Chapter 6 – Roadway Operational
Page 1 of 2
In theory, the problems of congestion, safety, mobility, etc. would dissolve
with increases in capacity (i.e., adding more lanes, and new facilities)
and the reconstruction of existing facilities (wider lanes and shoulders,
improved alignment and geometrics) to improve safety. However, such major
roadway improvements (briefly discussed in the previous chapter) introduce
significant economical, political, financial, and societal challenges,
many of which cannot (and perhaps should not) be overcome. Operational
improvements often provide alternative, less expensive, and more practical
approaches for addressing freeway problems.
The rest of this Handbook is devoted to a discussion of these operational
strategies and the enabling technologies, including ramp management and
metering (Chapter 7), managed lane concepts (Chapter 8), HOVs (Chapter
9), traffic incident management (Chapter 10), planned special event management
(Chapter 11), freeway management during emergencies and evacuations (Chapter
12), traveler information (Chapter 13), transportation management centers
(Chapter 14), surveillance and detection (Chapter 15), regional integration
(Chapter 16), and communications networks (Chapter 17). As noted in Chapter
1, each of these operational approaches and strategies works effectively
under specific conditions. And while most improvements individually achieve
only modest reductions in congestion, when combined together in an overall
freeway management program, they can provide significant improvements.
Moreover, they can significantly improve the efficiency of the existing
network, while increasing the reliability and safety of the transportation
It is interesting to note that all of the operational strategies and
technologies discussed in the subsequent chapters of this Handbook have
some relationship with the National ITS Architecture (Note: Additional
information regarding the National ITS Architecture, including references,
is provided in Chapter 3 herein), whether it be user services, market
packages, subsystems / links in the architecture diagram, or some combination.
But as emphasized in earlier chapters, Intelligent Transportation Systems
should not be confused with improved operations. ITS
can be a significant subset / enabler of improved operations – for example
ITS can provide real time information on the traffic flows; but operations
is knowing what to do with this information to improve traffic flow, safety,
and mobility. Moreover, several other strategies – that have little or
nothing to do with ITS – can be implemented to enhance the operation of
the freeway. Examples include low-cost roadway improvements (as discussed
in Chapter 5); and the actions discussed in this Chapter.
6.1.1 Purpose of Chapter
This chapter provides a high-level overview of potential actions – specifically
static signing, pavement markings, and roadway lighting – that do not
modify the roadway footprint or geometry; nor are they usually considered
in the context of "real-time" freeway management strategies and enabling
technologies. Nevertheless, such operational improvements can improve
the operation of the freeway, particularly in terms of safety and driver
convenience and comfort. An overview of Travel Demand Management (TDM)
strategies is also provided at the end of this chapter. It is emphasized
that this chapter only provides an introduction to these other operational
improvements and strategies. For additional details and design guidelines,
the practitioner should consult a variety of references, many of which
are identified at the end of this chapter.
6.2 Background and Overview
Static signing, pavement markings, and roadway lighting can enhance the
safety of the freeway and overall driver convenience. Signs and markings,
by their very nature and purpose, serve to regulate, to guide, to warn,
and to channelize traffic into proper position on the roadway; and this
contributes to safer operation of the freeway. Improved visibility – as
provided by illumination – can also contribute to safety and driver comfort.
6.2.1 Key Considerations During Freeway Management Program Development
Static signing (including speed zoning), pavement markings, and roadway
lighting are all integral parts of the initial process to plan, design,
and construct (or reconstruct / rehabilitate) a freeway facility. In many
respects, they may be considered a "prerequisite" for freeway operations,
as the freeway infrastructure really can't be opened to traffic without
including the appropriate signing, markings, lighting, etc. However, just
because they have been in place and operating since "day 1", does not
preclude the need to routinely evaluate the performance of these attributes
(and all freeway management and operation strategies for that matter),
and to identify and implement improvements as may be required.
