cjack, I just did some experimenting with D2 and it does seem to serve a purpose in the circuit.
With D2 in circuit you can hold the block detection contact closed as long as you want. When released the red then goes through it's complete cycle, then yellow cycles, then green.
With D2 removed, a momentary contact closure of the block detection device results in a normal cycle, red, yellow, green. But, if you hold the block detection contact closed longer than the red's timing cycle, when block detection contact is opened it goes immediately to the yellow cycle and then to green when the yellow cycle completes.
So, with D2 removed from the circuit, depending on how long the block detection contact is closed, the red cycle is either shortened or eliminated after the block detection device's contact opens.
Hope that makes sense...
