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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-152
Guidelines for the Use of Raised Pavement Markers
SECTION 2. RPM GUIDELINES
The following section will present a collection of recommended practices along with future research issues, which can be used to complete an official set of guidelines for RPM use.
GENERAL DELINEATION REQUIREMENTS
Before discussing how RPMs should be used to delineate roadways, it is necessary to establish in general the type of delineation that is required by motorists.
Justification: Blaauw (1985) states that, although a centerline provides lateral vehicle control inside the lane, it does not provide sufficient preview information–edgelining is therefore required.(17) Good and Baxter (1986) found that continuous treatments were favored over discrete, and that wide edgelines were most beneficial to driver observation–control processes.(18) Freedman et al. found that edgelines on roadways greater than 6.1–m (20–ft) wide are recommended by several sources when they conducted a literature review. Although they found mixed results in research concerning overall accident reduction, a significant reduction in fatal and injury–producing accidents was found.(3) The MUTCD currently states that a normal width of a longitudinal line should be 101.6–mm (4–in) to 152–mm (6–in) wide (1986).(1) Pennell (1993) recommended that painted stripes (including edgelines) be applied with a 101.6–mm (4–in) width.(19) It has been noted by the Roadway Delineation Practices Handbook (1994), however, that edgelines as wide as 203.2 mm (8 in) be considered to provide the elderly driver with better preview distances.(2) However, this possibility was investigated by Hall (1987), who found no significant improvement in run–off–the–road accident occurrence; as a result, it was recommended that the use of these 203.2–mm (8–in) edgelines be discontinued.(20) Good and Baxter also investigated the use of wide edgelines and found that a width of 150 mm (5.9 in) was in fact beneficial to the driver; a combination of wide edgelines and Post–Mounted Delineators (PMDs) (or RPMs)–for short–range and long–range information, respectively–would be ideal.(18) Both lane line boundaries should therefore be delineated with RPMs supplementing the center line. According to Pennell, RPMs should not supplement right edgelines unless: (a) there is a fixed obstruction (e.g., guardrail, trees, poles, bridge rails, ditches) or there are difficult traveling surfaces exceptionally close to the edge of the roadway, or (b) there is a lane drop or road width reduction.(19)
4. Guidelines should include specific information as to when and where left edgelines should be supplemented by RPMs, perhaps using lighting and ADT criteria.
5. Good and Baxter indicated that further studies with larger sample sizes should be conducted to confirm their results.(18) It is also imperative that the study include a sample of older drivers to ensure that the benefits of larger, more clear delineation to the older population is not discounted. Studies should also be conservative enough to include potential benefits of wide edgelines to drivers when faced with adverse weather conditions.
The Roadway Delineation Practices Handbook needs to be more specific about where to and where not to use RPMs.(2) The traffic engineer should be able to assess whether RPMs are needed on the basis of variables such as ADT, street lighting, road geometry, and road structure. Some research suggests the use of RPMs under the following situations:
Justification: Freedman et al. (3) reviewed several studies with respect to the effects of RPM delineation on lane position and speed, and found that the above practices produced positive results. For example, Niessner's study (1984) (cited in Freedman et al.) found that delineation of center and edgelines, where there are pavement width reductions at a narrow bridge, produced reductions in 85th percentile speeds and centerline encroachments. Niessner also reports that RPMs reduce erratic maneuvers when used at painted gores, exits, and bifurcations. Taylor et al. found that RPM delineation of gores, exits, and bifurcations provides advance warning and geometry definition of roadways, plus an added rumble effect if the vehicle strays.(13)
Justification: Pennell recommends the above practices.(19)
6. It would be useful to provide separate guidelines for those States that will invariably install snowplowable RPMs as opposed to non–snowplowable RPMs-especially since the cost–effectiveness may vary between States on the basis of weather conditions.
7. The cost–effectiveness/performance tradeoff should be investigated in order to determine whether RPMs should be used on road sections with broken lines. If RPMs are used to delineate a passing zone, they will be subjected to many impacts, reducing their life span when compared with other uses. However, a recent study by Zwahlen and Schnell (1996) suggested that longer preview distances would be beneficial where passing is allowed on both sides of the road (dashed line sections); their study showed that the driver had the worst and shortest preview conditions (2.14 s) when driving through these sections.(21) More information is required before making a recommendation.
The Roadway Delineation Practices Handbook outlines which color of RPM should be used on roadways and whether double or single RPMs are required. This information is presented in a set of 11 figures (figures 40 through 50) in chapter 7 of the handbook.(2) The following guidelines are included.
