4.2 Portable Changeable Message Signs (PCMS) & Arrow Boards (Display Modes, Placement, & Messages)

4.2 Portable Changeable Message Signs (PCMS) & Arrow Boards (Display Modes, Placement, & Messages)

Portable Changeable Message Signs (PCMS) Specifications

Portable Changeable Message Signs (PCMS) are dynamic, electronic traffic control devices designed to convey real-time, emergency, or variable traffic information to road users. Governed by MUTCD Section 6L.05, PCMS units are deployed when static warning signs cannot adequately convey changing field conditions, such as unexpected queues, lane closures, detours, adverse weather delays, or altered speed limits.

A standard full-size PCMS consists of a trailer-mounted LED matrix display panel powered by a battery bank supplemented by solar charging panels. The key hardware and operational parameters recommended by the MUTCD (these are Guidance, not Standard) include:

• Matrix Architecture: Displays feature either character-matrix (discrete 5x7 or 3x15 pixel character blocks) or full-matrix LED layouts capable of displaying text and approved graphical symbols.
• Display Capacity: Standard full-size PCMS displays are formatted to show a maximum of 3 lines of text per phase, with up to 8 characters per line (including spaces). This 3-line, 8-character matrix (24 characters total per phase) represents the optimal cognitive processing threshold for drivers traveling at highway speeds.
• Character Height & Legibility Distance:
  • All facilities: Letter height should be a minimum of 18 inches. The PCMS itself should be visible from 1/2 mile under both day and night conditions.
  • Incident-response and service patrol vehicles: A letter height as short as 10 inches may be used. Shorter letters may also be used on low-speed facilities provided the message is legible from at least 650 feet.
• Automatic Dimming Control: PCMS units must incorporate a photoelectric sensor that automatically adjusts LED brightness. The MUTCD requires only that PCMS automatically adjust their brightness under varying light conditions; the 50-percent dimming requirement applies to arrow boards, not PCMS to prevent driver glare and haloing, while ramping up intensity under bright sunlight to maintain high contrast.

PCMS Message Design, Phase Timing, & Prohibited Formatting

Because drivers have only a few seconds to read dynamic sign text while maintaining control of their vehicles, message composition is governed by strict human factors guidelines. MUTCD Section 6L.05 establishes the following operational guidance:

1. Phase Limitations: A PCMS message cycle must contain no more than 2 phases (screens). MUTCD Guidance is that a message should consist of no more than two phases; if more than two phases are needed, additional PCMS should be used, separated by at least 1,000 feet on freeways and expressways and at least 500 feet on other highways. If a driver approaching at 65 mph cannot read the message in 2 phases, the information is too complex and must be simplified.
2. Phase Display Timing:
   • Each phase must be displayed for a minimum of 2.0 seconds.
   • The total cycle time for a 2-phase message (Phase 1 + Phase 2) must not exceed 8.0 seconds.
   • Each phase should be capable of being understood by itself, regardless of the order in which it is read, and messages should be centered within each line of legend.
3. Standard 3-Line Message Formatting Rule:
   • Line 1 (The Problem / Hazard): Identifies what is occurring ahead (e.g., LEFT LANE, ROAD WORK, ACCIDENT).
   • Line 2 (Location / Distance): Specifies where the hazard is located (e.g., AT EXIT 12, 1 MILE, AHEAD).
   • Line 3 (Recommended Action): Instructs the driver on what action to take (e.g., MERGE RIGHT, PREPARE TO STOP, USE DETOUR).
4. Prohibited PCMS Practices:
   • No Scrolling or Moving Text: Text must never scroll horizontally across the screen or slide vertically.
   • No Dynamic Display Techniques: Animation, rapid flashing, dissolving, exploding, scrolling, and horizontal or vertical travel across the sign face are prohibited outright, except when the PCMS is used to simulate an arrow board display.
   • No Advertising or Non-Traffic Messages: PCMS displays must never present commercial messages, public service announcements, or generic text such as DRIVE SAFELY or HAVE A NICE DAY during active work zone operations.

+-------------------------------------------------------------------------+
|                    PCMS 2-PHASE MESSAGE DISPLAY STANDARD                |
+-------------------------------------------------------------------------+
|           PHASE 1 (Display >= 2.0 s)   |   PHASE 2 (Display >= 2.0 s)   |
|           +------------------------+   |   +------------------------+   |
|  Line 1   |  L E F T   L A N E     |   |   |  M E R G E             |   |
|  Line 2   |  C L O S E D           |   |   |  R I G H T             |   |
|  Line 3   |  1   M I L E           |   |   |  A H E A D             |   |
|           +------------------------+   |   +------------------------+   |
+-------------------------------------------------------------------------+

Arrow Boards: Specifications, Sizes, and Display Modes

Arrow Boards (also referred to as Arrow Panels) are high-intensity light displays used to supplement static advance warning signs and channelizing devices in temporary traffic control zones (MUTCD Section 6L.06). They provide crisp, directional guidance to advise approaching drivers of lane closures and split movements.

