As of noon o’clock on the first real day of operation, the Ashford Dunwoody Diverging Diamond Interchange is operating well. There are some hiccups, illustrated above. These were expected and are quite normal for this sort of change in traffic control. Why? It has to do with the signals.
When the interchange signal at the westbound off/on ramps was converted to the DDI configuration, it went from a three-phase signal to a two-phase. A three phase signal has a left turn phase for the main line of travel, a through phase for the main line, and a through/right/left for the ramp. Now that we have a DDI, our two-phase signal is for through travel south (with ramp turning north) and through travel north (with ramp turning south). Just by eliminating that phase for the left turn, we’re saving time every signal cycle by removing the necessity for a yellow, then an all-red signal. However, it comes with complications.
In order to make the signal play well with the others on Ashford Dunwoody, the signal engineer has to model the traffic and come up with a timing plan for the phasing. Once that is installed, it the has to be tweaked to match the field conditions. The congestion you see pictured above, from this morning’s AM commute1 shows some of that tweaking going on. The signal engineer noticed that the Hammond Drive left turners were backing into the intersection and reduced some of the through time for the northbound movement accordingly. This takes time however; it’s not instant. The tweaking will continue for several days and weeks.
Another complication is that all of the other signals along Ashford Dunwoody are three- or four- (or eight-) phase signals. Getting a two-phase signal to coordinate well with the others is something of an art, which I won’t try to describe here.
I was watching today’s AM commute for the majority of the “thick” period and there were numerous tweaks to the timing. Tomorrow will be even better.
This picture is a great illustration of why you need to have a signal engineer in the field, watching the traffic, and adjusting things on the fly.2
The morning commuting period worked pretty well according to all the press reports and my own humble observations. I’ll be back out for the evening commute to see how that goes.
More pictures and commentary later.