The Roadway Delineation Practices Handbook Standards:
8. Pennell recommended that guidelines for RPMs also include the following: clear markers should supplement white lines, and when two–way markers are used for motorist guidance, the nonreflective side should face wrong–way motorists as opposed to a red reflective side.(19) These factors contradict the current guidelines; further studies are needed to determine which approach is best.
9. Color coding with RPMs is not a current delineation practice, but it was suggested for use by earlier work in areas such as no–passing zones or merge/diverge areas. For example, a no–passing zone might be delineated with white markers at long spacing and yellow markers at short spacing, or by delineating a solid yellow line with markers as a warning where a solid white line is currently used (Taylor et al.).(13) Current standards outlined by the Roadway Delineation Practices Handbook only describe the use of colored RPMs to supplement the color of paint.(2) It might be useful to determine whether the color of RPMs might provide the driver with additional information.
The Roadway Delineation Practices Handbook presents a set of 11 figures (figures 40 through 50) that outlines where on the road RPMs should be placed.(2) Several roadway geometries and situations are included; the delineation handbook figures should be consulted for exact placement requirements and for visual reference.
The Roadway Delineation Practices Handbook Standards:
The Following Guidelines Should Also be Included:
Justification: Pennell suggested these two additional guidelines after reviewing several research studies.(19) The emphasis on snowplowable RPMs is useful to ensure that all States can be accommodated.
Justification: According to Zwahlen (1987), if a 0.31–0.61 m (1–2 ft) solid body of grass or snow exists on both sides of the curve, the view of the curve's inner edgeline may be limited. RPMs on the outer edge would provide the driver with a longer arc, which is exposed to a direct line of sight.(22) This would allow the driver to have a longer preview time of the curve and improved outline for easier perception. The angle of the RPM should be adjusted for curves as recommended in the second guideline above because when the front surface of the RPM is perpendicular to the driver's line of sight, which intersects the outer edgeline at 30.5 m (100 ft) in front of the car, the photometric performance of the RPM will be improved.(22)
Justification: Results by Blaauw show this configuration to be best.(17)
10. The current standards need to be specific for all situations. Currently, the Roadway Delineation Practices Handbook indicates in figure 40 (page 88) that RPMs should be placed inside double–yellow centerlines when used on two–lane, two–way roads, yet figures 44, 45, 47, and 48 all show situations where RPMs are placed outside double–yellow centerlines when used on two–lane, two–way roads.(2) The Handbook needs to clarify which pattern should be used under which circumstances.
11. Although recommendations currently suggest that ramps are delineated with RPMs on the left side only, according to Taylor et al., the "pathway to be driven within" should be defined as opposed to the "line to stay next to." They suggested placing RPMs on both edges for ramps or diverging roadways.(13) According to Kahn (1979) (cited in Migletz et al.) the level of guidance for night driving is increased when RPMs are placed on the inside and outside of curves.(2) These possibilities require further investigation.
12. Those preparing final guidelines for the various regions should consult the recommended guidelines made in New Jersey by Pennell, which support the current standards for: placing RPMs in line with and gap of skip lines, to offset RPMs from a solid edgeline, to offset RPMs to the traffic side of left edgelines, and to place RPMs between double–yellow centerlines.(19)
RPM SPACING IN TRAFFIC ZONES
The MUTCD states that broken lines are usually formed on the basis of a 3–1 segment–to–gap ratio, with a 12.2–m (40–ft) cycle length of N (gap + segment) where the gap is 9.2 m (30 ft) and the marking segment is 3.1 m (10 ft) in length.(1) Figures 40 through 50 in the Roadway Delineation Practices Handbook also assume these dimensions; these figures should be consulted for exact placement requirements and for visual reference.(2)
The Roadway Delineation Practices Handbook Standards for Tangents and Horizontal Curves:
Specific Horizontal Curve Patterns
Justification: The borderline cases (when the degree of curvature is equal to 3 or 15 degrees) in the specific horizontal curve pattern guidelines have been moved to the next, more conservative level, as recommended by Pennell.(19) The maximum spacing for curves of 3 degrees or more is therefore 12.2 m (40 ft), and the maximum spacing for curves of 15 degrees or more is 6.1 m (20 ft).
