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17 500 ft (152 m). Treatments on curves studied included PRPMs best of 25 initial delineation treatments. The visibility distances on the centerline (Treatments 3 and 11) or on the edgeline for six of the best treatments (Treatments 5, 6, 9, 10, 11, and 12) and centerline simultaneously (Treatment 4) as well as treat- did not differ with respect to statistical significance. Amongst ments with and without chevrons and post-mounted delin- those treatments, the one recommended on a benefit-cost basis eators. Measures were recognition distance and time spent was the treatment with a 4-in. (100-mm) yellow line, no edge- looking at the roadway. Measurements were taken using a line, and high-intensity post-mounted delineators (Treatment visual occlusion device and using subject assessment. 10). The treatments with PRPMs were more costly for an equal The first phase of the study involved a laboratory test of visibility benefit and therefore were not recommended. simulated nighttime driving. In the second phase of the study, a subset of the best treatments was then field-tested with the youngest and oldest age groups (see Figure 2-4). As is so 2.3.3 Driver Behavior in Response to PRPMs often the case, the treatment that improved performance for the older drivers also improved performance for younger In addition to measures of visibility distance, various mea- drivers. The treatment with the highest recognition distance for sures of driver behavior have been used to evaluate the effec- both groups was Treatment 12, which did not use PRPMs. tiveness of PRPMs. These measures include visual workload Treatment 12 had a 4-in. (100-mm) yellow centerline with a (as determined by number of looks required for comfortable measured coefficient of retroreflected luminance (RL) of 100 driving), speed, speed variation, lane position, lane position mcd/m2/lux (in-service level of brightness), a 4-in. (100-mm) variation, and encroachments into adjoining lanes. Some stud- white edgeline, and T-posts with engineering-grade reflec- ies involve measuring the behavior of a group of subjects on tivity with standard spacing (65 ft or 19.8 m). test courses, while others involve measuring the behavior of There were significant differences between left- and right- unsuspecting drivers on public roads where test installations curve recognition distances for some treatments and between have been set up. The greatest number of studies of the impact older and younger drivers for other treatments. On average, the of delineators on driver behavior were directed to horizontal older drivers had 14 percent less recognition distance than the curves. PRPMs on tangents and on ramps have been examined younger drivers. Furthermore, it has been demonstrated that in one study (29). Another study concerned gore areas and older drivers who volunteer for testing are likely to have sub- deceleration lanes (30), and two studies looked at approaches stantially better vision than the average older driver. Conse- to narrow bridges (31, 32). These studies are described in the quently, differences amongst the driving public between older paragraphs below. and younger drivers are likely to be much more pronounced. 220.127.116.11 Driver Lane Position 18.104.22.168 Visibility Distance and Speed for PRPMs on Curves and Benefit-Cost Analysis Table 2-7 summarizes the effects that PRPMs on curves In the Pietrucha et al. study (28), a benefit-cost analysis that have on driver behaviors. In a before-and-after study compar- used visibility distance to determine the benefit, examined the ing delineation with and without centerline snowplowable 1000 900 Old Young 800 MEAN RECOGNITION 700 DISTANCE (ft) 600 500 400 300 200 100 0 1 4 3 2 7 8 5 11 6 9 10 12 TREATMENT Figure 2-4. Recognition distance: old versus young groups (combined curves) (28).
