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Coordinating, Developing, and Delivering Highway Transportation Innovations

 
REPORT
This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-17-048    Date:  May 2018
Publication Number: FHWA-HRT-17-048
Date: May 2018

 

Enhancing Safety and Operations at Complex Interchanges With Improved Signing, Markings, and Integrated Geometry

Chapter 5. Practices Evaluation

The project team conducted a practices evaluation prior to the final site selection process and at the beginning of the field study and simulator study. The purpose of the practices evaluation was to determine, by means of site visits and a scan of photographs and videos available to the project team, the variations in the application of engineering design undertaken by various States.

Subsequent to examining signing and pavement marking practices in various States, the project team assessed uniformity between different applications to help identify persistent inconsistencies in design and fabrication and to develop recommendations. The summary of practices here is the foundation for the recommendations in chapter 9, including recommended future research and policy evaluation activities.

Because the practices evaluation was not conducted as a research activity with data collection for statistical analysis, traditional methods of evaluating the applicability and effectiveness of practices were not suitable. Instead, the project team developed a new methodology for evaluating traffic engineering practices on the basis of a logic model. This model is referred to here as the consistency principle.

Defining Inconsistency

The outcomes from TCD installations are typically evaluated in the field using various proxy measures of effectiveness, including crash experience, conflicts, volume, and speed. In laboratory tests of TCDs, legibility, comprehension, and subject reaction to devices in a driving simulator are typically used as measures of effectiveness.

One significant drawback to these studies is that they are limited to the devices being used in the laboratory studies or the devices in place in the field. A review of the literature indicates that some devices considered effective by agencies are not included in laboratory tests, and the results of those tests occasionally indicate the use of a device that may, in fact, be less effective than devices already in use in the field. In addition, devices in laboratory tests and field tests are occasionally not compliant with the MUTCD or exhibit properties that are inconsistent with accepted traffic engineering practice.

The project team developed an independent logic model for evaluating TCD installations. This logic model is intended to facilitate comparisons between settings and implementations.

A setting is defined as a geometric design or layout with discrete characteristics in terms of number of lanes or the arrangement of lanes. The layouts in the simulator study are examples of settings. An implementation is defined as the use of a TCD or a system of devices with specific characteristics. The use of a particular warning sign in a particular location is an example of an implementation. An alternative implementation would be the use of a different warning sign in the same location or perhaps the use of the same warning sign but in a different location. In practice, implementations are applied to settings.

The consistency principle holds that implementations are reserved for specific settings and the use of an implementation across multiple, discrete, differing settings reduces the application of logic in identifying the setting based on the implementation. If left-hand curve warning signs are used only for left-hand curves, the left-hand curve sign (an implementation) will always indicate the presence of an upcoming left-hand curve (a setting). If a left-hand curve warning sign were used to indicate a right-hand curve, the road-user expectancy is violated.

To identify the various inconsistencies between settings and implementations, the project team created an applications matrix (see table 40). Using the inconsistency matrix, logic-based generic examples were created that can be applied to field cases.

Table 40. Applications matrix for inconsistency identification.
Prefix Name Description
C Consistent application Different settings
Different implementations
D1 Diametric inconsistency Different settings
Swapped implementations
D2 Broadening application Different settings
Identical implementations
S Erratic application Identical settings
Different implementations
U Unrelated application Identical or different settings
Unrelated implementations

Using the generic logic models, five consistency matrices can be created. One matrix illustrates true consistency while the others illustrate two types of inconsistency. The two that illustrate inconsistency among different settings are classified as implementation-dependent, using the prefix “D.” The two that illustrate inconsistency among identical settings are classified as setting-dependent, using the prefix “S.”

This logic model can be used to compare applications across locations with identical settings, across locations with dissimilar settings, and in segments leading up to an interchange where cross-sectional characteristics change along the length of the roadway.

