Traffic Control Plans (TCP's) play a vital role in providing continuity of
safe and efficient traffic flow, to the extent interruptions in normal flow
are necessary for temporary traffic control operations or other events that
must temporarily disrupt normal traffic flow. Important auxiliary provisions
that cannot conveniently be specified on project plans can easily be
incorporated into Special Provisions within the TCP.
A TCP describes traffic controls to be used for facilitating vehicle and
pedestrian traffic through a temporary traffic control zone. The plan may
range in scope from being very detailed, to merely referencing typical
drawings contained in the MUTCD, standard approved highway agency drawings
and manuals, or specific drawings contained in contract documents. The
degree of detail in the TCP depends entirely on the complexity of the
situation, and TCP's should be prepared by persons knowledgeable about the
fundamental principles of temporary traffic control and the work activities
to be performed.
Traffic control planning requires forethought. Provisions may be
incorporated into the project bid documents that enable contractors to
develop alternate traffic control plans, which may be used only if the
responsible agency finds they are as good as those provided in the
plans/specifications. For maintenance and minor utility projects that do not
require bidding, forethought must be given to selecting the best traffic
control before occupying the temporary traffic control zone. Also,
coordination must be made between projects to ensure that duplicate signing
is not used and to ensure compatibility of traffic control between adjacent
projects.
Modifications of TCP's may be necessary because of changed conditions or
determination of even better ways of handling traffic safely and
efficiently, while permitting efficient temporary traffic control activities
to progress.
The temporary traffic control zone includes the entire section of roadway
between the first advance warning sign through the last traffic control
device, where traffic returns to its normal path and conditions. Most
temporary traffic control zones can be divided into four areas: the advance
warning area, the transition area, the activity area, and the termination
area. Figure VI-1 illustrates these four areas. The four components
that constitute a temporary traffic control zone are described in the order
that drivers encounter them. They include the following:
Advance Warning Area
In the advance warning area, drivers are informed of what to expect. The
advance warning may vary from a single sign or flashing lights on a
vehicle to a series of signs in advance of the temporary traffic control
zone transition area. On freeways and expressways, where driver speed is
generally in the higher range (45 mph or more), signs may be placed from
500 feet to 1/2 mile or more before the temporary traffic control zone.
The true test of adequacy of sign spacing is to evaluate how much time
the driver has to perceive and react to the condition ahead. In this
regard, the use of speed, roadway condition, and related driver
expectancy must be considered in order to derive a practical sign
spacing distance. As a guide, table II-1 in section 2C-3 should be used
in conjunction with consideration of actual or anticipated field
conditions. Effective placement of warning signs for urban and rural
locals is as follows:
(1) Urban
Warning sign spacing in advance of the transition area normally range
from four to eight times the speed (mph) in feet, with the high end of
the range being used when speeds are relatively high. When single
advance warning signs are used (as in the case of low-speed
residential streets), the advance warning area can be as short as 200
feet. When two or more advance signs are used on higher-speed streets
such as major arterials, the advance warning area should extend a
greater distance. (See table VI-3.)
(2) Rural
Rural roadways are characterized by higher speeds. Spacing for the
placement of warning signs is substantially longer-from 8 to 12 times
the speed (mph) in feet. Two or more advance warning signs are
normally used in these conditions, the advance warning area should
extend 1,500 feet or more in open highway conditions. (See table
VI-3.)
Advance warning is normally not needed when the activity area is
sufficiently removed from the driver's path that it does not interfere
with traffic.
Transition Area
When redirection of the driver's normal path is required, traffic must
be channelized from the normal path to a new path. This redirection is
intended to occur at the beginning of the transition area. In mobile
operations, this transition area moves with the work space. Transition
areas usually involve strategic use of tapers, which (because of their
importance) are discussed in more detail in section 6C-3.
Activity Area
The activity area is an area of roadway where the work takes place. It
is composed of the work space and the traffic space, and may contain one
or more buffer spaces.
(1) Work Space
The work space is that portion of the roadway closed to traffic and
set aside for workers, equipment, and material. Work space may be
fixed or may move as work progresses. Long-term work spaces are
usually delineated by channelizing devices or shielded by barriers to
exclude traffic and pedestrians.
(2) Traffic Space
The traffic space is the portion of the roadway in which traffic is
routed through the activity area.
(3) Buffer Space
The buffer space is an optional feature in the activity area that
separates traffic flow from the work activity or a potentially
hazardous area and provides recovery space for an errant vehicle.
Neither work activity nor storage of equipment, vehicles, or material
should occur in this space. Buffer spaces may be positioned
longitudinally and laterally, with respect to the direction of traffic
flow.