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18 TABLE 2-7 Summary of literature on driver performance and PRPMs (horizontal curves) Study Ref. Installation PRPM Spacing Effect on Speed Effect on Lane Position Other /Location Location Mullowney Centerline, NA Smoother speed profile through Centerline and edgeline When combined with (1982) (30) edgelines the site encroachments were reduced by illumination, greater New Jersey separately significant amounts (3.7 to 12%) reduction of encroachments (Night only) was measured Niessner Centerline, Recommended: · No significant difference in · Centerline encroachments Mixture of centerline and (1984) (31) edgeline · 80 ft (24 m) mean speeds nighttime 85th reduced by 50% (daytime and edgeline markings appear USA on curves up percentile speeds reduced nighttime) confusing at some sharp (Day and to 3 degrees significantly · Vehicle placement variability curves night) · Speeds increased on one reduced significantly · 40 ft (12 m) on curves up approach and decreased on vehicles shifted significantly to 15 degrees the other toward centerline during · Smoother speed profile (only daytime and toward edgeline · 20 ft (6 m) on at nighttime curves with night results) more than · Centerline and edgeline 15 degrees encroachments reduced of curvature significantly (only nighttime results) Agent & Centerline · 10 ft (3 m) Daytime and nighttime speeds Decreased encroachments for None Creasey apart reduced daytime and nighttime (no (1986) (33) · 20 ft (6 m) (p < 0.01) significance testing) Kentucky apart (Day and night) Zador et al. Both sides of · 80 ft (24 m) · Overall mean speed increase · 6 cm away from centerline: None (1987) ( 21 ) centerline (in apart of 1 km/h at nighttime Mean shift 12 cm away from 54 sites in conjunction · 40 ft (12 m) · Daytime and nighttime mean centerline, 30 m before curve Georgia & with other apart for speed increase 30 m before · Mean shift 21 cm away from New measures) sharper curves and 30 m into curve (from centerline, 30 m into curve Mexico graph) (similar effects both daytime (Day and and nighttime) night ) Krammes & Centerline NA · Speeds were higher with new · Vehicles placed further from None Tyer (1991) PRPMs in place centerline (34 ) · Speeds went down over time · Fewer opposite lane 5 sites encroachments USA (Night only) Hammond & Both sides of · 40 ft (12 m) No significant difference · Significant reduction in None Wegmann centerline apart encroachments (2001) ( 35 ) · 20 ft (6 m) · No significant difference 2 sites in apart between 6- and 12-m spacing Tennessee (Day only) PRPMs, Mullowney (30) measured impacts on encroach- tion) in centerline encroachments was measured. There was ments, on speeds, and on speed variance. Only nighttime data also a statistically significant reduction (p < 0.01) of 5.7 per- were collected for this study. Speeds were measured using a cent in edgeline encroachments at the sites with PRPMs after handheld radar unit. An on-site observer collected encroach- installation as compared with before. There was no statisti- ment data. Centerline encroachments were measured at three cally significant difference at two control sites for this com- sites, including one control site. Two of the three sites were parison. There is no indication of how long after installation on the same roadway. Edgeline encroachments were mea- measurements were made, nor is it indicated where installa- sured at two sites on the same roadway, one of which was a tions were centerline only or a combination of edgeline and control site, presumably the same delineation but without centerline. The former is assumed. PRPMs, although this presumption was not stated explicitly. During Mullowney's study (30), speeds were collected at After the PRPMs were installed at the treatment sites, they two sites, both on horizontal curves. At the first site, speeds were compared with the control sites, and a statistically sig- were collected for traffic in both directions at four different nificant reduction (p < 0 .01, p < 0.05) of 12 percent (site with locations around the curve. The presence of PRPMs appears street illumination) and 3.7 percent (site without illumina- to have resulted in a smoother speed profile through this site
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19 in both directions, as evidenced by less variation in speed At night, average speeds were lowered from 23 to 20 mph between the data collection points after installation of PRPMs (37 to 30 km/h) and from 24 to 22 mph (38 to 35 km/h) after than before installation of PRPMs. Speeds were higher at the installation of the PRPMs, for the different approaches. These apex of the curve than at the curve entrance or at the exit of the reductions were statistically significant (p < 0.01). The per- curve. At the second site, speeds were collected at three centages of encroachments were reduced from 44 to 22 per- locations around the curve. The speeds at the entrance and exit cent and from 13 to 7 percent during the day and from 52 to of the curve after PRPM installation were slower than those 18 percent and from 8 to 7 percent at nighttime. There was no speeds before PRPM installation, resulting in a smoother mention of tests of significance for encroachment data. speed profile throughout the site. (Note that the speed data At the regular curve with only PRPMs