National Academies Press: OpenBook

Safety Evaluation of Permanent Raised Pavement Markers (2004)

Chapter: Chapter 5 - Discussion of Study Results

« Previous: Chapter 4 - Safety Impact Analysis of PRPM Installations
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Suggested Citation:"Chapter 5 - Discussion of Study Results." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
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Page 40
Page 41
Suggested Citation:"Chapter 5 - Discussion of Study Results." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
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Page 42
Suggested Citation:"Chapter 5 - Discussion of Study Results." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
×
Page 42
Page 43
Suggested Citation:"Chapter 5 - Discussion of Study Results." National Academies of Sciences, Engineering, and Medicine. 2004. Safety Evaluation of Permanent Raised Pavement Markers. Washington, DC: The National Academies Press. doi: 10.17226/13724.
×
Page 43

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40 CHAPTER 5 DISCUSSION OF STUDY RESULTS This chapter discusses the results presented in Chapter 4 in conjunction with related human factors issues. The purpose of the discussion presented here is to link the statistical results of this research study with other findings of past research studies that have analyzed PRPM installations and observed effects in driver behavior. The discussions are presented for two-lane roadways and four-lane freeways separately. 5.1 TWO-LANE ROADWAYS 5.1.1 Overview of Human Factors Issues The purpose of PRPMs is to provide improved delineation at night. Studies have shown that drivers on approaches to curves need 3 to 5 seconds of preview distance in order to feel comfortable with the changes in the road path (25). At night, such long preview distances cannot be provided by paint, but are possible using PRPMs, post-mounted delin- eators, and chevrons. It is expected that the improved visi- bility produced by PRPMs will affect crash rates by affecting two types of driver behavior: • Lane control and positioning and • Speed control. 5.1.1.1 Lane Control and Positioning Previous studies of conventional PRPMs have found that PRPMs on curves cause drivers to shift away from the cen- terline at night (21, 34, 42). However, the impact on lane position during the day is not conclusive: one study (35) shows a shift toward the centerline and another study (21) shows a shift away from the centerline. The conventional PRPMs analyzed in the above studies protrude much higher above the road surface than do the snowplowable PRPMs analyzed in this study. Because of this protrusion, conventional PRPMs provide an auditory warning of lane crossing that is not found with snowplowable PRPMs. Therefore, any change in lane position at night with snow- plowable PRPMs is expected to result from improved delin- eation. Kallberg (42) found that drivers move away from con- spicuous post-mounted delineators. Kallberg’s finding may support this report’s hypothesis of snowplowable PRPMs’ effect on lane control and positioning. 5.1.1.2 Speed Control When the preview of the road ahead is reduced, as it is dur- ing nighttime with low-beam headlights, lane control becomes more difficult and driver workload increases, causing drivers to compensate by reducing their speed. Conversely, when the preview of the road is improved through delineation, driver workload decreases and drivers may compensate by increasing speeds. Harms (43) investigated speed choice in fog and found that drivers tend to undercompensate (i.e., not reduce speeds enough) in poor visibility conditions. Because of this and other studies of driver speed choice, Rumar and Marsh (44) predict that drivers overcompensate (i.e., increase speeds too much) in improved visibility conditions. Studies have found that speed increases at night after the implementation of PRPMs (21, 34). Improved delineation, in the form of post-mounted delineators, was associated with nighttime speed increases and increased crash frequency on roads with low design standards, but not on roads with high design standards (42). A driver who increases speed, especially at night, is responding inappropriately. While PRPMs improve the vis- ibility of changes in the road path, they do not improve the visibility of other hazards, such as pedestrians, bicyclists, animals, and debris. Higher speeds lead to longer stopping dis- tances and greater crash potential. Higher speeds in curves will result in an increase in lateral acceleration and a greater potential for run-off-road crashes. The issue of speed is likely to be more of a problem on curves with small radii. On high-speed roads, such curves force drivers to make large speed reductions. However, stud- ies of driver lateral acceleration in curves show that drivers drive closer to the safety margin on tight curves than on gen- tle curves (45). This suggests that drivers are reluctant to drop speed too much and trade off comfort for time savings. Any small increase in speeds associated with PRPMs will have a greater negative safety effect when drivers are closer to the safety margin. This greater negative safety effect may be the reason underlying the Kallberg study’s finding (42),

