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5 These specifications provide the performance require- Some states have evaluated the performance of recessed ments for pavement markers in terms of the coefficient of markers. The state of Maine ceased the installation of recessed luminous intensity before abrasion, abrasion resistance, color, markers because when the recessed grooves become filled lens impact strength, adhesive bond strength, compressive with snow and ice, the markers are ineffective. Investigations strength, and ramp hardness of holders. These specifications by the Pennsylvania DOT (PennDOT) found that recessed also provide guidelines on how to test the performance of markers on downgrades are not as visible as recessed mark- pavement markers. ers on inclines if water accumulates in the recessed slots. As Many states have developed special provisions and standard a result, PennDOT has decided to stop the installation of specifications for pavement markers that are generally based recessed markers on its roadways. on the above ASTM specifications. While the ASTM specifi- cations provide minimum and maximum values, state guide- lines tend to identify more precise values that fall within the Nonsnowplowable Pavement Markers ranges of acceptable values specified in the ASTM guidelines. For example, ASTM D 4383 specifies that the installed Raised nonsnowplowable markers are used extensively in height of the casting shall not exceed 0.43 in. (10.9 mm) and states where snowfall is not a concern, such as Texas and Cal- shall not be less than 0.06 in. (1.5 mm) above the road sur- ifornia. Other states--such as Illinois, New Jersey, Oregon, face, while the guidelines for the state of Illinois specify that Michigan, Maryland, and Massachusetts--use only snow- the height should be 0.3 in. (7.6 mm) and the guidelines for plowable pavement markers. the state of Maryland specify a maximum height of 0.25 in. (6.4 mm). 2.1.3 Implementation Criteria and Maintenance Procedures Snowplowable Pavement Markers States extensively use the MUTCD (3) and FHWA's "Roadway Delineation Practices Handbook" (2) as guides for In the United States, there are two types of snowplowable the implementation of PRPMs. The MUTCD mainly provides pavement markers: raised and recessed. According to the guidelines on the desired spacing of PRPMs, while the "Road- iTRANS state surveys and literature reviews, recessed mark- way Delineation Practices Handbook" also provides general ers are not as popular as raised markers are. Some of the states guidelines on PRPM colors, materials, installation, and main- that have installed or are currently installing recessed mark- tenance procedures. The "Roadway Delineation Practices ers are Kansas, Maine, Maryland, Oregon, Virginia, West Vir- Handbook" provides guidelines on the desired layout of ginia, and Pennsylvania. These states, with the exception of PRPMs for various roadway infrastructure elements (e.g., Oregon, also install raised snowplowable pavement markers. curves, intersections, and tangents ramps) on different road- Illinois, Indiana, Massachusetts, Michigan, New York, Ohio, way types (e.g., two-lane roadways, four-lane undivided road- and Wisconsin almost exclusively use raised snowplowable ways, and four-lane divided roadways). According to these markers. guidelines, the spacing between consecutive PRPMs on tan- Hofmann and Dunning (8) found that, although recessed gents should be 80 ft (24 m). For horizontal curves between snowplowable markers last on average 12 months longer 3 and 15 degrees, a spacing of 40 ft (12 m) is recommended. than raised snowplowable pavement markers, they do not For curves greater than 15 degrees, the recommended spac- perform as well as raised markers. This finding confirms ing is 20 ft (6 m). It is not recommended that centerline and Endres's (9) conclusion that raised pavement markers edgeline PRPMs be used together because this may create out-perform recessed markers under dry and wet weather confusion on some sharp curves. Most states, in accordance conditions. with these guidelines, install one two-way yellow marker on A variety of problems are associated with recessed markers the centerline of two-lane roadways only. In some states, because the collection of debris, rain, and snow in the recessed such as Illinois and Pennsylvania, a group of two markers can slots obscure the reflective surface of the markers. Pigman and be used on the centerline of high-volume, high-speed, two- Agent (10) evaluated the performance of recessed snowplow- lane roads. On divided multilane facilities, the most common able markers by observing the marker's visibility during practice in the iTRANS states surveyed during this current snow and ice conditions. It was found that following snow- research study is to install one-way white PRPMs on the lane plow operations, the groove retained snow and ice. How- lines only. An exception is New Jersey, where PRPMs are ever, because of the passing traffic, the snow and ice melted also installed on the left edgelines of multilane facilities. and the water was swept away in a short period of time. States have developed PRPM installation criteria. In the Pigman and Agent observed that vehicle tires cleansed the states of Ohio, Texas, and California, PRPMs are installed non- top third of the marker, but the bottom portion remained selectively on all state-maintained highways. Other states-- obscured. It was concluded that although nighttime visibility such as Maryland, Massachusetts, Wisconsin, Pennsylvania, was reduced, the recessed markers remained visible. Illinois, Indiana and Kansas--have a combination of selective

