I notice that there are large fluctuations in the traffic in certain lanes at certain places in certain times of day, days of week etc. If waze knew what lane you are in you could use that information to make recommendations as to the best lane for the moment. If you just add a multi lane attribute on a road segment when you are driving on that segment (if you have the option enabled) you would get a prompt to input what lane you are now in, after which waze could use the map, speed and accelerometer data to detect lane changes and prompt for confirmation. You could even help on the client end with a lane change calibration option. Invaluable as sticking to one lane compared to strategically changing at the right time can save a lot of time, even more on long commutes
Lane detection would be extremely important but not for just knowing which lane is moving faster.
I've noticed several times that because only one of the lanes in an highway is jammed (usually approching an exit) and Waze is not able to distinguish between lanes, it suggests taking unnecessary detours.