installed, average were presented only in graph form--no statistical analysis night speeds were reduced from 30 to 27 mph (48 to 43 km/h) was performed.) on one approach and from 30 to 24 mph (48 to 38 km/h) on the The effects of PRPMs at three horizontal curve sites were other approach after installation of the PRPMs. These reduc- investigated by Niessner (31) before and after the installation tions were significant (p < 0.01). Encroachments increased on of PRPMs. At the first site, which was an S-curve, conven- one approach from 22 to 26 percent and decreased on the tional markers were spaced 40 ft (12 m) apart, two on the cen- other approach from 32 to 29 percent. There was no mention terline and one on each edgeline. There was no statistically of the radii of the curves or of the tests of significance for significant difference in daytime or nighttime speeds, but encroachment data. nighttime 85th percentile speeds were significantly reduced. The effects of a number of commonly used curve delin- Centerline encroachments were reduced by 50 percent dur- eation treatments on vehicle speed and placement were ing both day and night. At the second site, a single row of examined in a study by Zador et al. (21). Treatments were PRPMs was installed on the centerline as well as on both implemented at 51 rural two-lane highway sites. Sites with edgelines. Speed measurements were taken at three points for chevrons, post-mounted delineators, and raised pavement both directions. Mean speeds decreased for one approach to markers were compared with unmodified control sites. Obser- the curve, but increased for the other approach. During the vations were taken at each modified and control site several daytime, the vehicle lane position shifted significantly toward weeks before and several weeks after the modifications were the center of the curve, while at night, the vehicle lane position put in place. Speeds and vehicle placement were taken 100 ft shifted toward the edgeline. At the third site, which was an (30 m) before and 100 ft (30 m) into the curve. S-curve, snowplowable PRPMs were installed along both Of the 51 sites, 12 used standard 44 Stimsonite PRPMs edgelines and on both sides of the centerline. Speed measure- installed on both sides of the double yellow centerline. The ments were taken at four locations along the curve. The snow- markers were usually spaced 80 ft (24 m) apart. Along sharper plowable PRPMs appear to have resulted in a smoother speed curves, where three markers could not be seen at one time, profile (i.e., there was less variation in speed through the the markers were spaced 40 ft (12 m) apart. The results indi- curve) for both directions. In addition, both centerline and cated that the PRPMs caused the largest shift from the cen- edgeline encroachments were reduced significantly. terline as compared with the other countermeasures and the Agent and Creasey (33) investigated the ability of various control condition. In advance of the curve 100 ft (30 m), the traffic control measures to delineate horizontal curves so mean displacement was approximately 0.4 ft (12 cm); 100 ft drivers would perceive the curve, slow to an appropriate speed, (30 m) into the curve, the mean displacement was approxi- and then receive guidance through the curve. A before-and- mately 0.7 ft (21 cm). after analysis was carried out to test the performance of In comparison, when chevrons were used, the displace- PRPMs, transverse pavement stripes, rumble strips, post delin- ment from the centerline was less. With post-mounted delin- eators, and chevron signs. Speed and encroachment (centerline eators, vehicles moved toward the centerline. Nighttime mean and edgeline) data were taken at all sites before and after vehicle speeds were increased by approximately 0.68 mph installation, and a before-and-after crash analysis was per- (1.1 km/h) with the use of raised pavement markers; daytime formed at some of the sites. The after data were taken more mean vehicle speeds increased by a similar amount. than 1 year after installation of the traffic control measures. In a nighttime study comparing the impact of PRPMs sup- PRPMs were applied at two regular curves and two plementing the existing centerline with post-mounted delin- S-configurations, each consisting of two 90-degree curves. eators, Krammes and Tyer (34) determined that drivers placed The PRPMs were applied in pairs along the centerline at inter- their vehicles further from new PRPMs than from older post- vals 10 or 20 ft (3 or 6 m) apart, depending on the location. mounted delineators and made fewer encroachments on the Two sites--one S-curve and one normal curve--had only adjacent lane. Two factors are operating here: (1) the new PRPMs and no other countermeasures installed. The other two PRPMs would be more conspicuous and (2) the new PRPMs sites had PRPMs and other traffic control measures as well. would be placed closer to the traveled lane than the post- At the two S-curves with only PRPMs, average speeds were mounted delineators would. lowered from 23 to 20 mph (37 to 30 km/h) and 25 to 23 mph With respect to changes over time, speeds were significantly (40 to 37 km/h), depending on the approach, during the day. higher with the new PRPMs than with PRPMs that had been