(a) Longitudinal Buffer Space
The longitudinal buffer space may be placed in the initial portion
of a closed lane in advance of the work space, as shown in figure
VI-1. When a protection vehicle is placed in advance of the work
space, only the space upstream of the vehicle constitutes the buffer
space.
The longitudinal buffer space, as depicted in figure VI-2, should be
used where a closed lane separates opposing traffic flows.
Typically, it is formed as a traffic island and defined by
channelizing devices.
A guide for the length of longitudinal buffer space is shown in
table VI-1. The length may be adjusted to satisfy individual agency
needs.
(b) Lateral Buffer Space
A lateral buffer space may be used to separate the traffic space
from the work space, as shown in figure VI-1, or a potentially
hazardous area, such as an excavation or pavement drop- off. A
lateral buffer space also may be used between two travel lanes,
especially those carrying opposing flows. The width of the lateral
buffer space should be determined by engineering judgment.
Figure VI-1. Component parts of a
temporary traffic control zone.
*Posted speed, off-peak 85th percentile speed prior to work starting, or the anticipated
operating speed in mph.
________________ 1Based upon American Association of State Highway and Transportation Officials (AASHTO)
braking distance portion of stopping sight distance for wet and level pavements (A Policy on Geometric Design of
Highways and Streets, AASHTO, 1990, p. 120). This AASHTO document also recommends adjustments for the effect Of
grade on stopping and variation for trucks.
(4) Incident
Management Vehicle Storage Space
When work occurs on a high-volume, highly congested facility in an urban
area, it is optional to allow space to store emergency vehicles (e.g., tow
trucks) to respond quickly to traffic incidents. The storage space is
typically provided at the beginning or end of the activity area, or both.
An emergency vehicle storage area should not extend into any portion of
the buffer space.
Termination Area
The termination area is used to return traffic to the normal traffic path.
The termination area extends from the downstream end of the work area to
the END ROAD WORK signs, if posted. Conditions may be such that posting of
END ROAD WORK signs is not helpful. For example, the END ROAD WORK signs
should normally not be used if other temporary traffic control zones begin
within a mile of the end of the work space in rural areas, or about a
quarter-mile within urban areas. For normal daytime maintenance
operations, the END ROAD WORK SIGN is optional.
A common important element of a temporary traffic control zone is a roadway taper. Tapers may be used in both the
transition and termination areas. Tapers are created using a series of channelizing devices or pavement markings
placed to move traffic out of or into its normal path. Whenever tapers are to be used near interchange ramps,
crossroads, curves, or other influencing factors, it may be desirable to adjust the length of tapers. Longer
tapers are not necessarily better than shorter tapers (particularly in urban areas characterized by short block
lengths, driveways, etc.) , because extended tapers tend to encourage sluggish operation and to encourage drivers to
delay lane changes unnecessarily. The real test of taper length involves observation of driver performance after
traffic control plans are put into effect. Types of taper lengths are presented in table VI-2. The maximum space
between devices in a taper normally approximates the distance in feet of the speed in miles per hour ( i.e.: a 55
mph speed road should normally have devices spaced about 55 feet apart). Types of tapers are shown in figure VI-2
and the two-way traffic taper is shown in figure VI-3:
_________________ *L = Taper length in feet.
W = Width of offset in feet.
S = Posted speed, off-peak 85th percentile speed prior to work starting, or the anticipated operating speed in
mph.
Merging Taper
A merging taper requires the longest distances because drivers are required to merge with an adjacent lane
of traffic at the prevailing speed. The taper should be long enough to enable merging drivers to adjust their
speeds and merge into a single lane before the end of the transition. For freeways, expressways, and other
roadways having a speed of 45 mph or greater, the minimum length for merging tapers should be computed by a
formula L = W x S. For residential, urban, and other streets with speeds less than 45 mph, the formula L = (W X S2)/60
should be used. Under either formula, L is the taper length in feet, W is the lateral shift of traffic due to the
partially or fully closed lane (in feet), and S is the posted speed, the off-peak 85th percentile speed
prior to work starting or the anticipated operating speed. The formula L = (W X S2)/60 is used for
speeds less than 45 mph because slower traffic can merge safely in a shorter distance.
Shifting Taper
A shifting taper is used when merging is not required, but a lateral shift is needed. Approximately one-half L has
been found to be adequate. Where more space is available, it may be beneficial to use longer distances. Guidance
for changes in alignment may also be accomplished by using horizontal curves designed for normal highway speeds.
Shoulder Taper
A shoulder taper may be beneficial on high- speed roadways with improved shoulders that may be mistaken for
driving lanes (when work is occurring in the shoulder area). If used, shoulder tapers approaching the activity
area should have a length of about one-third L. If a shoulder is used as a travel lane either through practice or
during a temporary traffic activity, a normal merging or shifting taper should be used. An example of a shoulder
taper is presented in figure VI-2.
