffic light in 3D map space and
helps you acquire an accurate ground truth data source for the traffic light
classifier by sending the current color state of all traffic lights in the
simulator. When testing on the vehicle, the color state will not be available. You'll need to
rely on the position of the light and the camera image to predict it.
https://github.com/udacity/CarND-Capstone/blob/master/ros/src/tl_detector/tl_detector.py#L29-L35
/vehicle/traffic_lights provides you with the location of the traffic light in 3D map space and helps you acquire an accurate ground truth data source for the traffic light classifier by sending the current color state of all traffic lights in the simulator. When testing on the vehicle, the color state will not be available. You'll need to rely on the position of the light and the camera image to predict it.