Implementing Consistency

It is important to note that, whether or not a practice represented a departure from established standards such as those found in the MUTCD, the project team considered all practices that exhibited any inconsistency to be candidates for evaluation. Inconsistent application of implementations to one or more settings is the foundation of nearly all violations of what is commonly referred to as driver expectancy. Quite simply, a driver expects to see implementation A associated with setting A and not setting B, where the application of implementation B is expected. Failure to observe the consistency matrix in freeway design is especially critical because driver expectancy is often relied on in the absence of other cues, in situations where heavy traffic blocks the view of pavement markings or where large vehicles prevent signing from being visible.

Both broadening application and erratic application lead to inconsistent road-user expectations. Some practitioners argue that prescribing specific use cases for TCDs and indicating clear designs in the MUTCD is some type of a “secret code” that only practitioners will know and that few will practice. This viewpoint fails to consider that consistently applied TCD treatments, with narrow use cases and uniform applications, will lead to road users adapting to the treatments, recognizing the relationships, and reacting appropriately when presented with information in the form of TCD treatments.

Inconsistency Examples

An example of inconsistency that can be studied from the perspective of both field implementations and traffic engineering policy is the use of various arrows on overhead guide signs in interchanges with option lanes.

Option Lane Signing

The signing of option lanes on freeways and expressways is a contentious issue in traffic engineering practice today, having been the topic of numerous proceedings in the meetings of both the National Committee on Uniform Traffic Control Devices and the AASHTO Subcommittee on Traffic Engineering. The option lane is illustrated in depiction A3 in figure 10.

Figure 10 shows that there are indeed four distinct cases of freeway exit types. For each of these cases, the guide signing must be treated differently; common signing methods follow.

Configurations of exiting lanes. This line-art graphic shows that there are four distinct cases of freeway exit types. For each of the four cases shown, guide signing must be treated differently. The first case, labeled A1, depicts a one-lane exit. The second case, labeled A2, depicts a one-lane mandatory exit. The third case, labeled A3, depicts a two-lane exit with a mandatory-exit lane and an option lane. The fourth case, labeled A4, depicts two mandatory exit lanes.

Source: FHWA.

Figure 10. Graphic. Configurations of exiting lanes.

Figure 11 shows example signing for an exit with an option lane. Although this practice is included in the MUTCD, the MUTCD does not include the use of what will be referred to in this report as the discrete arrow method of signing for option lanes (see figure 12), which includes the yellow ONLY panel to distinguish between the option lane and the exit-only lane. Nonetheless, in current practice, approximately 35 States use the discrete arrow method of signing option lanes. Several States have also begun using the so-called APL signing, which this report will refer to as the blended arrow signing. Both the discrete arrow and blended arrow methods can use one arrow per lane, so the term “APL” can be considered a technical misnomer. An example of the two signing methods is illustrated in figure 12.

Example of exit-direction signs based on Manual on Uniform Traffic Control Devices figure 2E-12. This photo shows the angled-up arrow method with five arrows for three through lanes to the left and two exit lanes to the right. The overhead sign on the left includes a route shield (interstate 35) and the cardinal direction (North) and has three angled-up arrows indicating the three lanes that can be used to travel toward 35 North. The overhead sign on the right is an exit sign (Exit 233A) and includes the text “Southwest Blvd, Mission Rd” with two angled-up arrows indicating the two lanes that can be used to take the exit. There are red arrows placed within each lane, showing the direction that vehicles can travel from each lane.

Source: FHWA.

Figure 11. Photo. Example of exit-direction signs based on MUTCD figure 2E-12.

Figure 12-A. Graphic. Discrete arrow guide sign (conventional practice). This exit direction sign includes angled-up arrows over the two lanes that can be used to exit, with the right arrow inside of a yellow “ONLY” panel and the left arrow on the green background of the sign.

Source: FHWA.