The Following Guidelines Should Also be Included:
Justification: Since current guidelines state that "RPMs usually do not supplement solid right edgelines," instructions for RPM use and spacing provided by Pennell for hazardous situations would appear to be an asset. RPMs should not supplement right edgelines unless: (a) there is a fixed obstruction (e.g., guardrail, trees, poles, bridge rails, ditches) or difficult traveling surfaces exceptionally close to the edge of the roadway or (b) there is a lane drop or road width reduction.(19)
Justification: Current guidelines do not provide an upper boundary for RPM spacing, simply a recommendation based on the cycle length of road markings. If the cycle length of 12.2 m (40 ft) is changed, it will impact RPM spacing criteria. Blaauw indicates that, for straight sections, RPM spacing distances up to 24 m (78.7 ft) required information less often.(17) The 36–m (118–ft) configuration was less favorable. Therefore, it was suggested that the RPM spacing distances be restricted to no greater than 24 m (which is 78.7 ft-a little less than the spacing suggested in the guideline above, but since segment–to–gap ratios in the United States are based on units of feet, the recommended maximum spacing is given as 80 ft) on straight tangents unless further research is done to provide a more suitable upper boundary. Blaauw also indicates that, for 200–m (655.7–ft) radius curves, spacing distances for RPMs must be restricted to 12 m (~40 ft); spacing distances of 24 m (78.7 ft) and 36 m (118 ft) were found to lead to speed reductions and lane excursions. In general, it was found that "total observation time increases and driving performance deteriorates when less delineation information is present per unit of road length".(17) Upper or lower boundaries should be made for other degrees of curvature when there is sufficient information.
13. The suggestion to use supplemental RPMs on right solid edgelines under hazardous conditions states that the edgeline should have a "reasonable distance of treatment before the hazard." This reasonable distance needs to be quantified, either through empirical research or by analytical means.
14. The current guidelines specify that centerlines are supplemented to indicate a transition from passing in both directions to no passing zone, on a two–lane, two–way road, with RPMs spaced at 2N-24.4 m (80 ft). These are placed between skip lines, and RPMs spaced at N-12.2 m (40 ft)-are placed outside of double solid–yellow lines. However, the guidelines provided by Pennell specify that special treatments should not be used for situations of passing, no passing, or a combination of the two.(19) Although recommendations made by Pennell support certain spacing standards, conflicting recommendations may reflect the need for region–specific guidelines. States that use snowplowable RPMs may require different guidelines for certain circumstances.
15. In figures 41, 42, and 44 in the Roadway Delineation Practices Handbook, alternate examples are given for spacing RPMs with a separation of N instead of 2N.(2) The Handbook needs to be more specific as to when or where this alternate spacing should be used.
16. In figures 41, 42, and 43 in the Roadway Delineation Practices Handbook, examples are given for RPM use without supplementing the painted delineation.(2) The Handbook needs to be more specific as to when or where this alternate system should be used.
17. When referring to segment–to–gap ratios, the MUTCD also states that dimensions other than the 3–1 ratio "may be used as best suits traffic speeds and need for delineation."(1) Since the placement of RPMs is directly dependent on the segment–to–gap ratio, it is suggested that the following two sources be considered to determine if an upper or lower boundary can be specified.
Two sources indicated the possibility that insufficient delineation may induce drivers to speed up in order to increase information flow rate. Allen et al. reported that contrast thresholds reduce under dynamic conditions and reach a maximum at a frequency region of 2.5 Hz.(5) Therefore, in order to receive information at an optimal frequency of 2.5 Hz, the current 12.2–m (40–ft) cycle length would require a driver's speed to be 109.5 km/h (68 mi/h). If the cycle length were reduced to 7.3 m (24 ft), the optimum speed would be 43.5 km/h (27 mi/h). The study found that, when driving with a 12.2–m (40–ft) cycle length, there was an increased driver time delay at slow speeds, which resulted in the driver's "desire" to speed up.(5) Good and Baxter also found that the addition of some, but insufficient, delineation seemed to result in excessive speeds.(18) It was suggested by Allen et al. that if cycle lengths were kept small, perhaps it would avoid inducing inappropriate high speeds under adverse visibility conditions.(5) Recommendations for using a higher segment–to–gap ratio (i.e., shorter cycle length) would change the current 1:3 ratio with a cycle length of 12.2 m (40 ft) (producing a 3.05–m (10–ft) length with a 9.2–m (30–ft) gap) to, for example, a 3:5 ratio with a cycle length of 7.3 m (24 ft) (producing a 2.7–m (9–ft) line length with a 4.6–m (15–ft) gap).
According to these sources, if the ratio is lowered too much, RPMs would be spaced too far apart and would not provide sufficient information. Ultimately, the delineation guidelines need to set strict criteria so that they require little speculation.