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20 in place for 11 months, suggesting that drivers had longer pre- 40- and 80-ft (12- and 24-m) spacing distances lead to speed view distances and therefore were comfortable with higher reductions and lane errors. The authors recommend a mini- speeds. mum spacing of 80 ft (24 m) on tangents and 40 ft (12 m) on In a recent daytime study, Hammond and Wegmann (35) curves. These results provided a basis for guidelines for the examined the effects on PRPMs at two spacings on curves by use of PRPMs recommended to FHWA (5). measuring changes in speed and encroachment distances into the opposing travel lane. Two minor arterial sites were chosen for this study. Six data points were collected for each vehicle, 22.214.171.124 Driver Speed, Lane Position, including speeds at the beginning, middle, and end of the and Encroachments at Hazardous Locations curve, as well as the distance of opposing-lane encroachment. Levels of encroachment were categorized on a scale of zero to In 1982, a study was carried out by 12 state highway agen- eight, with zero being low and eight being high. A shift in one cies to evaluate the effectiveness of raised pavement mark- level of encroachment translates into a 4-in. (10-cm) change ers at hazardous locations (31). Among the test sites were of the vehicle travel path. rural curves on two-lane roadways, narrow bridges, stop Speeds and encroachment distances were measured before approaches, through approaches, two-lane sites with left-turn and after the installation of the PRPMs at a 40-ft (12-m) spac- lanes, interchange gores, four- and six-lane undivided sites, ing and again after the installation of additional PRPMs, cre- multilane divided highway sites, and four- to two-lane sec- ating a 20-ft (6-m) spacing, for a total of 3 experimental con- tions. Several locations were tested for each site type. ditions. Stimsonite LifeLite 88A PRPMs were placed in pairs The report confirmed that PRPMs provide improved night- immediately on either side of the painted centerline. time pavement delineation when compared with and used in Speed variance was not found to be affected by the pres- conjunction with conventional paint stripes. For rural two- ence or spacing of the PRPMs. However, effects on encroach- lane curves, the report recommended that the double yellow ment were statistically significant. At Site 1, the control con- centerline be delineated with one row of PRPMs between the dition, the 40-ft (12-m) spacing, and the 20-ft (6-m) spacing two centerlines, with PRPM spacings of 80 ft (24 m) on yielded levels of encroachment of 4.0, 3.0, and 2.8, respec- curves up to 3 degrees, 40 ft (12 m) on curves between 3 and tively. At Site 2, the control condition, the 40-ft (12-m) spac- 15 degrees, and 20 ft (6 m) on curves with more than 15 ing, and the 20-ft (6-m) spacing resulted in levels of encroach- degrees of curvature. Visual observations indicate that two ment of 2.7, 1.9, and 1.3, respectively. markers may be needed to provide adequate delineation for locations with curves in excess of 20 degrees. The mixture of centerline and edgeline markings appeared confusing at 126.96.36.199 Driver Speed, Visual Workload, some sharp curves. and Lane Position Relative The study determined that PRPMs can significantly reduce to PRPM Spacing on Curves instances of erratic maneuvers of vehicles through painted gores at exits and bifurcations. This finding was true whether As discussed above, Hammond and Wegmann (35) com- or not overhead lighting was present. The study recommended pared operating speeds and encroachment distances for PRPMs that PRPMs be introduced slightly in advance of the highway spaced 40 ft (12 m) and 20 ft (6 m) apart. The authors found problem area to prepare motorists for the guidance technique no significant difference between the 40-ft (12-m) and 20-ft that is to be encountered. (6-m) spacings in terms of operating speed or centerline encroachments. Blaauw (36) examined drivers' observation strategy and per- 188.8.131.52 Driver Speed and Lane Position formance on road sections with various delineation arrange- in Relation to PRPM Spacing ments. Delineation treatments included PRPMs on edgelines on Tangents and Ramps and centerlines at spacings of 40, 80, and 118 ft (12, 24, and 36 m) on straight and curved (radius 656 ft [200 m] and radius Optimal spacings for PRPMs along tangent sections and 3,280 ft [1,000 m]) sections. A visual occlusion technique on interchange ramps of Interstate highways in Ohio were was used to determine changes in visual strategy as a func- determined by Zwahlen (29) (see Table 2-8). The first step in tion of road delineation. Drivers were equipped with glasses this process was to predict the illumination reflected back to with lenses that could be changed from translucent to opaque the driver's eyes on the basis of the following: almost instantaneously. These glasses allowed them half a second to look at the road on the press of a control switch. The · Headlight output; researchers found that total observation time increases and · Geometry with respect to the PRPM, headlight, and driver driving performance deteriorates when less delineation infor- eye positions; mation is present per unit of road length. This finding was par- · Photometric qualities of the PRPM; and ticularly striking on the 656-ft (200-m) radius curve where the · Transmissivity of the atmosphere.