A. Discrete arrow guide sign (conventional practice).

Figure 12-B. Graphic. Blended arrow guide sign (arrow-per-lane). This sign provides a route shield and cardinal direction for both the through destination and the exiting lanes, separated by a vertical divider line. The sign includes upward-facing arrows above each lane, and the rightmost arrow is surrounded by the E11-1a Exit panel and E11-1b Only panel indicating the exit only lane.

Source: FHWA.

B. Blended arrow guide signs (APL).(22)

Figure 12. Graphics. Two examples of guide signs for option lane signing.

A U.S.-wide review of practices undertaken as part of a previous research effort identified four examples of practices and policy that serve as evidence of inconsistency in the application of option lane signing:(30)

Design of blended arrow guide signs can be challenging for legend arrangements and other key factors. Achieving effective guide sign design may require training to convey the principles of guide sign design and the appropriate use of sign design software.

The exit-direction sign shown in figure 13 matches the design in MUTCD figure 2E-11.(22) It shows two exit-only lanes at the gore, although one of the lanes is an option lane. One potential reason given for the use of the signs in figure 13 in option-lane applications is that, if the sign is placed far enough down the gore, both lanes are indeed exit-only. However, the road user’s attention is focused on the upstream geometrics that they encounter and the sign should adequately display those conditions so that the user’s decision, made in advance of the gore, is informed by an appropriate indication of the geometrics.

Example of exit-direction signs. This photo shows signing for two exit-only lanes at the gore, although one of the two lanes is an option lane. There is a red circle around the exit-direction sign, drawing attention to the fact that the directional arrows show that both lanes exit. However, the arrows do not indicate that one of the lanes is an option lane. Four red arrows are placed within each of the lanes showing the direction that each lane travels; the left lane continues through, the center lane is an option lane that can be used to continue through or to exit, and the right lane is an exit lane.

Source: FHWA.

Figure 13. Photo. Example of exit-direction signs.

Observed Practices

The project team visited several major metropolitan areas not included in the partner States to identify additional situations that are not directly addressed in the MUTCD.

Geometric Design and Traffic Control Devices

The following subsections describe several geometric designs and signing options for exit and entrance ramps.

Closely Spaced Exit Ramp Terminals

In some instances where closely spaced exit ramp terminals exist, the strategy of co-locating advance primary guide signs with the distances to each exit was used. Where the exit-direction sign for the first exit was placed overhead, an advance primary guide sign for the second exit was also included, often with a distance to the exit given in feet rather than fractions of a mile.

Several States, including Minnesota, provided full roadside delineation between some closely spaced exits, typically in the form of panel-style, post-mounted, white delineators. In cases where shoulders were in place, transverse, angled markings were placed across the shoulder to discourage its use and clearly identify the point where the second exit taper begins.

States addressing closely spaced exit ramp terminals generally appeared to address navigation issues and guidance issues by using a combination of additional guide signs, explicit distance information, and increased pavement markings and delineation.

Entrance Ramp into an Upstream-Extant Exit-Only Lane

The TCDs used to address this scenario varied most widely among all of the scenarios studied. In some States, the auxiliary lane was treated with overhead “EXIT ONLY” advance primary guide signs and exit-direction signs in combination with dotted lane line and arrow pavement markings; regulatory signing; and occasionally, preemptive signing on the entrance ramp into the auxiliary lane. This scenario can also be addressed with a “THRU TRAFFIC MERGE LEFT” warning sign, which when placed on the entrance ramp, provides an unambiguous message.

Cloverleaf Interchanges

Most cloverleaf interchanges include short auxiliary lanes between the ramp terminals of the entering loop ramp and the subsequent exiting loop ramp. In some States, these auxiliary lanes are marked with standard broken lane lines, and overhead signing does not indicate an exit-only movement.

In other States, dotted lane line pavement markings are used to separate the mainline lanes from the short auxiliary lane. Few States use “EXIT ONLY” overhead signing for these movements. Generally, the stated reason is that the auxiliary lanes are very short, and because the sign is visible prior to the beginning of the lane, it may be mistaken as applying to the right-hand lane upstream of the loop ramp entrance and development of the auxiliary lane.