It is therefore important that the freeway performance monitoring effort
(discussed in Chapter 4) include measures that can help identify problems
associated with inadequate signing, markings, and / or lighting. For example,
a large number of night-time accidents might indicate the need for improved
lighting; a large number of accidents involving single vehicles running
of the road can indicate the need for better signage (e.g., chevron alignment
signs) and edge treatments (e.g., markings, rumble strips); and a history
of collisions and / or a wide variance in speeds in the vicinity of and
interchange might point to problems with guide signing.
6.2.2 Relationship to Other Freeway Management Activities
Static signing, markings, and lighting are integral parts of the other
operational improvements discussed in this Handbook. For example:
- The Manual on Uniform Traffic Control Devices (MUTCD – Reference 1)
includes specific sections on signing for preferential lanes and HOV
lanes (2B.48 – 50), signing for emergency management (2I), markings
for preferential lanes and HOV lanes (3B.22 – 23), as well as signing
for ramp metering.
- Signing is an important element of many incident management programs,
such as overpass names and 0.1-mile markers to help cellular phone
users in identifying the specific location of crashes and other incidents
that they report.
- In an area of inadequate geometrics that is experiencing a significant
number of collisions, signing (speed and / or warning) may prove effective
as an interim solution until the geometric deficiencies can be corrected.
6.3 Traffic Control Devices
Communication with the freeway motorist is a complex problem. The Manual
on Uniform Traffic Control Devices (MUTCD – Reference 1) is the national
standard for traffic control devices and their application. The Manual
presents criteria for the design and uniform application of signing, signalization,
painted channelization, and pavement markings. This Manual's text also
specifies the restriction on the use of a device if it is intended for
limited application or for a specific system.
The purpose of traffic control devices, as well as the principles for
their use, is to promote highway safety and efficiency by providing for
the orderly movement of all road users on streets and highways throughout
the Nation. Traffic control devices notify road users of regulations and
provide warning and guidance needed for the safe, uniform, and efficient
operation of all elements of the traffic stream. To be effective, a traffic
control device should meet five basic requirements:
- Fulfill a need;
- Command attention;
- Convey a clear, simple meaning;
- Command respect from road users; and
- Give adequate time for proper response.
The following aspects of traffic control devices should be considered
to ensure that the above criteria are met: design; placement and operation;
maintenance; and uniformity. Vehicle speed should be carefully considered
as an element that governs the design, operation, placement, and location
of various traffic control devices (1).
6.3.1 Static Signs
Section 2 of the MUTCD addresses traffic signs (Note: Signs for work
zones are addressed in Section 6 of the MUTCD), defined as a device
mounted on fixed or portable support whereby a specific message is conveyed
by means of words or symbols placed for the purpose of regulating, warning,
or guiding traffic. The MUTCD defines signs by their function as follows
– regulatory, warning, and guide.
220.127.116.11 Regulatory Signs
Regulatory signs give notice of traffic laws or regulations, the violation
of which constitutes a misdemeanor or felony. Freeway examples include
speed limit signs, lane use control signs, truck prohibitions, DO NOT
ENTER, and preferential (Diamond / HOV) lane signs.
One of the most common regulatory signs on freeways is the speed limit.
For those segments where the statutory speed limit (e.g., 55, 65, or 70
mph, depending on the State) is inappropriate, a reasonable speed limit
must be established and posted via speed zoning. These speed zones are
based on normal traffic conditions and favorable weather. Drivers have
the responsibility to adjust their speed in adverse conditions (or variable
speed limits may be implemented as discussed in Chapter 8 on Managed lanes).
Reference 2 (Fundamentals of Traffic Engineering) addresses speed zones,
stating that (per state vehicle codes) a systematic survey of physical
and traffic conditions on the highway section is required before a speed
zone is established. The following items should be considered in the survey:
- Spot Speed Distribution. The behavior of traffic provides
one indication of the appropriate speed limit on a highway section.