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21 TABLE 2-8 Summary of literature on driver performance and PRPMs (tangent sections) Study Ref. Installation PRPM Effect on Effect on Other /Location Location Spacing Speed Lane Position Zwahlen Tangent 59, 121, 240 ft No 5-in. (13-cm) None (1987) ( 29 ) lane line (18, 37, 73 m) significant shift away Ohio difference from (night testing centerline only) Zwahlen Ramp 13, 27, 50 ft No No significant Placement of (1987) ( 29 ) edgeline (4, 8, 15 m) significant difference PRPMs on outside Ohio difference edgeline not (night testing recommended (no only) significant difference) It was assumed that the headlight output was less than · Tangent sections included no PRPMs and PRPMs at 100-percent efficient (how much less was not stated). Also, spacings of 60, 120, or 240 ft (18, 37, or 73 m). to account for wear during the life cycle, the specific inten- · Ramp sections included no PRPMs and PRPMs at spac- sity value of the PRPM was assumed to be 50 percent of its new ings of 12.5, 25, or 50 ft (4, 8, or 15 m). value. Once an illumination threshold for human observers of 98-percent probability of detection was reached, the devices On tangent sections, there was a slight but consistent shift were assumed to be visible. of about 5 in. (13 cm) toward the right edgeline for PRPM A rain intensity of 1 in. per hour was assumed, since the spacing of 60 ft (18 m) compared with 120 ft (36 m). No sta- probability of having a greater rainfall than this within a 30-day tistically significant effects on vehicle speed were found. The period decreases rapidly (e.g., a rainfall of 2.6 in. per hour study concluded that a 120-ft (36-m) spacing should be rec- can be expected once every 25 years). The theoretical calcu- ommended on tangent sections. The slight improvement in lations indicated that the Stimsonite PRPMs would be visi- lane positioning for the 60-ft (18-m) spacing was not felt to ble in rain at 1 in. per hour at a distance of 480 ft (147 m). justify the doubling in cost of installation. A model of driver lane position standard deviation on tan- Statistical analysis showed no significant difference in gent sections was then used to predict lane position deviation speed or lane positioning related to the presence or spacing in relation to the number of PRPMs visible. Once there were of PRPMs on ramp sections; thus, the placement of PRPMs four or more delineation devices visible, the model predicted on the outer edgelines of cloverleaf interchange ramps was little change in standard deviation. Given a visibility of 480 ft not recommended. (147 m), to have four devices visible requires a spacing of Zwahlen's study (29) suggests that the problem with PRPMs 120 ft (36 m). on very sharp curves (i.e., with a radius curve 240 ft [73 m] On ramp sections in Ohio, the radius of curvature is typi- or less) is the lack of preview distance (i.e., lacking a preview distance of 120 ft [37 m] or shorter). Related work on chevron cally 240 ft (73 m) corresponding to a 24-degree curve. An spacings (37) used a paradigm in which subjects viewed assumption was made that a solid body of grass or snow 1 to curves with up to 12 equally spaced chevrons in laboratory 2 ft (0.3 to 0.6 m) high existed on the inner edge of the ramp conditions that simulated light levels and a size equivalent to curve, limiting the driver's view ahead. Given this geome- 90-degree curves with typical radii seen at night. The subject try, the illumination distance for PRPMs will be best if placed task was to determine whether the curve viewed was sharper on the outer edgeline of the pavement. Low-beam headlights or gentler than a standard curve. Results showed that perfor- provide the most light to the right; therefore, the left edge of mance reached a plateau when four or more equally spaced a left curve has the shortest illumination distance--115 