The project team is aware of some international sites that were signed with a type of diagrammatic sign showing the entering ramp, a short segment of concurrent roadway, and the exit ramp.

Signing for Exit-Only Lanes Near an Exit Ramp

Many locations inappropriately use “EXIT ONLY” panels on signing at or past the theoretical gore of an interchange. Per chapter 2E of the 2009 MUTCD, “EXIT ONLY” panels on overhead APL guide signs shall not be located at or near the theoretical gore. This inconsistency violates user expectancy where in one case (e.g., sign is not located at or near the theoretical gore) the exiting lane continues, and where in another case (e.g., sign is located at or near the theoretical gore), the lane is dropped and exits the highway.

Signing of Short Continuing Auxiliary Lanes After Exit Ramps

MnDOT has used short continuing auxiliary lanes, or “escape lanes” in geometric design for more than 20 yr. These lanes are typically a short-distance continuation of an auxiliary lane that allows traffic in the auxiliary lane to continue straight, if necessary, even though that practice is discouraged by signing. In most cases in Minnesota, the lane reduction for the escape lane is not signed, as it is typically signed as an exit-only lane at the point of departure. In New Mexico, recent freeway reconstructions have incorporated escape lanes into the design of interchanges with an auxiliary lane.

In figure 14, signing for a lane reduction near the exit is shown. Note that the option lane is signed as an exit-only lane because the lanes have been fully formed.

Escape lane signing that does not adhere to the consistency principle because the continuing lane is signed as an exit-only lane. This photo shows signing for a lane reduction at the point of the exit gore.

Source: FHWA.

Figure 14. Photo. Escape lane signing that does not adhere to the consistency principle because the continuing lane is signed as an exit-only lane.

Including Exit Numbers on Interchange Sequence Signs

Some agencies include exit numbers on interchange sequence signs. This practice appears to be more prevalent in dense urban centers where multiple exits occur in succession and the supplemental guide signing for the exits cannot uniformly be placed in sequence prior to just one exit ramp. In all observed cases, the exit numbers were placed to the left of the street name or route marker (the destination of the exit), and adequate space was provided between the exit number and the destination or route marker.

Arrangement of Legend on Exit Gore Signs

There are several methods for arranging legends on exit gore signs. In figure 15, the left-hand image displays an observed exit gore sign. Note that the legend “EXIT” is centered on the sign panel, and the exit-direction arrow is positioned in the lower right-hand corner of the sign, not aligned with any legend on the sign. An alternative used by some agencies is to group related elements together. To reduce the width of the sign, which is desirable in most gore areas, the right-hand image is used as the design practice in several States, including Minnesota. This modified design mitigates the issue of legend grouping each element of the sign on a separate line.

Figure 15-A. Photo. Exit gore sign with standard arrangement. This sign includes the directional arrow placed to the right of the exit number and letters.

Source: FHWA.

A. Exit gore sign with standard arrangement.

Figure 15-B. Photo. Exit gore sign with legend grouping applied. This sign includes the directional arrow placed below the exit number and letters.

Source: FHWA.

B. Exit gore sign with legend grouping applied.

Figure 15. Photos. Exit gore signs.

Applications of exit gore sign delineation were also found to vary with differing legend sizes, arrow sizes and types, and arrow angles.

Omitting Intermediate Advance Guide Signs With Lane Drops

One practice observed in multiple States was the omitting of intermediate advance guide signs in advance of lane drops—that is, where a mandatory exiting movement occurs. While the installation of three signs (at 1 mi, at ½ mi, and at the departure point) is desirable, some States omit the ½-mi sign, even when a continuing lane is being dropped. In addition, several States do not use distances on “EXIT ONLY” signing, a practice that appears to be related to maximum sign size for cantilever trusses. It should be noted the MUTCD reserves use of distances on lane drop signing to distances of ½ mi or greater.