Speed limits are typically set at the 85th percentile speed (subject
to the maximum speed established by state law).
- Standard Deviation of the Speed Distribution. The dispersion
or spread of its speeds is a good indicator of the efficiency and safety
of a traffic stream. Locations with broad speed distributions often
indicate an artificially low speed limit and the need to modify the
- Accident Experience. Crash patterns on a road segment may
indicate the need for a lower or higher speed limit. For example, a
higher limit may result in a more uniform traffic flow, increasing some
speeds at the low end of the distribution and reducing the speed of
some fast drivers because of the more reasonable limit; while excessive
speed is often cited as a contributing factor to single-vehicle run-off-the-road
- Traffic Volumes. At higher volumes it is especially important
that the speed distribution's spread be as low as possible, both for
capacity and safety reasons.
Other factors to be considered include interchange frequency and spacing,
the simultaneous impacts of horizontal and vertical curve alignment, and
the roadside environment and potential distractions (e.g., advertising
18.104.22.168 Warning Signs
Warning signs call attention to unexpected conditions on or adjacent
to a highway that might not be readily apparent (e.g., physical features
such as curves, grades, low clearances, etc., and intermittent conditions
such as gusty winds and icy roads). Warning signs alert road users to
conditions that might call for a reduction of speed (i.e. speed warning)
or an action in the interest of safety and efficient traffic operations.
In addition to the "diamond"-shaped warning signs, chevron alignment
signs are often appropriate on sections of roadway and ramps where curvature
requires additional warning.
The MUTCD (1) states that "the use of warning signs should be kept to
a minimum as the unnecessary use of warning signs tends to breed disrespect
for all signs. In situations where the condition or activity is seasonal
or temporary, the warning sign should be removed or covered when the condition
or activity does not exist." Additional guidance is provided as summarized
- The use of warning signs shall be based on an engineering study or
on engineering judgment.
- Warning signs should be placed so that they provide adequate PIEV
time, where PIEV time represents the total time needed to perceive and
complete a reaction to a sign; the sum of the times necessary for Perception,
Identification (understanding), Emotion
(decision making), and Volition (execution of decision).
A table of advance placement distances is provided for various posted
speed limits and conditions (e.g., stopping required, high judgment
required, deceleration to a posted advisory speed).
- Minimum spacing between warning signs with different messages should
be based on the estimated PIEV time for driver comprehension of and
reaction to the second sign.
- Warning signs should not be placed too far in advance of the condition,
such that drivers might tend to forget the warning because of other
driving distractions, especially in urban areas.
- The effectiveness of the placement of warning signs should be periodically
evaluated under both day and night conditions.
In many instances, it may be possible to have "activated" warning signs
– that is, signs coupled with surveillance devices that automatically
indicate to a driver that he or she is moving faster than the recommended
speed or is overheight (as indicated on the warning sign).
22.214.171.124 Guide Signs
Guide signs show route designations, destinations, directions, distances,
services, points of interest, and other geographical, recreational, or
cultural information that may help drivers on their trips. The MUTCD (1)
states that guide signs on freeways and expressways should serve distinct
functions as follows:
- Give directions to destinations, or to streets or highway routes,
at intersections or interchanges;
- Furnish advance notice of the approach to intersections or interchanges;
- Direct road users into appropriate lanes in advance of diverging or
- Identify routes and directions on those routes;
- Show distances to destinations;
- Indicate access to general motorist services, rest, scenic, and recreational
- Provide other information of value to the road user.
The 15th edition of "Fundamentals of Traffic Engineering" (2) addresses
directional signing on high-speed highways as follows: "High traffic speeds
and the complex interchanges found on modern freeways present a special
directional signing problem. Control devices must guide motorists to the
correct lane or off-ramp, and must begin this guidance sufficiently early
to minimize last-minute lane changes. Unfamiliar drivers must rely almost
entirely on signing to reach their destination." The MUTCD (1) further
notes that "route signs and directional signs should be used frequently
because they promote safe and efficient operations by keeping road users
informed of their location". Examples of directional signing include:
126.96.36.199 Sign Location and Placement
The standard placement for most signs is on the right-hand side of the
roadway facing approaching traffic; supplementary signs in other locations
may be used. Signs should be placed to be visible only to the traffic
for which they are intended. The following principles should govern the
longitudinal and lateral placement of signs.