ft chevrons were used. Chevrons can be seen considerably fur- (35 m). This is about one-fourth of the visibility available on ther than PRPMs because of their orientation, and there is no a tangent section. To have four delineators in view in the reduction in visibility with rain. Consequently, chevrons 115-ft (35-m) distance, an optimal spacing of 25 ft (7.6 m) seem to be preferable to PRPMs on sharp curves. was selected. The theoretical analysis led to field testing with 11 young subjects in wet and dry conditions. To better replicate the illu- 184.108.40.206 Driver Response to PRPMs mination levels for PRPMs that have been in service for some in Deceleration Lanes and at Gore Areas time, the new reflectors were cut in half. Vehicle speed and lane position were measured. Conditions tested included spac- One study examined the impact of PRPMs at gore areas ings both wider and narrower than the predicted optimum: (see Table 2-9). Mullowney (30) found that six out of nine
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22 TABLE 2-9 Summary of literature on driver performance and PRPMs (freeway exits) Study Ref. Site Type Installation PRPM Effect on Speed Effect on Other /Location Location Spacing Lane Position Mullowney Deceleration Gore, 40 ft (12 m) Two of the three Not None (1982) ( 30 ) lanes lane line, lane line, sites exhibited a determined New Jersey 40 ft (12 m) significant edgeline (night only) edgeline increase (p < 0.03, p < 0.01) in early entry into deceleration lane Painted gore Gore 20 ft (6 m) Six of nine sites Not None experienced determined statistically significant reductions (p < 0.02 or less) in cars that cut through painted gore Zwahlen Interchange Edgeline 13, 26, 50 ft No significant No significant Placement of (1987) ( 29 ) ramps (4, 8, 15 m) difference difference PRPMs on Ohio 240 ft (73 m) outside (night only) radius edgeline not recommended (no significant difference) sites where PRPMs had been implemented had statistically termeasures evaluated were combinations of advance warn- significant reductions in vehicles that cut through the painted ing signs, pavement markings, PRPMs, roadside delineators, gore. When PRPMs were placed on the lane and edgeline of object markers, and adhesive delineators. Measurements of deceleration lanes, two out of three sites showed a significant vehicle speed and lateral placement were made using FHWA's increase in early entry into the deceleration lane. fully automated Traffic Evaluation System (TES). These countermeasures did not result in statistically significant changes in the mean speeds at the p < 0.1 level. However, 220.127.116.11 Driver Response to PRPMs the countermeasures significantly reduced speed variation at Narrow Bridges when all the vehicle types and time periods were analyzed together. A before-and-after analysis of vehicle speed and lateral In the Niessner (31) report discussed above, for narrow placement at 18 narrow bridge approach sites was conducted bridges on rural two-lane roads, a PRPM spacing of 80 ft by Bowman and Brinkman (32) (see Table 2-10). The coun- (24 m) decreasing to 40 ft (12 m) approaching the bridge TABLE 2-10 Summary of literature on driver performance and PRPMs (narrow bridges) Study Ref. Installation PRPM Effect on Speed Effect on Lane /Location Location Spacing Position Niessner Edgeline, 80 ft (24 m) Two sites: Two sites: (1984) ( 31 ) centerline decreasing 1. Significant reduction 1. Moderate and severe USA to 40 ft in nighttime 85th encroachments over the (12 m) percentile speeds--no centerline were reduced significant difference in for both daytime and daytime speeds nighttime 2. Vehicle speed at night 2. No significant increased (no mention of difference for daytime daytime speeds) or nighttime Bowman and Centerline Unknown Speeds reduced No significant Brinkman (with other (p < 0.1) difference (1988) ( 32 ) counter- USA measures)