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6 and nonselective implementation practices. PRPMs are imple- rain); low roadway illumination; and evidence of vehicles mented nonselectively on certain roadway types, such as free- leaving the roadway, such as excessive wear of pavement ways, and selectively on other roadway types on the basis of markings or excessive skid marks. In Michigan, PRPMs are one or more of the following parameters: installed only on nonfreeways where there is a concentration of crashes and only after other countermeasures such as sign- Roadway type, ing, pavement markings, and roadside delineation (e.g., Traffic volume, chevrons and post-mounted delineators) have been unsuc- Illumination, cessful in improving the safety of the locations. Safety record, Illinois and Maryland install PRPMs at horizontal curves Speed limits, and where it is necessary for motorists to decrease their travel Horizontal curves. speed by more than 10 mph (16 km/h) in order to traverse the curve safely. Some states implement PRPMs at other cross-section ele- For example, Maryland implements PRPMs nonselec- ments. For example, Illinois installs PRPMs at lane reduc- tively on all Interstate highways and other freeways. Mary- tion transitions; freeway gores; rural left-turn lanes; and land, Massachusetts, and Wisconsin use the speed limit of a two-way, left-turn lanes. Maryland has detailed standard roadway as a primary criterion for deciding where to imple- design drawings for PRPM installations at one-lane ment PRPMs. In Maryland, PRPMs are implemented on all bridges; intersection approaches; two-way, left-turn lanes; two-lane roadways that have a speed limit exceeding 45 mph left-turn lanes; acceleration lanes; deceleration lanes; and (72 km/h), irrespective of the traffic volume. In Massachu- lane transitions. setts, PRPMs are installed on undivided roadways that have One of the primary maintenance problems with retro- a speed limit of 50 mph (80 km/h) or greater. In Wisconsin, reflective PRPMs is maintaining the reflectivity level. The PRPMs are installed on all roadways that have a speed limit reflectivity retention of retroreflective PRPMs tends to depend of more than 65 mph (100 km/h), which includes all multi- mostly on cumulative vehicular exposure since the time of lane freeway facilities. installation (11). A study by Ullman (12) evaluated several Missouri, Pennsylvania, and Massachusetts implement models of corner-cube reflectors for factors such as volume PRPMs on all freeways. Michigan's PRPM guidelines rec- of vehicle exposure, degradation in reflectivity, damage, and ommend implementation on all freeways that lack roadway missing percentages. The "Roadway Delineation Practices illumination. Handbook" (2) states that it is difficult to precisely predict The criteria for implementing PRPMs in Illinois, Indiana, the service life of retroreflective PRPMs. and Kansas relate to traffic volume thresholds for different In response to the iTRANS state practices survey, some roadway types. PRPMs are only installed on roadways where states provided information on their PRPM maintenance the average daily traffic (ADT) volumes exceed these thresh- practices. Pennsylvania and Ohio replace PRPM lenses on a olds. Table 2-1 provides a summary of the traffic volume fixed 2-year and 3-year cycle, respectively. In some states, thresholds for different roadway types. the replacement cycle depends on the roadway type and traf- The majority of surveyed states implement PRPMs at fic volume. Table 2-2 shows the PRPM replacement cycle for locations with actual or potentially poor safety records. In Indiana. Texas provides guidelines for when to schedule the Maryland, PRPMs are implemented where the crash rate for maintenance of PRPMs based on the results of a nighttime "correctable" guidance-related crashes is significantly higher test inspection (Table 2-3). The replacement cycle of PRPMs than the statewide average on similar road types. In Indiana, in Texas, based on ADT volumes, is summarized in Table site selection for the implementation of PRPMs is based pri- 2-4. Colorado and Iowa removed all existing PRPMs and marily on the need for additional alignment delineation in interrupted any future installations because of the high areas of frequent inclement weather (e.g., fog, smoke, and maintenance costs. TABLE 2-1 PRPM guidelines based on traffic volume for different roadway types (source: iTRANS state practices survey) State Guidelines for rural two- Guidelines for multilane lane roadways roadways Illinois ADT > 2,500 veh/day ADT > 10,000 veh/day Indiana ADT > 2,500 veh/day ADT > 6,000 veh/day Kansas ADT > 3000 veh/day and TADT > 450 veh/day ADT = Average daily traffic (both directions). TADT = Truck average daily traffic.