The Roadway Delineation Practices Handbook Standards for Lane Exits and Gores:
18. Pennell suggests that supplemental RPMs be spaced at 6.1 m (20 ft) for painted gores unless the gore exit is shorter than 24.4 m (80 ft), in which case, spacing should be 3.05 m (10 ft).(19) The Roadway Delineation Practices Handbook specifies a spacing of N/4-3.05 m (10 ft)-for the gore area of exit and entrance ramps.(2) This suggestion may have been made because snowplowable raised pavement markers (SRPMs) are used extensively in New Jersey. Consideration should be given as to whether this recommendation should also be implemented in other States where SRPMs are used.
19. Consider the progress of research concerning paint markings, for it might affect the current recommendation for RPMs (use and/or spacing). For instance, Fitzpatrick, Lance, and Lienau (1995) found that drivers moved into or out of the exiting lane further upstream of the lane drop gore after pavement markings were used at freeway lane drop exits. (23) The number of maneuvers also decreased; the largest decrease was in the number of one–lane changes through the gore. If the gore markings are supplemented with RPMs, as recommended in the subsection above, the areas with more frequent lane changes would suffer the greatest damage and loss of markers. Additional research should be done to address the best combination of paint and RPM location/spacing, as required for both day and night delineation.
20. Zwahlen considered the use of RPMs on cloverleaf entrance and exit ramps.(22) He suggested that four RPMs would be required in view to provide adequate preview time for the driver. The optimal spacing would therefore be 7.6 m (25 ft). He concluded that RPMs were not useful as delineation for cloverleaf entrance and exit ramps.(22) However, since he used a sample size of only 11 people, it is recommended that the experiment be redone with a larger sample size. Also, since RPMs were severed to achieve 50 percent of the initial reflectivity and left as long as 2 years, it is recommended that the experiment be redone with frequent measures of RPM reflectivity.
The Roadway Delineation Practices Handbook Standards for Narrow Bridges (on two–lane rural roads):
The Roadway Delineation Practices Handbook Standards for Left–Turn Lanes:
The Following Guidelines Should Also be Included:
Justification: Taylor et al. suggest this additional guideline.(13)
21. Pennell recommends that continuous center left–turn lanes should not be identified by specially spaced RPMs.(19) This suggestion may have been made because SRPMs are used extensively in New Jersey. Consideration should be given as to whether this recommendation should also be implemented in other States where SRPMs are used.
The Roadway Delineation Practices Handbook Standards for Intersections:
22. Pennell recommends using the same spacing before intersections as is used for through movement; (19) however, previous studies by Taylor et al. (13) and Bali, McGee, and Taylor (1976) (24) support the notion of reduced spacing before intersections. Pennell also made recommendations specific to signalized and unsignalized intersections, a distinction that may be useful for guidelines.(19)
23. One area of research considers the use of coded delineation to provide the driver with advance information about upcoming curves or intersections, but in such a way as to induce the driver to slow down. An initial review of the following research indicates that coded delineation may help the driver maneuver the vehicle both at intersections and through curves.
The use of coded stripes as advance warning for curves was studied by Witt and Hoyos (1976) in a simulation experiment.(25) They found that with coded stripes: (a) the precision of course–following increased, especially in the approach zones to curves; (b) steering wheel turning became more steady and smooth; and (c) the drivers drove slower on difficult road sections and faster on easy road sections. Witt and Hoyos suggested follow–up field research in order for the results to be generalized to the real world.(25) After reviewing literature that investigated the potential benefits of transverse stripes as warnings before curves or at stop approaches, Bali et al. considered them to be effective at reducing the average speed and the variability around that average.(24) Bali et al. suggested the use of transverse stripes painted across approach lane(s) at gradually decreased spacing where a required stop is unexpected or in high accident areas. The use of thermoplastics was also suggested (as paint would wear away too fast), along with a bar width of 609.6 mm (24 in) and an overall pattern length of 152.5 m (500 ft).(24) Taylor et al. suggested that the principle of perceptual modification techniques might be applied to spacing of RPMs or PMDs. They thought that deliberate distortions of relative motion between marker and vehicle might produce the deceleration profiles required.(13) Blaauw also suggested relating the spacing of RPMs to road curvature like that used for stripes and PMDs.(17) In fact, a study by Zwahlen (1993) used an optical illusion in an attempt to reduce driver speeds.(26) He arranged PMDs to provide a perceptual illusion of increased curve sharpness by ascending (or descending) delineation heights. The height increments needed to be relatively large (e.g., 1.07 m (3.5 ft) to 1.65 m (5.41 ft) over a 72–m (236–ft) curve length); the results found a reduction in speed with significant center speed reduction from 49.69 km/h (30.88 mi/h) to 44.09 km/h (27.4 mi/h).(26) A study by Rockwell and Hungerford (1979) (cited in Zwahlen, 1993) also found a decrease in speeds through curves when a novel PMD system was used.(26) Further research is needed to determine whether the effect wears off over time.