Longitudinal Placement – The longitudinal placement
of a sign must be coordinated with roadside features, including existing
guardrail and other signs.
- Regulatory signs are normally placed at or near the location where
the regulation exists or begins (e.g., truck restriction, speed zone);
additional signs may be used where a regulation (e.g., speed limit)
continues over an extended section of highway.
- Warning signs are typically placed in advance of the hazard. Guidance
for the longitudinal placement of warning signs (e.g., PIEV time) is
discussed in previous section 188.8.131.52.
- Guide signs are often placed in advance of an intersection or junction;
others, such as street name signs and kilometer (mile) posts, are located
at the point where they apply.
Positive guidance principles require that messages be spread over time
and distance, with preference normally given first to regulatory, then
to warning, and finally to guidance messages. Within these categories,
priority should be given to more critical upcoming elements (e.g., CURVE
AHEAD) rather than general warnings (e.g., DEER CROSSING).
Lateral Placement – Laterally, signs should be placed
within the driver's cone of vision, but not so close that they constitute
a hazard to an errant vehicle. The MUTCD (1) states that overhead signs
should be used on expressways, where some degree of lane-use control is
desirable, or where space is not available at the roadside. Overhead signs
have value at many locations. The factors to be considered for the installation
of overhead sign displays are not definable in specific numerical terms.
The following conditions (not in priority order) may be considered in
an engineering study to determine if overhead signs should be used:
- Traffic volume at or near capacity
- Complex interchange design
- Three or more lanes in each direction
- Restricted sight distance
- Closely spaced interchanges
- Multilane exits
- Large percentage of trucks
- Street lighting background
- High-speed traffic
- Consistency of sign message location through a series of interchanges
- Insufficient space for ground-mounted signs
- Junction of two freeways
- Left exit ramps
Overhead sign installations should be illuminated unless an engineering
study shows that retroreflectorization alone will perform effectively.
The freeway practitioner needs to be careful not to end up with too many
signs. Regulatory and warning signs should be used conservatively because
these signs, if used to excess, tend to lose their effectiveness (1).
Additionally, locations with potential for information overload should
be identified and corrected (3). Urban freeways and expressways are prime
candidates for information overload. The MUTCD (1) identifies the following
special sign treatments that may be desirable in these instances:
- Use of Interchange Sequence signs instead of the Advance Guide signs
for the affected interchanges (Figure 6-2).
Figure 6-2: Interchange Sequence Signs (Figure
2E-23 from MUTCD) D
- Use of sign spreading to the maximum extent possible (Note – Sign
spreading is a concept where major overhead signs are spaced so that
road users are not overloaded with a group of signs at a single location.)
- Elimination of service signing (e.g., fuel, telephone)
- Display of advance signs at distances closer to the interchange, with
appropriate adjustments in the legend
- Use of overhead signs on roadway structures and independent sign supports
- Use of diagrammatic signs in advance of intersections and interchanges
Finally, the practitioner must recognize that signs are easily damaged
due to impact or vandalism. Over time their visual quality will degrade
due to dirt and normal reflectivity deterioration. An agency's decision
to install signs must also include a commitment to continually maintain
6.3.2 Pavement Markings
Markings on highways have important functions in providing guidance and
information for the road user. Major marking types include pavement and
curb markings, object markers, delineators, barricades, channelizing devices
and islands. In some cases, markings are used to supplement other traffic
control devices such as signs, signals and other markings. In other instances,
markings are used alone to effectively convey regulations, guidance, or
warnings in ways not obtainable by the use of other devices (1). Specifically,
markings are used to:
- Display regulation (e.g., no passing)
- Supplement signs and other devices (e.g., symbol arrows, stop bars
at metered ramps).