Downstream Taper
The downstream taper may be useful in termination areas to provide a visual cue to the driver that access is
available to the original lane/path that was closed. When a downstream taper is used, it should have a minimum
length of about 100 feet per lane, with devices spaced about 20 feet apart. An example of a downstream taper is
shown in figure VI-2.
One-Lane, Two-Way Taper
The one-lane, two-way traffic taper is used in advance of an activity area that occupies part of a two-way
roadway in such a way that a portion of the road is used alternately by traffic in each direction. Typically,
traffic is controlled by a temporary traffic signal or a flagger. A short taper having a maximum length of 100
feet with channelizing devices at approximately 20-foot spacings should be used to guide traffic into the one-way
section. An example of a one-lane, two-way traffic taper is presented in figure VI-3.
At detours, traffic is directed onto another roadway to bypass the temporary traffic control zone. Detours should be
signed clearly over their entire length so that motorists can easily determine how to return to the original
roadway.
At diversions, traffic is directed onto a temporary roadway or alignment placed in or next to the right-of-way,
e.g., median crossovers or lane shifts.
Figure VI-3. Example of one lane-two way traffic control.
Where traffic in both directions must, for a limited distance, use a single lane, provision should be made for
alternate one-way movement through the constricted section. Some means of coordinating movements at each end shall
be used to avoid head-on conflicts and to minimize delays. Control points at each end should be chosen to permit
easy passing of opposing lines of vehicles. At a "spot" obstruction, however, such as an isolated pavement
patch on roadways with lower speeds and adequate sight distance, the movement may be self-regulating.
Alternate one-way traffic control may be accomplished as appropriate by flagger control, a flag-carrying or official
car, a pilot car, traffic signals, or by using stop or yield control. This section discusses each of these traffic
control techniques. (See section 6E-2 for flagger qualifications.)
Flagger Method
Where a one-lane two-way temporary traffic control zone is short enough to allow visibility from one end to
the other, traffic may be controlled by either a single flagger or by a flagger at each end of the section. When a
single flagger is used, the flagger should be stationed on the shoulder opposite the obstruction or work space, or
in a position where good visibility and traffic control can be maintained at all times. When good visibility and
traffic control cannot be maintained by one flagger station, traffic may be controlled by a flagger at each end of
the section. One of the flaggers should be designated as the coordinator. Flaggers should be able to communicate
orally or with signals. These signals should not be mistaken for flagging signals. The use of radios may also be
desirable even though visual contact is possible.
Flag Transfer Method
Flag carrying is effective when the route is well defined. It should be employed only when the one-way
traffic is confined to a relatively short length of road, usually not more than 1 mile in length.
The driver of the last vehicle proceeding into the one-lane section is given a red flag (or other token) and
instructed to deliver it to the flagger at the other end. The opposite flagger, upon receipt of the flag, then
knows that it is safe to allow traffic to move in the other direction. The flag being carried should always be
clean and dry. A variation of this method is the use of an "official" car that always follows the last
vehicle proceeding through the section. The use of an official car eliminates the possibility of loss of the flag.
Pilot Car Method
A pilot car is used to guide a queue of vehicles through a normally complex temporary traffic control zone
or detour. Its operation must be coordinated with flagging operations or other controls at each end of the
one-lane section.
The pilot car should have the name of the contractor or contracting authority prominently displayed. The PILOT CAR
sign (G20-4) shall be mounted at a conspicuous location on the rear of the vehicle.
Two or more pilot cars may be used to guide two-way traffic through a particularly complex detour.
Temporary Traffic Signal Method
Traffic signals may be used to control vehicular traffic movements in temporary traffic control zones.
Traffic signals should also be considered for half-width bridge reconstruction on low- to moderate-volume
highways. Typical applications include highway or street intersections with a temporary haul road or equipment
crossing and through areas requiring alternating one-way traffic operations.
Stop or Yield Control Method
A yield or stop sign may be installed on low- volume, two-lane roads where one side of the
roadway is closed and the other side must serve both directions. The side that is closed should yield to or stop
for oncoming traffic on the side that is open. The approach to the side that is not closed must be visible (for a
distance equal to the safe-passing sight distance for that approach) to the driver who must yield or stop. See
section 3B-5, Warrants for No-Passing Zones at Curves.
Provision for effective continuity of transit service needs to be incorporated into the temporary traffic control
planning process. Oftentimes, public transit buses cannot efficiently be detoured in the same manner as other
vehicles (particularly for short-term maintenance projects). On transit routes, the TCP shall provide for features
such as temporary bus stops, pull-outs, and waiting areas for transit patrons.
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