The signal in your picture is cleared for the rightmost depot track, hence the semaphore arm on the right is pointing upwards. I'm preparing more instructions for the next update so the signals will be easier to understand. In addition there will be a new helper that displays the signal as green or red blob in the route minimap depending on whether you can pass it or not.
In the meantime you can always access the helpers while running the sim by pressing ESC and going to the driving settings screen. If you temporarily enable the key helper, you'll get an idea of whether you should just accelerate past the signal or stay put.
The sim should be able to handle without problem the case where you overrun a signal and then reverse back.
I hope this clears things up!