- Guide traffic (e.g., lane lines and edge lines)
- Warn traffic (e.g., delineate gore areas).
The standards and recommendations contained in Section 3 of the MUTCD
(1) provide a basis for achieving uniformity of markings. Those markings
most applicable to freeways are summarized below:
- Lane lines are helpful in guiding traffic and in
achieving efficient utilization of the roadway. They are required on
all freeways and Interstate highways. Lane lines are normally broken
white lines, but a single solid line may be used to discourage lane
changes. Where crossing the lane line markings is prohibited, the lane
line markings shall consist of two normal solid white lines.
- Lane reduction transition markings should be used
whenever the number of lanes decreases. The transition is accomplished
using a taper with a length given by equations included in the MUTCD.
- Pavement edge lines are required by the MUTCD on
all freeways and expressways. Pavement edge lines on the right side
of the road are white and those on the median side are yellow.
- Channelizing lines (often referred to as gore striping)
may be used to form channelizing islands where traffic traveling in
the same direction is permitted on both sides of the island. A channelizing
line shall be a wide or double solid white line, and other pavement
markings in the channelizing island area shall also be white. New striping
can provide more visible patterns in ramp gore areas. Gore striping
can be very important on urban freeways, especially on drop lanes. As
shown in Figure 6-3, channelizing lines at exit ramps define the neutral
area, direct exiting traffic at the proper angle for smooth divergence
from the main lanes into the ramp, and reduce the probability of colliding
with objects adjacent to the roadway. At entrance ramps (Figure 6-4)
they promote safe and efficient merging with the through traffic. White
chevron markings may be placed in the neutral area for special emphasis.
These markings need to be highly visible, wear well, and often have
some type of tactile element to them (e.g., raised markers) to alert
drivers that they are crossing the neutral zone.
Figure 6-3: Exit / Lane Drop Markings (Figure 3B-8
/ Sheet 1 from MUTCD) D
Markings have limitations. Visibility of the markings can be limited
by snow, debris, and water on or adjacent to the markings. Marking durability
is affected by material characteristics and traffic volumes. Accordingly,
markings must be maintained and replaced on a recurring basis. In fact,
an important consideration for freeway management and operations is the
material used in markings. Paint, while the least expensive material,
wears the quickest and requires the most frequent renewal. Moreover, moving
painting operations can only occur at night on heavily traveled freeways.
The cost of renewed pavement markings and the potential for collisions
during these repainting operations can make paint less attractive than
methyl-methacrilite or thermo-plastic.
Figure 6-4: Entrance Ramp Markings (Figure 3B-9 from
184.108.40.206 Other Markings
In addition to pavement markings, other forms of marking are addressed
in Section 3 of the MUTCD (1) that can enhance roadway delineation and
hazard awareness. These include:
- Raised pavement markings are defined as devices with
a height of at least 10 mm (0.4 in) mounted on or in a road surface
that are intended to be used as a positioning guide, or to supplement
or substitute for pavement markings. The MUTCD addresses several applications,
including their use as vehicle positioning guides with longitudinal
markings, as a supplement to other markings, and as a substitute for
pavement markings. With respect to the latter application, retroreflective
or internally illuminated raised pavement markers, or nonretroreflective
raised pavement markers supplemented by retroreflective or internally
illuminated markers, may be substituted for markings of other types;
although raised pavement markers should not substitute for right edge
- Object markers are used to mark obstructions within
or adjacent to the roadway. These markers may consist of yellow retroreflectors
mounted symmetrically, horizontal, or vertically on a panel; or a striped
marker consisting of a vertical rectangle with alternating black and
retroreflective yellow stripes sloping downward at an angle of 45 degrees
toward the side of the obstruction on which traffic is to pass.