There are many coded delineation issues that remain unclear, such as what part of the curve (e.g., radius, degree of curvature) reveals the most essential elements of a turn (e.g., sharpness, width) and whether RPMs present this information early enough to warn the driver. The results of a study by Fildes and Triggs highlight a potentially dangerous illusion in the perception of curvature during the negotiation of bends in the road.(7) They found that, when drivers are asked to make judgments about curvature-similar to a situation commonly experienced as a driver moves along a curved two–lane highway-first, the driver will primarily make that judgment on the basis of the curve's deflection angle, and second, the radius of curvature is likely to be misinterpreted by drivers in their assessment. Subjects in the experiment responded to small–radius, small–angled curves as least curved when in fact they were the most curved.(7) Thus, it seems important to ensure that the driver has as much information about the most important part of the curve as far ahead of time as possible. The curve–assessment process was shown by Cohen and Studach (1977), and McLean and Hoffman (1973) (cited in Fildes and Triggs) (7), and by Shinar et al. (4), to start well in advance of the vehicle entering the curve (at least 100 m (327.9 ft) before); far delineation is therefore essential to provide the necessary information.
The above research should be included in a thorough literature review, and further empirical studies should be conducted to answer questions such as: "Does the effect of coded delineation wear off over time?" and "How are accident rates affected by coded delineation?"
RPM SPACING IN CONSTRUCTION ZONES
The Roadway Delineation Practices Handbook provides guidelines as to how far apart to space Temporary Raised Pavement Markers (TRPMs) on the basis of road type and the length of time the road segment is under construction.(2) Some of these guidelines are outlined in the text, others in the figures in the Handbook.
The Roadway Delineation Practices Handbook Standards for Tangents and Horizontal Curves Outlined in the Text:
The Roadway Delineation Practices Handbook Standards for Tangents and Horizontal Curves Outlined in the Figures:
The Following Guidelines Should Also be Included for Bridges With Grooved Decks, Relocated Exit Ramps, and Pavement Drop–Offs:
Justification: Pennell made recommendations on the basis of an extensive literature review.(19) Since there are currently no guidelines that include this situation, it is suggested that this be included unless there is other research to show that it would not be advisable.
Justification: Davis (1993) presented recommendations for RPM use after a literature search, a survey of professionals who made recommendations for improvement, a driver survey of photographic slides to determine which designs merited further study, and a driver video survey used to verify results.(27) Those listed above were among the final suggestions resulting from this study. They should be used in conjunction with Part VI of the MUTCD: Standards and Guides for Traffic Controls for Street and Highway Construction, Maintenance, Utility, and Incident Management Operations, 1993 revisions, (28) and the Roadway Delineation Practices Handbook.(2)
24. The Roadway Delineation Practices Handbook lists the following guidelines in the text and refers to figure 50 for clarification of RPM placement in work zones.(2)
RPMs should supplement paint with a spacing of 12.2 m (40 ft) (a retroreflective RPM is placed midway between each 3.05–m (10–ft) paint stripe).
If RPMs substitute for painted skip lines, a cluster of four nonretroreflective RPMs with a retroreflective RPM every 12.2 m (40 ft) or N is recommended. The nonretroreflective RPMs should be placed 1 m (3 1/3 ft) or N/12 apart to provide the daytime appearance of a skip line.
However, both RPM type and spacing are depicted differently in figure 50 than is written in the text. According to the figure, the following guidelines should be used:
RPMs can be used to substitute paint on two–lane, two–way roads if work zones will last 14 days or less. A series of three RPMs will be intra–spaced at N/12 and inter–spaced at 10N/12 (or N distance from the first RPM in series 1 to the first RPM in series 2).
RPMs can be used to substitute paint on two–lane, two–way roads if work zones will last over 14 days. A series of three RPMs will be intra–spaced at N/12 and inter–spaced at 10N/12 (or N distance from the first RPM in series 1 to the first RPM in series 2), and accompanied by painted edgelines.
RPMs can be used to substitute for paint on two–lane, two–way roads with severe curvature if work zones will last 14 days or less. A series of two RPMs are used, intra–spaced at N/20 and inter–spaced at 18N/20 (or N/2 distance from the first RPM in series 1 to the first RPM in series 2).