41 which is an increase in crash frequency on roads with low design standards. Wet weather is another situation in which drivers are likely operating closer to the safety margin by not slowing suffi- ciently to compensate for increased braking distance. Thus, the negative impacts of any speed increases on tight curves may be exacerbated in wet weather. 5.1.2 Expected PRPM Impacts on Two-Lane Roadways To summarize, the substantial improvements in nighttime centerline visibility and the associated increase in driver com- fort after the implementation of PRPMs are expected to have the following impacts on driver behavior: • Reduced oncoming and left-lane encroachments at night, • Increases in shoulder encroachments at night, and • Small increases in speeds at night. These changes in driver behavior are expected to have the following impacts in turn: • Decreases in nighttime head-on crashes, with increasing benefits as traffic volumes increase; • Decreases in safety benefits as the degree of curvature increases; • Decreases in safety benefits as the vehicle moves closer to the edgeline; • Decreases in wet weather nighttime crashes; • Slight decreases in daytime wet weather crashes; and • Less positive effects of PRPMs on gentle curves and less negative effects on sharp curves on roads with illumina- tion when compared with roads without illumination. 5.1.2.1 Decreases in Nighttime Head-On Crashes, with Increasing Benefits as Traffic Volumes Increase The majority of head-on crashes are due to inadvertent excursions into the oncoming lane (only 4 percent of head- on fatalities are associated with overtaking) (46). The prob- ability that inadvertent excursions result in head-on crashes increases as traffic volumes increase. Given driver behavior, it is expected that (1) improved delineation of the centerline by PRPMs at night and the consequent movement away from the centerline will reduce head-on crashes at night and (2) the benefit of PRPMs will increase as traffic volumes increase. The safety impacts expected, as described above, are sup- ported by the results of the composite analyses (see Chapter 4) undertaken in this research study. These results are as follows: • There were statistically significant decreases in head-on crashes on two-lane roadways in New Jersey (nonselec- tive implementation). • There were statistically nonsignificant decreases in head- on crashes and a statistically significant increase in total crashes in two-lane roadway Illinois data (nonselective implementation). • Although available sample sizes did not permit a com- posite or disaggregate analysis of nighttime head-on crashes, the results in Table 4-3 (AMFs for two-lane roadways) show statistically significant improvements in the safety performance of PRPMs at night as traffic volumes increase. 5.1.2.2 Decreases in Safety Benefits as the Degree of Curvature Increases On sharper curves (i.e., with a higher degree of curvature), it is possible that the negative safety impact of speed increases is not offset by the positive safety impact of improved visibil- ity; failure to offset the negative safety impact would result in an increase of nighttime crashes. This proposition is sup- ported by the univariate analysis of two-lane roadways and by the results of the disaggregate analysis in Table 4-3, which show that PRPMs will have negative safety effects on road- ways with a degree of curvature exceeding 3.5. The negative safety effect holds true for all ranges of traffic volumes avail- able in this research study. 5.1.2.3 Decreases in Safety Benefits as the Vehicle Moves Closer to the Edgeline The risk of run-off-road crashes on two-lane roadways is hypothesized (as described in Section 5.1.1) to be higher on roadways with lower design standards (e.g., with higher degrees of curvature and narrower pavements widths) because vehicles move away from the centerline to the edgeline to avoid the PRPMs. For example, narrower shoulder widths reduce the recovery area for vehicles that leave the travel lane. The uni- variate analysis indicated a positive correlation between traf- fic volumes and pavement widths, meaning that higher-traffic- volume roadways are normally associated with higher roadway design standards. This may in part explain why the AMFs in Table 4-3 show decreases in safety benefits with decreased traffic volumes, which are in turn associated with roads with narrower pavement widths. 5.1.2.4 Decreases in Wet Weather Nighttime Crashes The significant improvement in visibility in wet weather at night would be expected to reduce run-off-road crashes and head-on crashes on gentle curves where small increases in speed would not significantly increase crash risk. The results of the safety composite analysis shown in Table 4-1 indicated a statistically significant decrease in wet weather nighttime

