I've come across this many times and have sat in the car pondering "why Waze direct's me left and then connects me back onto the same road I am currently on..."

I did some research and reading of the blogs on the international site and have discovered waze keeps an "average road speed" which comes into effect when calculating the shortest route to your destination.

The traffic light may cause delay which is why waze would direct you around the round about and continue straight. You should arrive at your destination earlier than if you stayed at the traffic lights waiting for it to go green (per say...)

Obviously if the light was green you wouldn't go around the round-a-bout however at the time of passing waze doesn't know the colour of the traffic light.

Hope this helps.