8.1 Emergency Vehicle Pre-emption (EVP)

5.1 Emergency Vehicle Pre-emption (EVP)

Emergency Vehicle Pre-emption (EVP) is a specialized traffic signal operational mode designed to grant immediate, safe right-of-way to emergency response vehicles—such as fire engines, ambulances, and police units—as they approach a signalized intersection. By transitioning conflicting signal indications to red and displaying continuous green to the emergency vehicle's approach, EVP significantly reduces emergency response times and minimizes the probability of broadside (angle) crashes at intersections.

Unlike traffic signal priority systems, emergency pre-emption is a non-discretionary, high-priority override that temporarily interrupts normal background signal coordination and phase sequencing. However, controller pre-emption logic must strictly enforce established safety clearance intervals to prevent hazardous phase transitions.

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Detection & Emitter Technologies

Modern traffic signal controllers interface with three primary types of emergency vehicle detection systems: optical, radio-frequency/GPS, and acoustic.

Optical Pre-emption Systems

Optical systems utilize vehicle-mounted emitter strobes that flash at precise, coded pulse frequencies. Roadside optical detectors mounted on mast arms or span wires receive the light pulses, decode the frequency, and send a discrete contact closure or DC logic signal to the controller cabinet.

• High Priority (14.035 Hz): Standard frequency reserved for fire department apparatus, ambulances, and law enforcement vehicles responding to critical emergencies.
• Low Priority (9.639 Hz): Standard frequency assigned to transit vehicles or secondary emergency units.

GPS and Connected Vehicle (CV) Pre-emption

GPS-based pre-emption combines on-board Global Positioning System receivers with 900 MHz or 2.4 GHz spread-spectrum radios (or Cellular Vehicle-to-Everything, C-V2X). The vehicle continuously broadcasts its real-time position, speed, heading, and emergency equipment status (light bar/siren active) to a roadside unit (RSU). The controller calculates the Estimated Time of Arrival (ETA)—typically initiating pre-emption when the vehicle is 25 to 30 seconds away—regardless of curves, buildings, or adverse weather obstructing line-of-sight.

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The Four-Phase Pre-emption Operational Sequence

When a valid pre-emption call is received by the traffic signal controller, the software executes a structured, four-stage operational sequence:

[Entry / Call Registration] ──> [Transition / Clearance] ──> [Hold Phase] ──> [Exit Phase]

1. Entry / Call Registration Phase

Upon receiving an active low 24 VDC logic signal at the controller preempt input (e.g., Preempt 2 input on a NEMA TS 2 BIU or Model 170 C1 input), the controller immediately logs the call. If the conflicting movement currently holds green, the controller initiates green termination.

• Minimum Green Rule: If configured by agency policy, the active green phase may be truncated immediately or allowed to satisfy a shortened minimum green (typically 2.0 to 4.0 seconds) before yellow change.

2. Transition / Clearance Phase

During transition, the controller safely clears conflicting traffic and pedestrian movements:

• Mandatory Vehicle Clearance: The Yellow Change Interval (3.0–6.0 seconds) and Red Clearance Interval (1.0–3.0 seconds) MUST NEVER be shortened, omitted, or truncated under any circumstances. Overriding these intervals violates MUTCD Section 4D.27 and causes catastrophic driver expectancy conflicts.
• Pedestrian Clearance Truncation: MUTCD standards permit the controller to immediately abort active Walk and Flashing Don't Walk (FDW) displays to Solid Don't Walk during emergency pre-emption entry, provided minimum vehicle clearance intervals are fully displayed.

3. Hold Phase

Once conflicting phases complete their clearance intervals, the controller displays solid green to the designated pre-emption approach (the Hold Phase).

• Green remains active throughout the duration of the emergency vehicle approach.
• Hold Delay Timer: After the vehicle passes the detector and the preempt input drops, a configurable hold delay timer (typically 2.0 to 5.0 seconds) holds the green signal open to ensure the emergency vehicle (and any trailing apparatus) fully clears the intersection Conflict Zone.

4. Exit Phase

After the hold delay expires, the controller exits pre-emption mode:

• The controller selects configured Exit Phases (typically main-street green or non-conflicting side-street phases).
• In coordinated networks, the controller enters Offset Transition Mode (such as Smooth Dwell or Add/Subtract transition) to re-synchronize its background cycle to the system master clock without causing abrupt phase skips.

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Controller Cabinet Hardware Interface & Input Logic

In standard NEMA TS 1, NEMA TS 2, Model 170, and Model 2070 controller cabinets, pre-emption commands are routed through dedicated hardware inputs:

• Preempt Input 1: By industry standard, Preempt 1 is hardwired and logic-isolated for Railroad Pre-emption, possessing higher priority than all other inputs.
• Preempt Inputs 2 through 6: Designated for Emergency Vehicle Pre-emption and assigned prioritized hierarchy (e.g., Preempt 2 overrides Preempt 3).
• Electrical Isolation: Optical receivers and pre-emption phase selectors utilize opto-isolated relay outputs to pull the controller's 24 VDC logic inputs down to cabinet logic ground (0 VDC), ensuring electrical noise does not trigger false pre-emption events.