This discrepancy needs clarification. It is also important that the guidelines specify under which circumstances paint should be replaced by RPMs rather than be supplemented by them.
25. Pennell recommended that TRPMs should be spaced at 1.5 m (5 ft) or N/8 with retroreflective units at 6.1 m (20 ft) when substituting stripes.(19) This should be considered when determining which source (text or figures) reflects the guidelines of the Roadway Delineation Practices Handbook.
26. The results of the study by Davis were used to make recommendation for the preferred way to delineate at curves, drop–offs, and relocated ramps. (27) The final recommendations for RPMs were listed in the subsection above; there were also a series of suggested study areas that emerged from the survey of professionals who had delineation experience. Some of these were not included in the final experiments, and may warrant further research. For horizontal and vertical curves, consider supplemental retroreflective RPMs spaced 6.1 m (20 ft) apart or retroreflective RPMs spaced 1.8–3.05 m (6–10 ft) apart when used without paint. For relocated entrance ramps, consider supplemental RPMs spaced at 3.05 m (10 ft) and RPMs spaced at 1.5 m (5 ft) when used without paint.
A description of the features that should be incorporated for nonretroreflective RPMs, retroreflective RPMs, snowplowable RPMs, and temporary (construction zone) RPMs should be included in the guidelines to provide a buyer's guide for RPMs. It would also be useful if the guidelines included a list of currently available RPMs that meet those requirements. The Roadway Delineation Practices Handbook provides such features for construction zone RPMs, but is lacking for the other two RPM types.(2) Some suggested features are listed below along with a list of RPM models that may fulfill future requirements.
The Following Features Should be Incorporated for Nonreflective RPMs:
(i) free from gloss or glaze
(ii) have a number of integrally formed protrusions of 1.27 mm (0.05 in)
(iii) protrusions project from the surface in a uniform pattern of parallel rows
(iv) protrusions shall have a flat surface parallel to the bottom of the marker with an area between 65.2 and 41.9 mm2 (0.101 and 0.065 in2)
(v) combined area of faces between 1419 and 2581 mm2 (2.2 and 4 in2)
(vi) protrusion shall be circular in section
(vii) number of protrusions should not be less than 50 or more than 200
(viii) sides of protrusions may be tapered (to facilitate forming and mold release)
(ix) must not exceed radius of 15 degrees from perpendicular to marker bottom
(x) markers manufactured with protrusions having diameter less than 0.38 cm (0.15 in) may have additional taper not exceeding 30 degrees from perpendicular to marker bottom and extending no more than one–half the total height of the protrusion
Justification: These are the specifications used by the California Department of Transportation (Caltrans), as cited in the Roadway Delineation Practices Handbook, for nonreflective ceramic markers.(2) This should be considered when compiling national guidelines for nonreflective RPMs.
The Following Features Should be Incorporated for Retroreflective RPMs:
Justification: There are several different types of retroreflective RPMs listed in the Roadway Delineation Practices Handbook. Blaauw and Padmos found that the corner-cube reflector with plastic mounting provided the driver with the greatest visibility distance (extrapolated for clear, dry, wet, and fog conditions) when compared with paint, thermoplastics, profile tape, and biconvex lens RPMs.(6) Additional research is required to give final specifications of each feature (see the future research issues discussed at the end of this subsection).
The Following Features Should be Incorporated for Snowplowable RPMs:
Justification: The Roadway Delineation Practices Handbook lists the above features when describing snowplowable RPMs.(2) Some of the above features may be specific to one model; more general information could be specified for incorporation into the guidelines.
