The Signal at Vista Avenue/I-84 Eastbound On-Ramp - A Graphic Illustration

Hello from the Road Wizard!

The graphic below illustrates how this signal works when it is operating as part of a synchronized system, which is between 7 a.m. and 11 p.m. seven days a week. We are talking January 2006. If this page is still floating around the web years from now, be warned! Things might have changed.

The graphic responds to Chris and several other Road Wizard readers puzzled about why the signal seemed to “skip” them as they waited to turn left. Some readers were heading for eastbound I-84 from Vista Avenue. Some were heading north from Wright Street. Columns discussing this were published on December 18, 2005, and January 15, 2006.

The graphic is about Vista/I-84, but the same principle applies to Wright Street’s left turn to Vista. To understand one is to understand both!

Legend:

• NB Northbound thru lanes (Boise bound]
• SB Southbound thru lanes [Airport bound]
• SBLT Southbound Left Turn lane to I-84 (Chris is in this lane]
• EB East-bound Off-ramp from I-84
• Cycle One cycle takes 110 seconds and then starts over. The numbers refer to the time in seconds that has elapsed in the cycle.

Colors:

• Solid green: “Guaranteed” to be green whether anyone is in the lane or not; may also designate “Minimum Greens”
• Hatched green: Additional time that MIGHT be given to the lane
• Yellow: The 4-second warning before a green light turns red
• Red: The amount of red time before the next movement turns green.
• Sometimes referred to as the "all red" and lasting about 1 second
  (For simplicity, the graphic doesn’t show the full red time, just the beginning of it.)

One more thing: Gaps. Actuated controllers rely on detector loops to sense the arrival of cars. The absence of cars is a “gap.” When a gap lasts longer than the programmed number of seconds (typically 2 seconds), it “gaps out.” The controller considers the lane to be “cleared” of its demand and moves on to serve demand elsewhere. If none exists, it will “rest in green” on Vista.

The first diagram shows how things go when heavy traffic awaits on all lanes:

1. North and southbound have 20 seconds of green Guaranteed, plus 15 more seconds because traffic is so heavy. Then they get yellow and red lights.

2. At 40 seconds, northbound goes red, while southbound continues green. This allows Chris to get a green arrow and turn left to enter I-84 eastbound.

3. At 70 seconds, traffic headed for the airport is still heavy, but people waiting to enter Vista from I-84 have been patient and now get their turn. It costs the system a lot, because their green requires that all other directions stop on red.

4. At 105 seconds, the system must prepare to begin a new cycle because it is synchronized with other signals down Vista Avenue. The green goes yellow, then all-red. The cycle begins again.

Please note: If Chris arrives in the left-turn lane in heavy traffic at the unluckiest moment – at second 70 – the wait for the next green arrow could last about 80 seconds. In light traffic, Chris might have better luck. The lines below the diagram show how Chris could trigger a green arrow by arriving as early as 25 seconds or as late as 90 seconds into the cycle. Same goes for the folks leaving I-84 and headed for town, except that they are allowed to arrive up to 100 seconds into the cycle.

Example A, the second diagram, depicts a specific morning when Chris has bad luck by arriving at the 83rd second of the cycle AND when the signal happens to be green for drivers exiting I-84 and turning left on Vista.

1. Vista north and south enjoyed about 55 seconds of green, 35 more than their “guaranteed” amount.

2. The controller noticed traffic stacking up on the off-ramp (and no one in the southbound left-turn lane), so at 60 seconds they got the green. At second 83, Chris arrives.

3. Sadly, Chris isn’t going anywhere. Among the rules the controller has to obey is one that says, “Left-turn moves from Vista must occur after Vista’s thru moves.” These are called “lagging lefts.” So the controller bows to this rule and gives Vista’s thru lanes a grudging Minimum Green lasting 5 seconds, followed by 4 seconds of yellow and 1 second of all-red. By this time, the cycle has reached second 100. Meanwhile, traffic has arrived from the off-ramp. The controller is squeezed by its logic to skip Chris. “I have only five seconds of green available before I have to turn it yellow and be ready to begin the next cycle with green for Vista’s thru lanes. I can’t give it to Chris because it will produce a “lagging” left sequence on Vista.”

4. Chris waits for the next cycle to begin. In light traffic, the wait will end at second 25 of the new cycle, a total wait of 52 seconds. With heavy Vista traffic, the wait is 67 seconds.


Example B, the third diagram, shows another way for Chris to be skipped. On this morning, Chris arrives at second 58, just after the controller had observed a gap in off-ramp traffic at second 50 and returned to green for Vista. A big airplane has disgorged, new departures are bringing people to the airport, and north/south demand is heavy. The signal very much favors clearing this traffic, as you already know if you read the Road Wizard column of January 15, 2006: avoiding traffic congestion in front of the terminal building is a major goal of the signal system.

1. The cycle begins with the usual 20 seconds of Guaranteed green for Vista thru lanes. Someone arrives from the off-ramp at second 24, and a momentary lull in Vista’s thru lanes gives the off-ramp a quick green.

2. The off-ramp gaps out, so the signal goes back to green for Vista’s thru lanes at second 55. Chris arrives at second 58. That jet was a jumbo, so it continues green until it gaps out at second 90.

3. Once more, the controller logic squeezes Chris out of luck. Not enough time remains in the cycle for the controller to serve both the off-ramp and Chris’s turn. Each of these moves would require 10 seconds, and only 15 are available. Remembering the “lagging left” sequence logic, the signal skips Chris.

4. The new cycle begins. Chris waits through Vista’s Guaranteed green and however much additional green it gets up to second 40. Combining the time in the old cycle and the new cycle, Chris waits between 77 and 82 seconds.

In conclusion, congratulations for coming this far in your understanding of signal logic in the airport area. If you and Chris can’t always have good luck when you get to Vista’s southbound left-turn lane onto eastbound I-84, at least you can spend those idling moments watching and understanding what the signal is doing and why.

Click to see larger version of graphic