9.3 Temporary Traffic Control and Work Zones

9.3 Temporary Traffic Control and Work Zones

Temporary Traffic Control (TTC) plans protect workers, pedestrians, and motorists by safely routing traffic through work zones. TTC design is highly regulated by MUTCD Part 6. For the PE Civil Transportation exam, candidates must master the structural components of a work zone, taper length calculations, and device spacing standards.

The Five Components of a Work Zone

A standard temporary traffic control zone is divided into five distinct longitudinal areas. Each component serves a unique function and must be designed sequentially along the direction of traffic flow:

1. Advance Warning Area: This area alerts road users to the upcoming work zone, allowing them to adjust their speed and prepare for merging or shifting.

   • Signs: Typically includes three sequential orange warning signs: "ROAD WORK AHEAD", followed by "RIGHT LANE CLOSED AHEAD" (or similar), and "ROAD WORK XX FT".
   • Spacing: On freeways, sign spacing is typically $1,000\text{ ft}$ to $1,500\text{ ft}$. On rural highways, $500\text{ ft}$ is standard. On urban streets, spacing varies from $100\text{ ft}$ (low speed) to $350\text{ ft}$ (high speed).

2. Transition Area: This area redirects traffic out of its normal path. It is where physical lane closures or lateral shifts occur using tapers.

   • Devices: Cones, drums, barricades, and arrow boards.
   • Design: The length of the transition area is determined by the calculated taper length ($L$).

3. Buffer Space: A safety clearance zone that separates traffic from the active work area.

   • Longitudinal Buffer: Provides recovery space for out-of-control vehicles before they enter the work space. It must remain completely clear of workers, equipment, and stored materials. Its length is typically based on AASHTO stopping sight distances.
   • Lateral Buffer: Separates traffic from the work space horizontally.

4. Activity / Work Area: The space closed to traffic where the actual work takes place. It contains:

   • Work Space: Where workers, equipment, and materials are situated.
   • Traffic Space: The lane(s) designated for traffic to pass through the zone.
   • Buffer Space: Internal safety clearances.

5. Termination Area: This area returns traffic to its normal path.

   • Devices: A downstream taper may be used to guide vehicles back.
   • Sign: An "END ROAD WORK" sign is placed at the end of the termination area.

Taper Length Calculations

Taper length ($L$) is one of the most critical design calculations on the PE exam. The formulas for $L$ depend on the design speed ($S$) and the lateral offset or lane width ($W$):

1. High-Speed Formula (Speed $\ge 45\text{ mph}$)

$L = W \times S$

2. Low-Speed Formula (Speed $\le 40\text{ mph}$)

$L = \frac{W \times S^2}{60}$

Where:

• $L$ = Taper length (feet)
• $W$ = Width of offset or lane width to be closed (feet)
• $S$ = Posted speed limit, off-peak 85th-percentile speed, or anticipated operating speed (mph)

Taper Types and Length Multipliers

The MUTCD defines several types of tapers, each with a different length requirement based on the calculated value of $L$:

• Merging Taper: Used to close a lane and force traffic to merge. Minimum Length = $L$.
• Shifting Taper: Used where a lateral shift of the travel path is required (no lanes closed). Minimum Length = $0.5L$.
• Shoulder Taper: Used to close a shoulder. Minimum Length = $0.33L$.
• Downstream Taper: Used in the termination area to guide traffic back. Length = $50\text{ ft}$ to $100\text{ ft}$ per lane.
• One-Lane, Two-Way Traffic Taper: Used when one lane is shared by opposing traffic (controlled by flaggers). Length = $50\text{ ft}$ minimum to $100\text{ ft}$ maximum.

Channelizing Device Spacing

Channelizing devices (cones, drums, barricades) must be spaced close enough to prevent drivers from penetrating the work area, but far enough apart to be cost-effective.

• Inside the Taper: The maximum spacing (in feet) between devices is equal to the speed limit $S$ (in mph). $\text{Spacing}_{\text{taper}} = S\text{ ft}$
• Along Tangent Sections (Active Work Area): The maximum spacing (in feet) is twice the speed limit $S$. $\text{Spacing}_{\text{tangent}} = 2S\text{ ft}$

On low-speed urban streets, spacing is often reduced to half of the speed limit (e.g., $15\text{ ft}$ to $20\text{ ft}$ apart) to improve delineation and block off cross streets or driveways.

Portable Changeable Message Signs (PCMS) and Arrow Boards

Portable Changeable Message Signs (PCMS) are real-time information systems used to advise motorists of unexpected traffic conditions, lane closures, or speed reductions ahead.

• Message Constraints: A PCMS message must be limited to no more than two phases. Each phase must consist of no more than three lines of text, with a maximum of eight characters per line.
• Display Time: The display time for any single phase must be at least 3 seconds, and the total cycle time for a two-phase message should not exceed 8 seconds.
• Arrow Board Modes: Flashing arrow boards are used to guide traffic during lane closures. They must operate in one of three modes:
  • Flashing Arrow or Chevron: Used only for lane closures (cannot be used for shoulder closures or shifting tapers).
  • Flashing Double Arrow: Used when traffic can pass on either side of a split lane.
  • Flashing Caution Bar (or Four Corners): Used for shoulder closures, shifting tapers, or work near the roadway where no lane closure is present.

Flagger Safety and Standards

Flaggers are used to control traffic in single-lane, two-way operations. They are positioned in the advance warning area, visible to approaching traffic before the transition area.

• Hand-Signaling Devices: The primary device for flagger control is the STOP/SLOW paddle. Paddles must be at least 18 inches wide with letters at least 6 inches tall. Flags are only permitted in emergency situations.
• Illumination: If flagger operations occur at night, the flagger station must be fully illuminated with temporary lighting (minimum $5\text{ foot-candles}$ or $54\text{ lux}$) to ensure safety.