The Following Features Should be Incorporated for Construction Zone RPMs:
Justification: The Roadway Delineation Practices Handbook lists the above features for incorporation into the design of construction zone RPMs.(2)
RPM Types Expected to Fulfill Future Requirements:
The following list of RPM types is only a sample and does not address cost–effectiveness, although the final version should consider such a variable when compiling a list of available RPMs:
Justification: Several models of corner–cube reflectors, a microprism high–intensity sheeting reflector, and a Swareflex reflector with 1/3 cm (1/8 in) glass beads were evaluated by Ullman (1994).(29) Factors such as volume of vehicle exposure, degradation in reflectivity, damage, and missing percentages were all included. Results concluded that moderately performing RRPMs (Apex 921, Apex 918, Apex 928, Batterson Reflective Button, Ray–O–Lite 8704(S), Ray–O–Lite 8704(R), Stimsonite 88) performed reasonably well under lower volume exposure (Site 4), but when exposed to higher volume conditions (Site 1), their reflectivity degraded below the SI value of 0.5 by about 7 months. High–performance RRPMs (Ray–O–Lite 9704, Ray–O–Lite 2002, Ray–O–Lite 2003, Stimsonite 911, Stimsonite 948, Stimsonite 953, Swareflex) provided at least minimum reflectivity at most sites for an entire year or more. Several still provided fairly high levels of reflectivity at low–volume sites after 2 years, but at the highest volume site, only Stimsonite models maintained minimum reflectivity up to and beyond 1 year. Of the high–performance models, the Stimsonite 911, 948, and 953 had the lowest percent of damage and number of missing markers (14.8 percent and 0 percent, 14.8 percent and 10 percent, and 14.8 percent and 3.6 percent, respectively).(29)
A literature review conducted by Pennell found two sources that indicate that the Stimsonite 96 LP was the most durable snowplowable RPM. This marker was the best of four types tested in a study done in California by Lougheed (1986), and was also found to be the best of three markers tested in a study by Bryden and Lorini (1981) (cited in Pennell).(19)
A study by Khan (1987) (cited in the Roadway Delineation Practices Handbook) proved that only one TRPM provides adequate day and night delineation for construction and work zones.(2) This study by Khan must be consulted directly to obtain the name of the TRPM; it was unavailable when the current report was prepared.
27. More information is required in order to compile a final list of features that should be incorporated for RPM types. A minimum reflectivity criteria for RPM effectiveness has yet to be established and will influence the level of reflectivity required for new RPMs. Cost analyses will determine whether a minimum half–life is required. Research to determine lens types, damage protectors, mountings and other special features will most likely follow production–driven studies. Documentation of acceptable features should be extrapolated from these sources.
RPM APPLICATION AND MAINTENANCE
Assuming that a step–by–step user manual accompanies RPMs when they are purchased, the following recommendations will focus on research–based issues. Just as when determining which type of RPM to use, the determination of application materials and installation procedures and maintenance will also most likely be product–driven. Ideally, the ability to match road type, RPM type, and bonding material will allow the most cost–effective combinations to be applied. The Roadway Delineation Practices Handbook outlines several limitations that should be considered when determining which combination of marker and bonding to use for certain road types.(2) However, more details are needed to create a national guide for installation, application materials, and maintenance of RPMs. The following information should be used to set guidelines to indicate which procedures and which materials should be used for particular circumstances (based on region, weather, road type, ADT, etc.)
Some Facts About RPM Installation:
Justification: The above installation limitations are mentioned in the Roadway Delineation Practices Handbook.(2) Other limitations outlined by the manufacturer should also be included in final guidelines.
Some Facts About Application Materials:
Some Facts About the Maintenance of RPMs:
Justification: The above information was taken from various sources. The first three points are outlined by the Roadway Delineation Practices Handbook.(2) Pennell gives snowplow and replacement recommendations in guidelines for highway delineation.(19) Tielking and Noel report that on a test section of road, markers that were replaced with epoxy, alternately in front of and on top of the failure spots in the skip stripe gaps, were found to all be in place 22 months later. The method used included using a shot of compressed air to blow debris out of the failure depression before filling it with epoxy and placing a marker on top.(30)
28. Zwahlen explains the use of OCARD (ODOT Computer Aided Road Delineation)-a knowledge–based system that computes the delineation layout using consistent and uniform delineation-in his study "Optimal Application and Placement of Roadside Reflective Devices for Curves on Two–Lane Rural Highways."(26) If it is possible to adapt this software system for the use of RPMs, it may aid the traffic engineer in the application of RPMs and make the task more efficient. The potential of this package should be investigated further.
29. As of yet, there are no national guidelines for when to replace an RPM. Since the criteria for determining minimum acceptable reflectivity for RPMs are not yet resolved, further research must be done in order to set a criterion in place. Currently, individual States determine which criteria to use for replacement procedures, based not on minimum reflectivity but on missing markers. Freedman et al. indicate several States that use different criteria to determine "effectiveness" of RPMs and therefore when to replace them: (3)
Since the effectiveness of RPM delineation requires both adequate reflectivity and adequate numbers, it would be ideal to determine a replacement criterion based on both factors. The Delineation Practices Handbook refers to a study by McNees (1987), which sought to determine a procedure that could be used to evaluate the effectiveness of RPM and raised traffic button systems.(31) The evaluating criteria include both reflectivity and loss of markers as indicators of replacement. However, the proposed procedure still requires further investigation. The experimental procedure and, thus, in turn, the proposed evaluation procedure does not account for additional variability.(31) Further research could determine whether evaluations might be biased by factors such as film processing, comparing scenes taken from different cameras or film sets, or whether color film produces significantly different representations from black and white film. Other factors that might bias the contrast or brightness of the photograph should also be investigated, so that a precise procedure might be provided. Lee, Hostetter, and Leibowitz (1991) include some information about controlling for contrast that is useful.(32)
If the method of evaluating photographs proves insufficient, it is still necessary to apply a criterion to determine whether RPMs should be replaced. One issue to consider, however, is how one accurately applies such a criterion to RPMs in the field while they are attached to the road. One study addresses some of the problems in measuring wet night performance and discusses the mobile laser retroreflectometer. This measuring device is described as being able to measure background luminance and it records both the coefficient and the contrast luminance of a line along significant distances of road (DeJaiffe, 1987).(33) The progress of this research should be monitored, along with the possibility of measuring RPMs with a mobile device with similar capabilities.