Omitting Distances on Advance Guide Signs

Omission of the distances on advance guide signs is becoming more common on freeways; the project team believes that the cause is a desire to reduce the area of the sign in an effort to control costs.

The problem is especially apparent in the large blended arrow signs being installed in many urban areas in the United States. Figure 16 illustrates signing with no displayed distances to the exit (or exits, in the case of multiple exits from the mainline, which are also signed with this same approach in other places). The lack of displayed distances is shown by the two red rectangles toward the bottom of the sign. In addition, the red square encompassing the right side of the sign depicts how this sign design does not separate out the right two lanes onto a separate panel. Dividing information on separate panels was found to produce greater upstream final lane selection confidence according to the simulator study (see chapter 6).

New installation of blended arrow guide sign in North Carolina. This photo illustrates composite signing with no distances displayed to the exit—or exits, in the case of multiple exits from the mainline, which are also composite-signed with this same approach in other places. In figure 16, two red rectangles toward the bottom of two of the three sign segments show the lack of displayed distances. In addition, in figure 16, a red square encompassing the right-side segment of the sign highlights how this sign design does not separate the two right-exit-only lanes into a separate panel.

Source: FHWA.

Figure 16. Photo. New installation of blended arrow guide sign in North Carolina.

Inconsistent Arrow Design

Use of short-shaft arrows in place of the type B arrow is becoming prevalent (see figure 17). Recall that types B, C, and D are not permissible on freeway and expressway guide signs. Interviews have revealed that sign fabrication contractors are not using the MUTCD-standard arrows or the SHS designs but, rather, are using software-provided arrows manipulated to approximate what appears on shop drawings.

Figure 17-A. Graphic. Arrow underneath route shield. This sign includes the words “North to” placed in between the two route shields, with the destination name “Selah” on a second line below these. The arrow is placed to the right of the word “Selah”, below the rightmost route shield.

Source: FHWA.

A. Arrow underneath route shield

Figure 17-B. Graphic. Arrow next to route shield. This sign includes the word “North to” on a top line, followed below by the two route shields, with the directional arrow placed to the right of the route shields. Below this is the destination name “Selah”.

Source: FHWA.

B. Arrow next to route shield

Figure 17. Graphics. Two examples of inconsistent arrow design.

Use of Guide Sign Arrows

The MUTCD prescribes arrows per use on guide signs, in MUTCD figure 2D-2 displayed in figure 18. A comprehensive policy on the use of guide sign arrows is not currently available at the national level, although some States have more guidance for practitioners.

Graphic. Arrows for use on guide signs (Manual on Uniform Traffic Control Devices 2009, figure 2D-2). This black line-art graphic illustrates six arrows for use on guide signs. Left-to-right, they include: “Type A” up arrow, “Type A-Extended” up arrow, “Type B” shortened up arrow–thick, “Type C” right-pointing arrow, “Type D” shortened up arrow–thin, and “Down Arrow.” There is also text in the bottom left corner of the graphic which states, “Note: The ‘Standard Highway Signs and Markings’ book contains the details of these arrow designs.”

Source: FHWA.

Figure 18. Graphic. Arrows for use on guide signs (MUTCD 2009, figure 2D-2).(22)

Omitting Interchange Sequence Signs

In urban areas, the use of interchange sequence signs assists motorists in spatially locating their desired destination along the length of an upcoming segment of freeway. This is especially important in situations where the exit for a major interchange occurs prior to the exit for a service interchange where the overcrossing of the intersecting roadway is upstream of the major interchange.

Lane-Reduction Signing

The design of acceleration lanes typically takes into account the entrance ramp geometric design, the expected approach speed of vehicles on the entrance ramp, the expected speed of vehicles on the mainline, and the mainline and ramp volumes. Auxiliary lanes shorter than optimal can affect traffic operations when vehicles encounter an unexpected reduction in the physical number of lanes.