1. Arrow Board Classification & Minimum Dimensions (MUTCD Figure 6L-3):

   • Type A (48 inches x 24 inches): Features a minimum of 12 lamps. Appropriate for use on low-speed urban streets. Minimum legibility distance is 1/2 mile (2,640 feet).
   • Type B (60 inches x 30 inches): Features a minimum of 13 lamps. Appropriate for intermediate-speed facilities and for maintenance or mobile operations on high-speed roadways. Minimum legibility distance is 3/4 mile (3,960 feet).
   • Type C (96 inches x 48 inches): Features a minimum of 15 lamps. Intended for use on high-speed, high-volume motor vehicle traffic control projects. Minimum legibility distance is 1 mile (5,280 feet).
   • Type D (Flexible In-Vehicle / Trailer): Used for mobile operations.

2. Operational Display Modes & Legal Usage:

   • Lane Closure Modes (Directing Traffic Merge):
     • Flashing Arrow: Displays a solid arrowhead flashing in the direction of the open lane (LEFT or RIGHT).
     • Sequential Chevron: Displays a series of flashing chevrons pointing continuously toward the open lane.
     • Usage Rule: Flashing Arrow, Sequential Arrow, and Sequential Chevron modes shall be used only for stationary or moving lane closures on multi-lane roadways. Arrow boards shall be used only to indicate a lane closure and shall not be used to indicate a lane shift. They must never be used on two-lane, two-way roads where flaggers or temporary signals govern alternating one-way traffic.
   • Split Traffic / Multi-Lane Shift Mode:
     • Flashing Double Arrow: Displays arrowheads pointing simultaneously to the left and right.
     • Usage Rule: Authorized strictly for center lane closures on multi-lane roadways where traffic is permitted to pass safely on either side of the work obstruction. It is strictly prohibited for single-direction lane merges.
   • Caution / Warning Modes (No Merge Authorized):
     • Alternating Four-Corner Flash: Flashes four lamps positioned at the outermost corners of the panel.
     • Alternating Diamond Caution: Alternately flashes two diamond-shaped groups of elements.
     • Usage Rule: Caution mode signifies that a work operation is present along the shoulder or within a closed lane, but does not authorize or direct a lane change. Drivers are advised to proceed with caution in their established lane.

+-------------------------------------------------------------------------+
|                    ARROW BOARD OPERATIONAL DISPLAY MODES                |
+-------------------------------------------------------------------------+
| Display Mode            | Visual Pattern         | Authorized Application|
+-------------------------+------------------------+----------------------|
| Flashing Arrow (R or L) | <--- or --->           | Single Lane Closure  |
| Sequential Chevron      | <<< or >>>             | High-Speed Merge     |
| Flashing Double Arrow   | <--------->            | Center Split Only    |
| Four-Corner Caution     | *                    * | Shoulder Work Only   |
|                         | *                    * | (No Lane Change)     |
+-------------------------------------------------------------------------+

Field Placement, Lateral Offset, and Sight Distance Rules

Proper field installation of PCMS trailer units and Arrow Boards is vital to prevent secondary crashes and ensure drivers process displays well in advance of merge tapers:

• Placement of Arrow Boards:
  • For a stationary lane closure, the arrow board should be located on the shoulder at the beginning of the merging taper; where the shoulder is narrow, it should be located in the closed lane. A separate arrow board shall be used for each closed lane.
  • For mobile operations, the Arrow Board is mounted on the rear of a shadow vehicle equipped with a Truck-Mounted Attenuator (TMA) trailing the work convoy.
• Placement of PCMS Units:
  • Positioned upstream of the work zone, well in advance of the first static advance warning sign or upstream of the expected traffic queue tail.
  • Placed off the shoulder and behind a traffic barrier where practicable; where no barrier is available, off the shoulder and outside the clear zone. If it must be placed on the shoulder or within the clear zone, it shall be delineated with retroreflective TTC devices.
• Clear Zone Offset & Delineation:
  • When placed on the shoulder within the roadside clear zone, trailer units present a potential fixed-object hazard. They must be delineated with retroreflective channelizing drums or protected by temporary crash cushions.
• Aiming Angle & Glare Elimination:
  • Trailer units must be leveled using jack stands to ensure the display panel is perpendicular to the vertical grade.
  • To maximize LED target value while eliminating specular nighttime reflection glare (headlight reflection off the polycarbonate screen), the display panel should be sited and aligned to provide maximum legibility and to allow time for road users to respond; the MUTCD prescribes no specific aiming angle.