Updated 2026-10-12

Why the control type matters to a MAP

A MAP describes geometry: lanes, nodes, connections and the signal group that governs each connection. A SPaT describes state: for each signal group, the current event state and when it is expected to end. The way the controller is run (pretimed, actuated, coordinated) does not change the MAP, but it changes what a vehicle should expect from the SPaT. The map builder ships four sample intersections, one for each of the common cases, so you can load one, generate the matching SPaT and compare.

The three drawn samples use real road centerlines and lane counts from OpenStreetMap, offset to 12 ft lanes. Their signal plans are written to illustrate each control type; they are not the agencies' timing sheets. The fourth is an unmodified MAP from the USDOT ITS Joint Program Office's Operational Data Environment sample data.

Pretimed (fixed-time) control

A pretimed controller runs a fixed cycle: each phase gets a set green, yellow and all-red every cycle, with no detection. Downtown grids and closely spaced signals on one-way pairs are often pretimed because the fixed cycle keeps progression predictable.

In SPaT, the controller knows exactly when every interval ends, so minEndTime and maxEndTime carry the same value (or maxEndTime is omitted). A vehicle can trust the countdown. The Park Ave sample runs a 60 s cycle: 32 s for the main street (phases 2 and 6), 20 s for the side street (4 and 8), with 4 s yellow and 1 s all-red at each change. All turns are permissive, so each shared lane's left, through and right connections point at the same signal group and the state is permissive-Movement-Allowed, never protected-Movement-Allowed.

Semi-actuated control

The main street rests in green. Detectors (loops, video, radar) on the side street and in the left-turn bay place calls; when a call arrives the controller serves that phase for a minimum green and extends it while vehicles keep arriving, up to a maximum. Typical for a collector or driveway meeting an arterial, and for most T intersections.

In SPaT the main street through phases show a minEndTime that keeps moving out while they rest in green. Once a side-street call is placed, the main-street maxEndTime becomes meaningful: the green will end by then. The Sunset Hills Rd & Business Center Dr sample runs the main street through phases 2 and 6 with a protected-only lead left (phase 5, which runs with phase 2 before 2 and 6 run together) and a single split phase 4 for the side street, serving its left and right lanes together.

Fully actuated control

Every approach has detection and every phase is called and extended by traffic. Cycle length floats with demand; phases that receive no call are skipped. This is the usual choice for isolated arterial intersections with heavy turning volumes.

SPaT from a fully actuated controller is the hardest for a vehicle to use: minEndTime is the earliest the green can end (after minimum green, or after the walk and pedestrian clearance if a pedestrian phase is timing) and maxEndTime is the maximum green. The true end lands between them and may jump when the controller gaps out. Agencies feeding SPaT to connected-vehicle applications often add a "likely time" from the controller (likelyTime and confidence in TimeChangeDetails) for exactly this reason. The Reston Pkwy sample publishes separate min and max times for all eight vehicle phases.

Coordinated control

Coordination ties a corridor of signals to a common cycle length and offsets so platoons meet green at each one. Each signal still runs actuated or pretimed locally, but the coordinated phases (usually 2 and 6 on the arterial) are guaranteed a window every cycle and the others are served in the time left over. In SPaT, the coordinated phases have reliable end times near the end of their split; the non-coordinated phases behave like actuated phases. A time-of-day schedule often switches between plans, so timing seen at 8 am and 2 pm differs.

Left-turn treatments

• Permissive. The left turn runs on the circular green with the through movement and yields to opposing traffic. In MAP the left connection carries the through movement's signal group. In SPaT the state is permissive-Movement-Allowed.
• Protected-only. The left turn runs only on a green arrow with the opposing through stopped. Required by most agencies when there are two left-turn lanes, when the opposing speed is high, or after a crash history. The left connection has its own signal group (1, 3, 5 or 7 in NEMA numbering) and the state is protected-Movement-Allowed while the arrow is on and stop-And-Remain otherwise.
• Protected-permissive. A green arrow (protected) followed by a permissive interval, often shown with a flashing yellow arrow. The same signal group changes from protected-Movement-Allowed to permissive-Movement-Allowed within one cycle. A vehicle must read the state, not only the group number.
• Lead and lag. A leading left runs before the opposing through (5 then 2 and 6); a lagging left runs after (2 and 6 then 5). Lag lefts suit protected-permissive operation and coordination; the SPaT sequence shows which the agency uses.
• Split phasing. Each approach of one street is served alone, with all of its movements together. Used when a left-turn lane cannot be fitted or when the approaches are offset. All lanes of the approach share one signal group, as the side street does in the T sample.

NEMA ring-and-barrier numbering

Most United States controllers number phases on two rings separated by a barrier. Ring 1 runs phases 1, 2, 3, 4 and ring 2 runs 5, 6, 7, 8; phases 1, 2, 5 and 6 are on one side of the barrier (the main street) and 3, 4, 7, 8 on the other (the side street). Even phases are through movements, odd phases are the left turns that oppose the next even phase: phase 1 opposes phase 2, phase 5 opposes phase 6. Pedestrian movements are usually numbered with the through phase they cross with (or offset, such as 22, 24, 26, 28 in these samples). The signal group in a MAP connection does not have to equal the controller phase, but almost every deployment keeps them the same because it makes the SPaT readable to the maintenance crew.

What to check when you build your own

1. Every connection has the signal group that actually controls it. A permissive left shares the through group; a protected left has its own.
2. Crosswalks have their own groups so a vehicle can see when pedestrians have the walk.
3. The intersection id and revision in SPaT match the MAP; see Mapping MAP to SPaT.
4. The control type is stated in the agency's timing sheet. Ask for the ring-and-barrier diagram, the time-of-day plans and the detector map; they answer most questions a MAP author has.

Where to read more

• FHWA, Traffic Signal Timing Manual (first edition FHWA-HOP-08-024, 2008; second edition published as NCHRP Report 812, 2015): the standard reference for phasing, left-turn treatments, actuated settings and coordination.
• FHWA, Signalized Intersections: Informational Guide (FHWA-SA-13-027): geometry and phasing choices by intersection type.
• Manual on Uniform Traffic Control Devices, Part 4 (Highway Traffic Signals): warrants, signal indications including flashing yellow arrows, and pedestrian signals.
• NEMA TS 2 and the ATC 5201 and 5401 standards: controller phase and ring definitions.
• SAE J2735, MAP and SPaT sections, and NTCIP 1202 v04 for controller-to-RSU SPaT data.
• Many state departments of transportation publish signal timing and design manuals (for example Virginia, Minnesota, Washington, Texas and Florida) that show their left-turn treatment rules and ring diagrams; search the agency's site for "signal timing manual" or "traffic signal design guidelines".