The primary issue causing complexity with acceleration lanes is the inconsistency in how they are signed and marked. Even within agencies, inconsistency of signing and marking practices leaves road users with insufficient information to form an expectation. Furthermore, the differences between treating short and long acceleration lanes, particularly with respect to signing, indicates a lack of understanding among practitioners of how explicit, case-specific pavement markings and the placement of warning signs and vertical delineation can assist road users in identifying the beginning of lane-reduction tapers for acceleration lanes.

In addition, the misapplication of signs in these areas can lead to road-user confusion and drastically increase the difficulty of comprehending the roadway geometric design. These misapplications include the following:

Other ongoing research is looking into these issues in more detail.

Pavement Markings and Delineation

The design of pavement markings for situations not specifically described in the MUTCD requires a practitioner to consider how drivers perceive the markings, and the importance of maintaining consistency of width, color, and pattern between applications. This is critical not only for pavement markings consisting of longitudinal and transverse lines, but also markings created with point markers (e.g., RRPMs and non-traversable areas created by means of delineator posts). The project team observed pavement marking implementations inconsistent with the consistency principle, including the following:

Typical Applications of Pavement Marking Patterns

Pavement marking patterns play an important role in providing road users with information on the status of a lane, whether continuing or not, whether there is a transition taper or not, and the type of restriction in the lane or restrictions for movement into and out of the lane. Of equal importance is the maintenance of markings, especially in areas where snow and precipitation are common, something undertaken to a higher degree of success in Europe for example.

In general, roadway facilities have six distinct classes of pavement marking pattern applications, outlined in table 41.

Table 41. Pavement marking patterns and typical uses.
Pattern Typical Dimension Use
Broken lane line 10-ft line/30-ft space Separates two continuing lanes
Wide dotted lane line (drop marking) 3-ft line/12-ft space Separates a continuing lane from a non-continuing lane subject to a downstream mandatory turn or exit movement
Dotted extension 2-ft line/6-ft space Separates a full-width lane from an area of transition, such as a lane development taper for a turn lane, continuance of an edgeline through an intersection, or between turning lanes within an intersection
Solid line Solid Separates a continuing lane from a non-travel lane, such as a shoulder or, when wider, separates a continuing lane from a non-continuing auxiliary lane such as a turn lane or other mandatory movement lane, or separates lanes designed for restricted use
Double solid line Solid Separates lanes where crossing from either side is prohibited
Solid line with
broken or dotted
lane line
Mixed Separates lanes where crossing from one side is permitted but crossing from the solid side is prohibited

The typical uses of pavement marking patterns here can be applied to various scenarios of continuing, non-continuing, and terminating lanes, using patterns in conjunction with each other, perhaps in double-line configurations.

In addition to width, color, and pattern, the texture of the pavement marking can also be important. In regions with limited snow removal activities, the use of textured and profiled markings has been used as a replacement for non-reflective raised pavement markers. These profiled markings cause a tactile sensation for road users, and the use of these markings, particularly in conjunction with roadside delineation, can be an effective mitigation against roadway departure crashes.

Figure 19 illustrates five geometric design settings. Depictions A, B2 and C show geometries that are commonly used in many States. An emerging practice observed in many States is the use of the dotted lane line for the geometry in depiction B1 and depiction BC. In heavy traffic, particularly in cases where the acceleration lane taper is of significant length, motorists may mistake the acceleration lane (BC) for an auxiliary lane (B1), and may fail to vacate the acceleration lane. Road users on the major facility may similarly mistake the lane and move into it, not realizing that the lane terminates. Even the use of a lane-reduction warning sign and lane-reduction arrows may be insufficient in heavy traffic, especially if auxiliary lanes are generally provided in locations along a corridor.

Graphic. Configurations of exiting and entering lanes. This black line-art graphic illustrates five geometric dotted-line design settings. From top to bottom, they include: A—separating thru and exit-only lanes; B1—dotted lines for exit lane with entering and exiting ramps; BC—dotted lines between thru land and entering ramp; and C—entering-ramp lane with combined dashed line/dotted line.