42 crashes (by 20 percent) in two-lane roadways in New York where locations were selected for PRPM installation on the basis of their nighttime wet weather crash history. 5.1.2.5 Slight Decreases in Daytime Wet Weather Crashes Snowplowable PRPMs may improve daytime visibility under wet weather conditions because of the profile of the PRPM housing above the film of water covering the painted markings. This improvement in visibility might contribute to a decrease in daytime wet weather crashes. The safety composite analysis of the two-lane roadways in New York indicated a 20-percent reduction in all wet weather crashes after selective implementation of snowplowable PRPMs. The composite analysis did not separately evaluate daytime wet weather crashes. 5.1.2.6 Less Positive Effects of PRPMs for Gentle Curves and Less Negative Effects for Sharp Curves on Roads with Illumination when Compared with Roads without Illumination The improvement in delineation visibility is expected to be more noticeable on roads without illumination. Illumination is expected to reduce both the positive effects of PRPMs on visibility and the negative effects on speed, since illumina- tion assists drivers in determining lane position and control. The presence of illumination on sharp curves is hypothesized to reduce the potential negative effect of PRPMs due to increased speeds. Because of limited sample sizes of two- lane curves with illumination, it was not possible to deter- mine the net effect of illumination and PRPMs. The presence of illumination on gentle curves and tan- gents could reduce the positive effects of PRPMs on forward visibility and could cause the results in Table 4-3 to be an overestimation of the effectiveness of PRPMs on illuminated roadways with gentle curvature. On these roadways, as on roadways with sharp curves, it was not possible to determine the net effect of illumination and PRPMs because of limited sample sizes of two-lane curves with illumination. 5.2 FOUR-LANE FREEWAYS 5.2.1 Overview of Human Factors Issues The common practice on four-lane freeways is to imple- ment PRPMs nonselectively with the aim of providing a comfortable driving environment and improving safety in con- ditions of decreased visibility (i.e., nighttime and wet weather conditions). As with two-lane roadways, the implementation of PRPMs on the lane line of freeways is expected to impact two types of driver behavior: • Lane control and positioning and • Speed control. 5.2.1.1 Lane Control and Positioning Increased delineation of the lane line is likely to cause drivers to stay better centered in lanes delineated on both sides. Where the lane line but not the edgeline is delineated, drivers are likely to position themselves farther from the delineated line toward the edgelines demarcating the median and the shoulder. The number of lane line encroachments, and there- fore the potential for sideswipe crashes, will decrease. Since the possibility that a lane encroachment resulting in a crash is higher at higher traffic volumes, a measure that reduces lane line encroachments will have a proportionally greater effect at higher traffic volumes. The safety benefits of reduced- lane-line encroachments are expected to be greater than the potential negative safety impact of increased shoulder encroachments, where there are wide shoulders and shoulder rumble strips. 5.2.1.2 Speed Control Improved visibility is likely to increase driver confidence and comfort to the extent that travel speeds will increase. Freeways have high design standards (e.g., high standards for degree of curvature, lane widths, and shoulder widths); there- fore, it is unlikely that small speed increases will cause drivers to operate at or close to the margin of safety with respect to these parameters. Speed increases, however, may result in increased crash occurrence due to increased stopping, decel- eration, and weaving distances required, especially during conditions of reduced visibility. 5.2.2 Expected PRPM Impacts on Four-Lane Freeways To summarize, the substantial improvements in visibility of delineation at night and during poor weather conditions, and the associated increase in driver comfort after the imple- mentation of PRPMs, could have the following impacts on driver behavior at night and poor daytime weather conditions: • Reduced encroachments over the lane line, • Increased shoulder encroachments, and • Small increases in speed at night. These changes in driver behavior are hypothesized to have the following impacts on crashes in turn:

43 • Decreases in nighttime crashes, with increasing benefits at higher traffic volumes; • Decreases in guidance-related crashes (e.g., sideswipes); and • Decreases in wet weather crashes. 5.2.2.1 Decreases in Nighttime Crashes, with Increasing Benefits at Higher Traffic Volumes The results of the composite analysis in Table 4-5 show that PRPMs had no overall effect on nighttime crashes. How- ever, the results of the disaggregate analysis, presented in Table 4-7, show that snowplowable PRPMs may only be effective in reducing nighttime crashes on four-lane freeways with AADTs exceeding 20,000 veh/day. 5.2.2.2 Decreases in Guidance-Related Crashes The results for four-lane freeways in Missouri (Table 4-5) show a statistically significant 10.3-percent reduction in guidance-related crashes after the implementation of snow- plowable PRPMs. A similar statistically significant result was not observed for Pennsylvania four-lane freeways. This differ- ence between the two states may be explained by two design attributes: rumble strips and shoulder width. On average, Missouri freeways have wider shoulders and a higher pro- portion of freeways with shoulder rumble strips than Penn- sylvania freeways have. Table 3-15 shows that the average shoulder width on Pennsylvania freeways is 6.3 ft (1.9 m) compared with the 9.3 ft (2.8 m) in Missouri. Table 3-25 shows that 53 percent of four-lane freeways in Missouri have shoulder rumble strips compared with 38 percent of Penn- sylvania freeways. 5.2.2.3 Decreases in Wet Weather Crashes The results of the composite analysis indicated that snow- plowable PRPMs were effective in reducing wet weather crashes in four-lane freeways in Missouri (12.8 percent) and Pennsylvania (5.4 percent).

Next: Chapter 6 - Guidelines for the Use of Snowplowable PRPMs »
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 518: Safety Evaluation of Permanent Raised Pavement Markers examines the safety performance of snowplowable permanent raised pavement markers on two-lane roadways and four-lane freeways.

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