4.3 Transit Priority & Multimodal Signal Operations (TSP)

Transit Priority & Multimodal Signal Operations (TSP)

Transit systems operating on urban arterial corridors experience significant travel time variability and delay at signalized intersections—often accounting for 30% to 50% of total transit route run time. Transit Signal Priority (TSP) modifies traffic signal timing in real time to expedite transit vehicles (buses, streetcars, Light Rail Transit [LRT]) while minimizing disruptive impacts to general traffic and pedestrians.

1. Transit Signal Priority (TSP) vs. Emergency Vehicle Preemption (EVP)

A critical distinction in traffic signal operations is the boundary between Priority and Preemption:

+-----------------------------------------------------------------------------+
|                   PREEMPTION (EVP) VS. PRIORITY (TSP)                       |
+-----------------------------------------------------------------------------+
| Operational Dimension      | Emergency Preemption (EVP) | Transit Priority (TSP)   |
+----------------------------+----------------------------+--------------------------+
| User Class                 | Fire, Police, EMS, Trains  | Transit Buses, LRT, BRT  |
| Control Type               | Hard Preemption            | Soft / Conditional Mod.  |
| Coordination Cycle Impact  | Drops Coordination / Abrupt| Preserves Cycle Length & |
|                            | Termination of Non-EVP     | Offsets (Smooth Transition|
| Safety Intervals           | Must serve Yellow/All-Red; | Must serve full Yellow,  |
|                            | may truncate Ped Walk/FDW  | All-Red, Walk, & FDW     |
| Recovery Process           | 2 to 4 cycles transition   | Zero to 1 cycle recovery |
+-----------------------------------------------------------------------------+

• Preemption (EVP): Immediately interrupts normal signal operation, terminates conflicting phases (abbreviating minimum greens and pedestrian clearances if programmed), and forces an immediate green signal to clear the emergency path.
• Priority (TSP): Modifies normal phase timing within existing cycle constraints, adjusting green splits or phase sequence without violating minimum green times, pedestrian Walk/Flashing Don't Walk intervals, or dropping coordination.

2. TSP Operational Strategies: Passive vs. Active

A. Passive Priority

Passive priority requires zero real-time vehicle detection. Signal timing plans are developed offline to favor transit operating characteristics:

• Progressive Offsets for Transit Speeds: Setting corridor progression offsets based on bus travel speeds ($12\text{–}18\text{ mph}$) rather than passenger-car free-flow speeds ($30\text{–}40\text{ mph}$).
• Transit-Weighted Optimization: Using optimization software (e.g., Synchro, TRANSYT-7F) where passenger-car delay penalty weights are multiplied by average vehicle occupancy (e.g., weighting a bus with 40 passengers 30 times higher than an SOV with 1.2 passengers).
• Shorter Cycle Lengths: Reducing system cycle lengths to shorten maximum red wait times for transit vehicles.

B. Active Priority

Active priority responds dynamically to real-time transit detection:

1. Green Extension: If a transit vehicle approaches near the end of the scheduled green interval, the green phase is extended by $\Delta g$ (typically $5\text{–}20\text{ s}$) up to a programmed maximum ($G_{max,ext}$), allowing the bus to clear without stopping.
2. Early Green (Red Truncation): If a bus arrives during a red phase, the duration of conflicting phases is shortened (subject to statutory minimum greens and pedestrian FDW clearance) to return to the transit phase earlier.
3. Phase Insertion: A dedicated, non-actuated phase (e.g., a 4-second transit queue jump phase) is inserted dynamically into the cycle only when a transit vehicle is detected.
4. Phase Rotation (Phase Swapping): Reversing the sequence of leading and lagging left-turn phases to align green time with approaching transit.
5. Conditional Priority: The Priority Request Generator (PRG) requests priority only if the bus is running behind schedule (e.g., $> 3\text{ minutes}$ late) or carrying a high passenger load, preventing unnecessary disruption to crossing traffic.

3. TSP System Architecture & NTCIP 1211 Standards

Modern TSP systems rely on connected communications between the vehicle, roadside controller, and central traffic management:

+-----------------------------------------------------------------------------+
|                        TSP SYSTEM ARCHITECTURE                              |
+-----------------------------------------------------------------------------+
| Component                  | Functional Role & Communication Protocol       |
+----------------------------+------------------------------------------------+
| Automatic Vehicle Location | GPS/Dead Reckoning on bus determines exact     |
| (AVL)                      | coordinates, speed, and schedule deviation.    |
| Priority Request Generator | On-board processor formats Priority Request    |
| (PRG)                      | message based on lateness and passenger load.  |
| Roadside Unit (RSU) / Comms| Transmits request via DSRC / C-V2X (SAE J2735) |
|                            | or cellular MQTT/NTCIP to wayside cabinet.     |
| Priority Request Server    | Software module (in cabinet or central server) |
| (PRS)                      | that arbitrates competing requests (NTCIP 1211)|
| Advanced Signal Controller | Executes split extension/truncation and        |
| (ASC - NTCIP 1202)         | maintains phase concurrency and min clearance. |
+-----------------------------------------------------------------------------+

Detector Placement & Advance Time ($t_{advance}$)

To execute an early green or green extension smoothly, the Check-in Detector must be located at a travel time distance equal to the desired advance notice ($t_{advance} \approx 10\text{ to }20\text{ s}$):

$D_{check\text{-}in} = 1.47 \cdot V_{\text{bus}} \cdot t_{advance}$

Where $V_{\text{bus}}$ is average bus approach speed (mph). The Check-out Detector is placed at the intersection stop line or downstream departure curb to immediately cancel the priority state and return excess time to non-priority phases.

4. Multimodal Signal Treatments: Queue Jumps, Far-Side Stops, & Pedestrian Intervals

Queue Jump Lanes & Leading Bus Intervals (LBI)

A queue jump lane combines a dedicated right-side transit/turn lane with a specialized signal phase. When the queue jump phase begins:

1. The transit signal displays a white vertical bar (MUTCD Part 4D) for $3\text{ to }5\text{ seconds}$ while all general-purpose traffic faces a steady red indication.
2. The transit bus accelerates into the downstream receiving lane ahead of queued general traffic, bypassing arterial bottlenecks.

Bus Stop Placement: Near-Side vs. Far-Side

Bus stop location is the single most critical factor in TSP reliability:

• Near-Side Stops (Upstream of Intersection): Highly variable passenger boarding dwell times occur between the check-in call and intersection arrival. If a bus checks in but dwells at the curb for 30 seconds, granted green extensions expire before the bus departs, wasting arterial split time.
• Far-Side Stops (Downstream of Intersection): The bus approaches and clears the intersection before dwelling. Travel time from check-in to stop line is highly predictable, maximizing TSP success rate ($> 90%$).

Multimodal Safety: Leading Pedestrian Intervals (LPI)

Under MUTCD 11th Edition standards, a Leading Pedestrian Interval (LPI) provides a WALK display for $3\text{ to }7\text{ seconds}$ before the corresponding vehicular green is displayed. This allows pedestrians to establish their physical presence in the crosswalk, dramatically reducing turning vehicle conflict crashes.