Source: FHWA.

Figure 19. Graphic. Configurations of exiting and entering lanes.

Inconsistent Use of Dotted Lane Lines

Preserving a distinction between these two patterns is critical to the effort engineers should undertake to provide consistency among usage cases. Some agencies use the same marking cycle for all dotted lines, regardless of the intended use. Other agencies have preserved the marking pattern for dotted lane lines that specifies a 12-ft gap, as opposed to a 9-ft gap, recognizing the importance of that larger ratio in preserving this distinction between the dotted extension marking pattern and dotted lane line marking pattern, especially when coupled with the use of wider lines for dotted lane line installations.

Comparison of the dotted lane line and dotted extension line applications in depiction BC of figure 19 reveals how the change in pattern is an effective way to indicate that the lane is approaching its termination point. The exclusive use of the dotted extension (i.e., avoiding broadening-usage cases) will ensure that road users interpret it as indicating an area of transition, a taper forming a lane or terminating a lane. In figure 20, the same marking pattern is used along the entire length of the left-hand acceleration lane, a left entrance from another freeway. The lack of advance lane-reduction arrows, roadside delineation, and signing for the lane-reduction taper could be mitigated with a transition in pavement markings, a cue to road users that the status of the lane is changing.

Photo. Left-hand acceleration lane terminating in a lane-reduction taper. The photo shows the marking pattern used along the entire length of a left-hand acceleration lane—a left entrance from another freeway.

Source: FHWA.

Figure 20. Photo. Left-hand acceleration lane terminating in a lane-reduction taper.

Omission of Dotted Extension Lines

The use of dotted extension lines in lane reductions was identified chapter 2 as a practice of several States. Many other States use dotted extension lines in lane addition tapers, as well, but typically only if the lane is a non-continuing lane. In figure 21, a freeway-to-freeway ramp is shown emerging from a tunnel in a left-hand curve. Added on to the left is a left-hand auxiliary lane, which serves as the deceleration lane for a left-hand exit from the C/D roadway.

Photo. Overhead view of entering ramp (lower right of image) with asymmetrical widening to the left in a left-hand horizontal curve. At its far left, this photo shows a freeway-to-freeway ramp emerging from a tunnel in a left-hand curve. To the left is a left-hand auxiliary lane, which serves as a deceleration lane for a left-hand exit from a collector–distributor roadway.

©Esri.

Figure 21. Photo. Overhead view of entering ramp (lower right of image) with asymmetrical widening to the left in a left-hand horizontal curve.(36)

In this location, the dotted extension line is not being used to delineate the edge of the continuing lane. However, in many States, such a marking would be applied in this case. A dotted extension line applied here would help guide left-turning traffic into the through lane, which atypically is to the right. In addition, while guiding traffic out of the left-hand lane, the dotted extension would also allow for visual perception of the widening by means of the increasing distance in the triangular area between the left edgeline and the dotted extension line. In figure 22, the superimposed red line graphic shows the location of the left edgeline in the area of the asymmetrical widening over and beyond a crest vertical curve.

Photo. Location of the left edgeline in an asymmetrical widening over and beyond a crest vertical curve. Figure 22 superimposes a red line graphic into the figure 21 photo to locate the left edge line in the asymmetrical widening over and beyond a crest vertical curve.

©Esri.

Figure 22. Photo. Location of left edgeline in an asymmetrical widening over and beyond a crest vertical curve.(37)

Observations at this location during free-flow traffic periods indicated that roughly two-thirds of vehicles continuing straight onto the freeway (not taking this left-hand service interchange exit) tracked into the auxiliary lane, such that at least one tire moved across the dotted lane line markings that begin beyond the railway overcrossing.

