4.2 Actuated Interval Timing and Volume-Density Features
Key Takeaways
- A fully actuated phase terminates one of three ways: gap-out when the passage timer expires, max-out when the maximum green expires with demand still present, or force-off when coordination logic terminates it.
- Passage time (also called vehicle extension, gap, or unit extension) restarts on every actuation and defines the largest headway the controller will tolerate before ending the green.
- Volume-density adds variable initial green, which extends the initial interval based on vehicles that arrived on red, and gap reduction, which shrinks the allowable gap from the passage time down to a minimum gap.
- Recall settings force a phase to be served without an actuation: minimum recall places a vehicle call, maximum recall runs the phase to its maximum green, and pedestrian recall places a pedestrian call every cycle.
- Presence-mode detection holds the call for as long as the vehicle occupies the loop, while pulse mode issues a short call on arrival regardless of how long the vehicle stays.
4.2 Actuated Interval Timing and Volume-Density Features
IMSA lists "signal phasing and timing" as a Level II Field training area, and a field technician is regularly asked to change a timing value in response to a complaint: the side street holds too long, the left turn drops a car, the mainline waits at 2 a.m. with nobody coming. Making that change safely requires knowing exactly which timer controls which behavior.
The Interval Structure of an Actuated Phase
An actuated phase is built from timers that run in a defined order.
| Interval | Common Names | What It Does |
|---|---|---|
| Minimum green | Initial, minimum initial | The shortest green the phase can display once it starts. Sized to clear vehicles stored between the detector and the stop line. |
| Variable initial | Added initial, seconds per actuation, computed initial | Volume-density feature that lengthens the initial interval based on the number of actuations received while the phase was red. |
| Passage time | Vehicle extension, gap, unit extension, preset gap | Restarts on every actuation. When it expires with no new actuation, the phase gaps out. |
| Gap reduction | Time before reduction, time to reduce, minimum gap | Volume-density feature that shrinks the allowable gap from passage time toward the minimum gap as the phase runs. |
| Maximum green | Max 1, Max 2, dynamic max | Caps the green when demand is continuous. The max timer starts when a conflicting call is registered, not when the phase turns green. |
| Yellow change | Yellow | Fixed; never shortened by actuation. |
| Red clearance | All red | Fixed; never shortened by actuation. |
The detail that catches technicians: the maximum green timer begins timing when a conflicting call is present, not at the start of green. A phase resting in green with no opposing demand does not accumulate max time.
Three Ways a Phase Ends
| Termination | Trigger | What It Tells You |
|---|---|---|
| Gap-out | Passage timer expired — traffic headways exceeded the allowable gap | The phase served its demand. Normal, desirable outcome. |
| Max-out | Maximum green expired while calls were still present | Demand exceeded the phase's allotted capacity. Repeated max-out means the max green or the split is undersized. |
| Force-off | Coordination logic terminated the phase at its force-off point | The phase is running in a coordinated pattern and hit its split boundary. |
Controllers log which termination occurred, and that log is the fastest diagnostic available. A left-turn phase that gaps out on the first vehicle every cycle almost always has a passage time that is too short or a stop-bar detector that is dropping the call. A side street that maxes out at 3 a.m. almost always has a stuck detector holding a constant call.
Passage Time in Practice
Passage time answers a single question: how long will the controller wait for the next vehicle before giving up on this phase?
With a stop-bar-only presence detector, passage time is short — typically 2.0 to 3.0 seconds — because the detector already tells the controller a vehicle is physically present. With a setback detector placed upstream, passage time must be long enough for a vehicle to travel from the detector to the stop line, or the phase will terminate under the vehicle. The travel-time calculation is straightforward:
where D is the detector setback in feet and V is the approach speed in mph. A detector 250 feet back on a 40 mph approach needs roughly:
Set passage time below that and the controller will strand vehicles that were legitimately approaching. Set it far above and the phase will hold green for traffic that never arrives, wasting cycle capacity.
Volume-Density Operation
Volume-density is the controller's answer to a phase that must serve both a single car and a long queue.
Variable Initial
While the phase is red, the controller counts actuations on that phase's detectors. Each actuation adds a programmed seconds per actuation to the initial interval, up to a maximum initial ceiling. A left-turn bay that stored six cars on red therefore starts with a longer green than one that stored a single car — without waiting for those cars to gap out one at a time.
Gap Reduction
Gap reduction makes the controller progressively less patient as the phase runs and opposing traffic waits.
- Time before reduction — the delay, measured from the start of the conflicting call, during which the full passage time still applies.
- Time to reduce — the period over which the allowable gap is linearly reduced.
- Minimum gap — the floor the allowable gap reduces to.
The effect: early in the phase the controller tolerates a 4-second headway; late in the phase, with a queue waiting on the cross street, it will terminate on a 2-second headway. Gap reduction never overrides minimum green, yellow, or red clearance.
Recalls, Detector Modes, and Modifiers
Recalls
| Recall Type | Effect |
|---|---|
| Minimum recall | Places a vehicle call so the phase is served every cycle for at least its minimum green |
| Maximum recall | Places a continuous call so the phase runs to its maximum green every cycle |
| Pedestrian recall | Places a pedestrian call every cycle, forcing walk and clearance |
| Soft recall | Returns the phase to green when there is no other demand, without forcing service |
| No recall | Phase is skipped entirely unless an actuation arrives |
Maximum recall is the field workaround for a failed detector, and it is the correct temporary action — but it must be logged and reversed. An intersection left on max recall for months after a loop failure runs as a fixed-time signal and generates delay complaints that get misdiagnosed as coordination problems.
Detector Modes
- Presence mode — the call persists as long as the vehicle occupies the detection zone. Standard for stop-bar detection and for left-turn bays.
- Pulse mode — a short fixed-duration call is issued when the vehicle arrives, regardless of dwell. Used for counting and for setback detectors where a stopped vehicle should not hold a permanent call.
Per-Detector Timers
- Delay — the detector must be occupied continuously for the delay period before a call is placed. Used on right-turn-on-red lanes so a vehicle that turns right on red does not needlessly call the phase.
- Extend / carryover — holds the call for a set period after the vehicle leaves the zone. Used to bridge gaps between short detection zones.
Phase Modifiers
- Dual entry — if one ring's phase is called after the barrier and the other ring has no call, the controller brings up a companion phase rather than leaving one ring dark.
- Conditional service — allows a left turn to be re-served within the same barrier group when time remains, typically re-serving phase 1 after phase 2 if the through gapped out early.
- Simultaneous gap-out — requires both rings to be in gap simultaneously before crossing the barrier. Without it, one ring can gap out, wait, and be re-extended by a late arrival, extending the whole barrier group.
A setback detector is located 200 feet upstream of the stop line on a 35 mph approach. What is the approximate minimum passage time needed so an approaching vehicle is not stranded?
A side-street phase maxes out every cycle throughout the night when almost no traffic is present. What is the most likely cause?
What does the volume-density variable initial feature do?
A right-turn lane with its own detector repeatedly calls the side-street phase even though most vehicles turn right on red and never wait. Which detector setting best addresses this?