You should get does not mean you get :-) For random placement of the lights, and for both directions simulataneously, it is near imposible to maintain green wave.
And even if it was possible, it cannot work simultaneusly for both straight ahead and turning directions.
Unfortunately, BRouter cannot detect your course at the traffic lights.
Imagine 2 otherwise comparable 15 km long primary roads. Once goes around the city, the other across the city with 10 traffic lights. You are cought by 3 of them, whaty costs you waiting, breaking and acceleration. Are you sure you do not prefer the former one ?
It is obvious that traffic light do not delay you always. But there is need to caunt with probability. As illustrative example, if it delays you e.g. 1 km ( you would be 1 km farther without it ) with probability 0.2, there is need to take as avg penalty 200 m.