Omission of Lane-Reduction Arrows

In addition to using a change in longitudinal pavement markings to indicate a lane change, lane-reduction arrows can be particularly helpful at providing another cue to drivers that a lane change is required.

Use of Yellow Delineators

The use of yellow delineation adjacent to white pavement markings may be contributing to guidance task failures, particularly during inclement weather. For example, yellow delineation used on the left edge of a freeway while also being used on both sides of an exit gore (where white delineation should be used adjacent to the mainline lanes) may lead to confusion over the corresponding edge of the roadway.

RRPMs

While a change in marking patterns is useful to road users, a change in the marking patterns may prove detrimental if the change is not progressive. Examining the transition from a broken lane line to a dotted lane line to a dotted extension, for example, leads to the conclusion that the pattern becomes visually more restrictive as the road user moves through the patterns. When RRPMs are placed, their installation cycles typically correspond with the associated longitudinal pavement markings. Longer spacing between RRPMs is logically associated with a less-restrictive marking, missing markers notwithstanding. In fact, most agencies do not use RRPMs in transition areas, those being the development tapers of turn lanes and the termination tapers associated with lane reductions. Even if those areas are marked with dotted extension lines, the markers are omitted, partially to preclude the intensive replacement cycle due to traversing traffic but also because movement across those areas is encouraged, when desired by the navigation and piloting directives of the road users.

WSDOT uses substitutionary markers for pavement markings in certain cases, typically consisting of round, 4-inch, non-reflective, domed markers and RRPMs. Comparison of the double lane line marking pattern (most restrictive) and dotted lane line pattern (intended for information) in figure 23 reveals that the less-restrictive marking (the dotted lane line) features reflectors spaced at roughly one-half the interval of the more restrictive marking (the double lane line). A short drive along any exit-only lane reveals how disorienting and counterintuitive this marking pattern can be, especially in areas of horizontal curvature where edgelines may be supplemented with RRPMs at 20-ft intervals as well.

Graphic. Excerpt from Washington State Department of Transportation (DOT) Standard Plan M20.50-02. This black line-art graphic illustrates substitutionary markers for pavement markings that Washington State DOT uses in certain cases, typically consisting of round, 4-inch, nonreflective domed markers and raised reflective pavement markers, or RRPMs, represented by circles in the graphic. Most restrictive double RRPMs in double center line and double lane lines are likewise depicted.

Source: WSDOT Standard Plan M20.50-02.

Figure 23. Graphic. Excerpt from WSDOT Standard Plan M20.50-02.(38)

Institutional Observations

In addition to examining technical policy and its application in practice, the project team also examined institutional operational and management policies and practices to determine how those might affect the design and operation of complex interchanges. While further examination of these issues is outside the scope of this project, the initial investigation revealed three major policy characteristics.

First, the project team found that, in States with consistent signing practices and where application of TCDs most adhered to the consistency principle, a formal sign design training course was often present and actively provided across all agencies ranging from the State transportation department–level to local agencies and consultants. MnDOT publishes the agency’s sign design course online and offers additional content, including sign support selection and the preparation of signing plans. States with strong central traffic sign design expertise, yet lacking the training course, also demonstrated improved consistency in practice. In some of those States, central office staff assisted region staff in reviewing consultant-prepared plans, aiding in consistent design practices and ensuring that State traffic engineering policy was addressed in project development.

Secondly, the project team discovered that States with long-standing traffic engineering expertise and institutionalized funding for traffic engineering activities generally had comprehensive policy manuals and more consistent applications of TCDs, particularly on freeways. Some States have demonstrated statewide consistency in traffic engineering, owing to the centralized nature of the traffic engineering publications and the presence of large urbanized areas with well-developed freeway systems. In States with limited freeway networks, common errors in guide sign design and application were observed far more frequently.

Finally, State transportation departments’ central office–level management of large-format sign design, as is the case in Minnesota and Washington State, seemed particularly correlated to a reduction in fabrication errors and incorrect field installations.

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