How do the street signal lights know to stay green a little longer or turn from red to green a little sooner when Metro Rapid and Metro Orange Line buses are approaching?
After our original post some of you asked about train signal preemption, which is not the same thing as the signal priority technology used for buses. It’s a totally different beast that we can certainly talk about later.
You also asked about the Orange Line. The Orange Line is not rail but bus service on an exclusive right-of-way. Because it’s a bus, it’s allowed priority like other Rapid lines. Priority does not mean the Orange Line or Rapids avoid getting red lights. It simply reduces the amount of time a bus will have to sit at a red light with early or extended green time.
Someone pointed out that the Orange Line seems to stop for a particularly long time at one or more intersections and wondered why that might be. The Orange Line runs through a few complicated intersections where buses do get red lights. The system was intentionally designed that way with safety in mind.
Also, the way signal priority works, if a bus is not granted priority or misses it for some reason, the bus will get a normal red light cycle, which can be quite long at some intersections. Only one bus approximately every 2-3 minutes will get priority so that the signal has cycle time to recover. So once one bus gets priority, it will lock out another in that 2-3 minutes. That could account for a waiting time that seems longer than you might expect.
Categories: How do they do that?