- Delineators (also referred to as guideposts) are
retroreflective devices mounted above the roadway surface and along
the side of the roadway in a series to indicate the alignment of the
roadway. They are considered guidance devices rather than warning devices.
Delineators are particularly beneficial at locations where the alignment
might be confusing or unexpected, such as at lane reduction transitions
and curves. Delineators are effective guidance devices at night and
during adverse weather. An important advantage of delineators in certain
locations is that they remain visible when the roadway is wet or snow
covered. The color of delineators must conform to the color of edge
lines. The MUTCD requires that single delineators be provided on the
right side of expressways and freeways and on at least one side of interchange
ramps, except in a few cases (e.g., on sections of roadways where continuous
lighting is in operation between interchanges. Guidance on delineator
placement and spacing for tangent and curved sections is also provided
in the MUTCD.
6.3.3 Rumble Strips
Addressed in Part VI of the MUTCD, rumble strips may be considered another
form pavement marking. Rumble strips are raised or grooved (indented)
patters installed on the pavement surface of a travel lane or a shoulder
intended to alert inattentive drivers through vibration and sound that
their vehicles are leaving the travel lane. On divided highways, they
are typically installed on the median side of the roadway as well as the
outside (right) shoulder.
Run-off-the road (ROR) crashes account for almost one-third of the
deaths and serious injuries each year on the Nation's highways. Inattentive
driving (due to distractions or fatigue) has been identified as a significant
causal factor in many of these crashes. A number of studies have demonstrated
the benefits of shoulder rumble strips an reducing death and serious injury
caused by inattentive drivers in ROR crashes (8). For example (as reported
in Reference 2), Caltrans has evaluated many of its safety projects to
determine what has been effective. On average, rumble strips resulted
in a 50% reduction in "drift off road accidents".
Rumble strips will not eliminate ROR crashes caused by excessive speed,
sudden turns to avoid on-road collisions, or high-angle encroachments.
Because they are intended to alert drivers "drifting" off the
road, rumble strips are most effective when installed near the edge line
adjacent to relatively wide shoulders. This placement provides motorists
leaving the traveled way at a shallow angle with both time and space to
steer back onto the roadway safely. Most states offset shoulder rumble
strips just outside the edge line of the travel lane by a distance of
100 mm (4 in) to 300 mm (12 in). This keeps the strip elements some distance
from the construction joint between the travel lane and shoulder; helps
reduce the number of inadvertent hits from passing traffic, especially
larger trucks; and allows for a substantial width of the paved shoulder
to remain available for other users of the shoulder. Rumble strips installed
at the outside edge of a shoulder with no useable recovery area beyond
the shoulder are of questionable value. Long sections of relatively straight
roadways that make few demands on motorists are the most likely candidates
for the installation of shoulder rumble strips (8).
Reference 8 (FHWA Technical Advisory on Roadway Shoulder Rumble Strips)
provides additional information regarding rumble strip types (e.g., milled-in,
rolled-in, formed, raised), design practices (e.g., location, spacing),
installation, and other references on the subject. It also provides the
- Continuous, milled shoulder rumble strips should be installed on rural
freeways and expressways as an effective means of reducing single vehicle,
run-off-road crashes caused primarily by any form of motorist inattention.
While they may be installed on a project-by-project basis, economies
of scale and timely implementation of shoulder rumble strips make system-wide
installation projects highly desirable.
- Regardless of the type of rumble strip element installed, shoulder
rumble strip usage should be coupled with continuing driver behavior
safety programs aimed at educating the general driving public on the
dangers of drowsy and inattentive driving.
- Rumble strips should not normally be used in urban or suburban areas
or along roadways where prevailing speeds are less than 80 km/h (50
- Where rumble strips are being installed for the first time or where
their use might be unexpected, appropriate signs and pavement markings
alerting both motorists and cyclists to their presence are advisable.