Photos of cars on freeway, speeding sign

Freeway Management and Operations Handbook

Chapter 6 – Roadway Operational Improvements
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6.1 Introduction

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 system operation.

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. 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 limit.
  • 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 crashes.
  • 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 signage). 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 below:

  • 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). 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 merging movements;
  • Identify routes and directions on those routes;
  • Show distances to destinations;
  • Indicate access to general motorist services, rest, scenic, and recreational areas; and
  • 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:

  • Signed route numbers. The MUTCD (1) now requires that all numbered routes be identified with signs.
  • Cross-street names, required in larger cities to inform motorists of their approximate location. Freeways often lack clearly distinguishable landmarks adjacent to the roadway.
  • Numbered Interchanges. The MUTCD (1) requires the numbering of interchanges on freeways, and recommends kilometer (mile) post numbering rather than consecutive numbering.
  • drawing showing an example of diagrammatic signing in advance of a freeway ramp where two routes diverge from one another

    Figure 6-1: Example of Diagrammatic Signing (Figure 2E-7 from MUTCD) D

  • Kilometer (Mile) posts must be place on all freeways (Ref. 1). Numbering begins at the south or west state border, or at the south or west beginning of the route. The MUTCD (1) also states that Reference Posts (e.g., mile markers at 0.1 mile intervals) may be installed along any section of a highway route to assist road users in estimating their progress, to provide a means for identifying the location of emergency incidents and traffic accidents, and to aid in highway maintenance and servicing.
  • Use of diagrammatic signs (i.e., guide signs that show a graphic view of the exit arrangement in relationship to the main highway, as shown in Figure 6-1) has been shown to be superior to conventional guide signs for some interchanges. (1)
  • Motorist service signs provide directions to food, fuel, lodging, hospitals, telephones, and similar services. This type of guidance is essential in advance of freeway exits because the business themselves may not be visible from the roadway.
  • Recreation and cultural interest area signs guide road users to a general area and then to specific facilities or activities within the area. Recreational or cultural interest areas are attractions or traffic generators that are open to the general public for the purpose of play, amusement, or relaxation. Recreational attractions include such facilities as parks, campgrounds, gaming facilities, and ski areas, while examples of cultural attractions include museums, art galleries, and historical buildings or sites. 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
  • 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).
drawing showing the sequence of signs for closely spaced freeway interchanges

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 the device.

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.
drawing showing pavement markings for two types of freeway exits: one involving a parallel deceleration lane, the other involving a tapered deceleration lane

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.

drawing showing pavement markings for two types of freeway entrances: one involving a parallel acceleration lane, the other involving a tapered acceleration lane

Figure 6-4: Entrance Ramp Markings (Figure 3B-9 from MUTCD) D 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 line markings.
  • 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 following recommendations:

  • 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 mph).
  • 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.