Parts of a temporary traffic control zone

Every temporary traffic control zone, from a two-hour curb lane closure on a city street to a months-long highway project, is built from the same four parts. Drivers meet them in order: first they are warned, then they are guided out of their normal path, then they pass the work, and finally they are returned to normal driving. When Mission Traffic Control LLC's traffic control supervisor, Nadia Kozak, trains new crew members, she starts by walking them along Larch Avenue in the direction of traffic, naming each part as they reach it. This lesson follows the same walk.

1. The advance warning area

The advance warning area tells drivers what is ahead before they reach it. It usually starts with a sign such as ROAD WORK AHEAD and continues with signs that explain what drivers will have to do: a lane closed ahead, a lane ending, a flagger ahead, a reduced speed or a detour. Drivers need time to read each sign, understand it, decide what to do and do it. That time translates into distance, which is why the advance warning area gets longer as speeds rise.

On a slow city street such as Larch Avenue, posted at 35 mph, the advance warning area might be only a block long, with signs a few hundred feet apart. On a state highway posted at 65 mph, it may stretch for more than half a mile, with signs 1,000 feet or more apart, and it may start with a portable changeable message sign several miles upstream warning of delays. The spacing comes from the manual's tables, which lesson 5 covers.

The advance warning area must account for the queue. If traffic is expected to back up past the first sign, drivers at the back of the queue get no warning at all and may run into stopped vehicles at full speed. Where queues are likely, the warning area is extended, signs are added further upstream, and on high-speed roads queue warning systems or a protection vehicle at the back of the queue may be used.

2. The transition area

The transition area is where drivers are moved out of their normal path. It is made with a taper: a line of channelizing devices, such as cones, tubular markers or drums, set at an angle to guide traffic out of the closed lane or shoulder and into the open lane. Different tapers do different jobs.

• Merging taper. Closes a lane on a multilane road, so drivers must merge into the adjacent lane. It is the longest taper because drivers must find a gap in moving traffic.
• Shifting taper. Moves traffic sideways into a new path without reducing the number of lanes, as when lanes are shifted onto a shoulder or temporary pavement. It can be about half as long as a merging taper.
• Shoulder taper. Closes a shoulder that drivers might otherwise use. It can be about a third as long as a merging taper.
• One-lane, two-way taper. Used with flaggers or temporary signals when one lane of a two-lane road is closed and traffic in both directions takes turns using the open lane. It is short, because drivers are stopped or slowed before they reach it.
• Downstream taper. Returns traffic to its normal path at the end of the work. It is short and is covered under the termination area.

Lesson 7 shows how to calculate taper lengths. The key point here is that the transition area is where drivers make their biggest change, so it must be long enough, clearly marked and free of surprises such as workers, parked trucks or materials.

3. The activity area

The activity area is where the work happens and where traffic passes it. It has three parts.

• Buffer space. An empty area between the transition area and the work space, or between the traffic and the work space at the side, that gives a driver who misjudges the taper room to stop or recover before reaching workers. A longitudinal buffer runs along the lane, upstream of the work; a lateral buffer runs between the traffic lane and the work space. Nothing should be stored or parked in a buffer, and workers should not stand in it. On high-speed roads, a protection vehicle with a truck-mounted attenuator is often parked at the upstream end of the work space, inside or just past the buffer, to absorb the impact of a vehicle that gets through.
• Work space. The area closed to traffic and set aside for workers, equipment and materials. It may be fixed, as for the water service connection on Larch Avenue, or it may move along the road, as for line painting or sweeping. It should be clearly marked with channelizing devices along its edge, and its own internal traffic, trucks backing in and out, is controlled by the internal traffic control plan.
• Traffic space. The part of the road where traffic passes the activity area. It must be wide enough for the vehicles that use the road, including buses and trucks, and free of drop-offs, debris and equipment that could catch a vehicle.