One alternative to measuring reflectivity in the field is to find a relationship between field and laboratory measurements such that laboratory–controlled studies can accurately reflect what would be found in the field. A study by King and Graham (1989) found that there is a strong relationship between test subject subjective evaluation of field luminance and laboratory evaluation of luminance. (34) This relationship can be expressed mathematically and the resulting equation used to calculate "field factors" relating laboratory–produced evaluations to actual field evaluations. If King and Graham's findings can be verified, future research could then be conducted in the laboratory under controlled and safe conditions with minimum field verification. The possibility of applying this method to RPM research should be considered.
As mentioned previously, there is no current standard for minimum RPM reflectivity on the basis of how much information the driver requires for controlled driving performance. However, several sources provide information that might be useful in determining this criterion. To determine how bright RPMs need to be relative to their surroundings to provide adequate delineation, several issues must first be established. Establishing a minimum preview distance for the average driver must account for the increased processing time and decreased discrimination ability of the older driver. Establishing which level of contrast will optimize driver performance must account for ambient lighting, weather conditions, headlight glare, and the complexity of the surroundings. The Roadway Delineation Practices Handbook discusses these issues and establishes the criterion of 100 millicandelas per lux per square meter as the minimum value for coefficient of retroreflected luminance, RL, for pavement markings on dry roads.(2) The same type of criterion is needed for RPMs, measured with the coefficient of retroreflection RA (as used for traffic signs). It is explicitly stated in the Roadway Delineation Practices Handbook that the two coefficients should not be compared as they are measured differently, but there are a few facts mentioned that should be considered when developing a criterion for RPM markings.(2)
Facts About Driver Visibility Requirements Made by the Roadway Delineation Practices Handbook:
Facts About Driver Visibility Requirements Made by Other Sources:
30. Given how the luminosity requirements for delineation are outlined (based on research performed by Freedman et al. (3)) in the Roadway Delineation Practices Handbook (2), if possible, the same basic method should be applied to determine minimum contrast levels for RPMs. If a minimum reflectivity criterion cannot be achieved through this method, information from the study by Blaauw and Padmos should be expanded to determine a criterion on the basis of visibility distances.(6)
31. If contrast can be used to determine a minimum reflectivity criterion, the results of the computerized headlight evaluation model (DETECT) used by Freedman et al. may be useful.(3) They showed that, for centerline visibility, if a single vehicle providing glare is between 100 and 300 m (327.9–983.7 ft) away, the contrast on dry pavement needs to be eight times greater than when there is no glare source in order to preserve a 3–s preview time. On wet tangents, for a single oncoming vehicle at 300 m (983.7 ft) away, the contrast needs to be four to eight times greater than the contrast needed when there is no glare source to preserve 2 s of preview time.(3)
32. Given that Blaauw and Padmos used a required preview time of 5 s, in clear weather the required preview distance for delineation would have had to be 110 m (360.6 ft) when the velocity was 80 km/h (49.7 mi/h) and 140 m (459 ft) when the velocity was 100 km/h (62.1 mi/h).(6) In fog, the required preview distance for delineation would have had to be 100 m (327.9 ft) when the velocity was 70 km/h (43.5 mi/h) and 110 m (360.6 ft) when the velocity was 80 km/h (49.7 mi/h). According to these findings, at a speed of 80 km/h (49.7 mi/h), the driver would have to be able to see delineation at least 110 m (360.6 ft) ahead in clear weather.(6) If these levels were adjusted using a preview time of 4 s (the upper limit for visibility distance to account for older or impaired drivers) and other conditions were taken into account (e.g., glare), it would be a starting point for determining a minimum reflectivity level based on visibility distance as opposed to contrast level.