I have yet to figure out/understand how the “diverging diamond” works. What is it supposed to do exactly?


I have yet to figure out/understand how the “diverging diamond” works. What is it supposed to do exactly?

In: Engineering

It prevents you having to make a left turn across traffic. Where before you would be stuck waiting for an opening (or the next light cycle to get a green arrow), with a diamond, as soon as you’re at the turn lane, you can go. It keeps traffic moving more smoothly since no one has to wait for that opening to come (or misjudge the amount of time they have and cause an accident turning in front of someone going too fast to stop).

Here is a pretty good video describing and illustrating this:


And TIL there was something called a “diverging diamond”.

Okay. I get it. I guess if you’re not making a left turn in them, it’s hard to notice any difference. Thanks!

The 2 factors that make it desirable are that it:

1. Generally uses the existing bridge. If a bridge is not structurally deficient (e.g. a 30-40 year bridge that’s only 10-20 years old) then continuing to use it saves money, rather than demolishing it and replacing it.

2. The roadways are routed so that there are only 2 phases at each traffic light. As opposed to the regular diamond interchange which has 3 or 4 phases. The less phases there are, the more green light time can be given to each phase.

What is a phase? It’s basically a “step” in the cycle of the light serving specified traffic movements. Simple traffic lights without any turn arrows usually have just 2 phases. For example, phase 1 is a green light for NB and SB traffic, phase 2 for EB and WB. Left turn arrows usually add phases, an intersection that has a left turn arrow from each direction has 4 phases. Phase 1, NB to WB left turn and SB to EB left turn; phase 2, NB and SB straight through; phase 3, EB to NB left turn and WB to SB left turn; phase 4, EB and WB straight through.

While each traffic light phase can be adjusted to make it longer or shorter, the overall problem still exists that having more phases can only take time away from other phases, and having 2 phases wastes the least time by allowing each movement to have the most green time possible.