The edge between the traffic space and the work space is the most important line in the whole zone. Every worker should know where it is and stay on the work side of it. When the work requires someone to cross it, for example to adjust a sign or move a cone, the move should be planned and quick, made facing traffic, and ideally made when traffic has been stopped.

4. The termination area

The termination area returns drivers to normal. It often has a short downstream taper to let traffic move back into the closed lane, and an END ROAD WORK sign or similar to tell drivers the zone is over. Without a termination area, drivers may not know when they can return to normal speed or lane use, and some will stay slow and confused while others speed up early, right beside the end of the work space.

Lengths add up

The total length of a work zone is the sum of its parts, and on a short city block the parts may not fit. Nadia uses a simple program to check whether a planned closure fits between intersections before she draws it.

# Does a curb-lane closure fit on one block of Larch Avenue? (illustrative lengths in m)
block_length = 260          # distance between intersections
advance_warning = 0         # signs placed on the approach block, not on this block
merging_taper = 60          # from the taper formula in lesson 7
longitudinal_buffer = 65    # stopping sight distance at 50 km/h
work_space = 75             # gate apron and pump truck footprint
downstream_taper = 15
needed = merging_taper + longitudinal_buffer + work_space + downstream_taper
print(f"Closure needs {needed} m of the {block_length} m block")
spare = block_length - needed
print("Fits" if spare >= 0 else "Does not fit", f"(spare {spare} m)")
# A driveway inside the taper would need its own treatment
driveways = [("cafe parking lot", 90), ("office loading dock", 230)]
for name, x in driveways:
    zone = "taper" if x < merging_taper else "buffer" if x < merging_taper + longitudinal_buffer else \
           "work space" if x < merging_taper + longitudinal_buffer + work_space else "after work space"
    print(f"  {name} at {x} m falls in the {zone}")

Closure needs 215 m of the 260 m block
Fits (spare 45 m)
  cafe parking lot at 90 m falls in the buffer
  office loading dock at 230 m falls in the after work space

The check showed that the closure fitted, with 45 m to spare, but also that the café's parking lot entrance fell in the buffer space. Drivers turning into or out of the lot would cross the buffer. Mission Traffic Control LLC adjusted the layout so the driveway stayed open with a gap in the cones and a sign for drivers leaving the lot, and the gate crew knew to watch for cars turning across the buffer.

The advance warning signs for the closure were placed on the approach block, west of the intersection, so drivers would see them before turning onto the closure block. Where intersections and side streets feed traffic into the middle of a work zone, drivers entering from the side street also need warning, usually a ROAD WORK AHEAD sign on the side street.

The principles behind every zone

Traffic control manuals in both countries rest on the same basic principles, and Nadia makes new crew members repeat them until they can say them without thinking.

• Safety for workers and road users comes first, and the zone must be planned before the work starts.
• Inhibit traffic as little as practical. Close the fewest lanes for the shortest time, and do noisy or disruptive work outside peak hours where possible.
• Guide drivers clearly. Use standard signs and devices, give enough warning, and avoid sudden changes.
• Inspect routinely. Check the zone at set-up, during the shift and at night, and fix anything missing, damaged or moved.
• Pay attention to roadside safety. Keep drop-offs, equipment, materials and parked vehicles out of the clear zone where errant vehicles could hit them.
• Train everyone. Everyone who works in or plans the zone should be trained for their role.
• Keep the public informed. Tell residents, businesses, transit and emergency services about closures and detours.

Side streets, driveways and businesses

Real streets are full of entry points. Every driveway, side street and alley inside a work zone is a place where a driver can enter the zone without passing the advance warning signs, or turn across the buffer or work space. The plan should show each one and say how it is handled: kept open with a gap in the devices and a sign for drivers leaving, closed with the owner's agreement and an alternative access, or controlled by a flagger during truck movements. Business owners care a great deal about access, and a short conversation before the closure, explaining when and how their entrance will be affected, prevents most complaints.